PUBLICATIONS

Natural design and fabrication strategies have long served as a source of inspiration for novel materials with enhanced properties. Less investigated is the prospect of leveraging the complexity of readily available, naturally occurring micro-/nanostructures as platforms for investigating functional materials. In the field of phononics, exploiting structural biocomposites is gaining traction; but finding natural phononic structures that interact with ultra- and hypersonic acoustic waves remains an open quest. In this context, the phononic behavior of natural Nacre, a biocomposite often looked at for inspiration due to its superlattice-like architecture of alternating organic and inorganic phases, is here characterized. To such end, a combination of non-contact pump-probe laser ultrasonics techniques and Brillouin spectroscopy are employed to interrogate Nacre’s hierarchical structure at the micro- and nanoscale and measure its phononic dispersion behavior in the MHz and GHz range. It is found that for wavelengths longer than the brick-and-mortar characteristic length, Nacre behaves as a dispersionless medium with effective transversely isotropic properties; but as the wavelengths become comparable to its structural periodicity an involved phononic spectrum arises which challenges the notion of a perfectly periodic, high mechanical-contrast biocomposite.

Machine learning (ML) is emerging as a transformative tool for the design of architected materials, offering properties that far surpass those achievable through lab-based trial-and-error methods. However, a major challenge in current inverse design strategies is their reliance on extensive computational and/or experimental datasets, which becomes particularly problematic for designing micro-scale stochastic architected materials that exhibit nonlinear mechanical behaviors. Here, we introduce a new end-to-end scientific ML framework, leveraging deep neural operators (DeepONet), to directly learn the relationship between the complete microstructure and mechanical response of architected metamaterials from sparse but high-quality in situ experimental data. The approach facilitates the inverse design of structures tailored to specific nonlinear mechanical behaviors. Results obtained from spinodal microstructures, printed using two-photon lithography, reveal that the prediction error for mechanical responses is within a range of 5 – 10%. Our work underscores that by employing neural operators with advanced micro-mechanics experimental techniques, the design of complex micro-architected materials with desired properties becomes feasible, even in scenarios constrained by data scarcity……

Efficient and nontoxic delivery of foreign cargo into cells is a critical step in many biological studies and cell engineering workflows with applications in areas such as biomanufacturing and cell-based therapeutics. However, effective molecular delivery into cells involves optimizing several experimental parameters. In the case of electroporation-based intracellular delivery, there is a need to optimize parameters like pulse voltage, duration, buffer type, and cargo concentration for each unique application. Here, we present the protocol for fabricating and utilizing a high-throughput multi-well localized electroporation device (LEPD) assisted by deep learning–based image analysis to enable rapid optimization of experimental parameters for efficient and nontoxic molecular delivery into cells. The LEPD and the optimization workflow presented herein are relevant to both adherent and suspended cell types and different molecular cargo (DNA, RNA, and proteins). The workflow enables multiplexed combinatorial experiments and can be adapted to cell engineering applications requiring in vitro delivery.

The mechanical and thermal properties of transition metal dichalcogenides (TMDs) are directly relevant to their applications in electronics, thermoelectric devices, and heat management systems. In this study, we use a machine learning (ML) approach to parametrize molecular dynamics (MD) force fields to predict the mechanical and thermal transport properties of a library of monolayered TMDs (MoS₂, MoTe₂, WSe₂, WS₂, and ReS₂). The ML-trained force fields were then employed in equilibrium MD simulations to calculate the lattice thermal conductivities of the foregoing TMDs and to investigate how they are affected by small and large mechanical strains. Furthermore, using nonequilibrium MD, we studied thermal transport across grain boundaries. The presented approach provides a fast albeit accurate methodology to compute both mechanical and thermal properties of TMDs, especially for relatively large systems and spatially complex structures, where density functional theory computational cost is prohibitive.

The potential of Kirigami-based metamaterials as versatile platforms in applications where shape-morphability is desired is presently well established. Interestingly, in-plane loading under simple tension has been the predominant actuation-of-choice, with other fundamental loadings (e.g., shear, torsion) being largely overlooked. In this work, we explore the instability landscape of a simple Kirigami motif under in-plane shear loading and demonstrate how the incorporation of shear actuation effectively augments both the design and output space of Kirigami-based metamaterials. The nonlinear response of Kirigami under shear is first elucidated parametrically via nonlinear Finite Element Analysis on a subset of geometrically asymmetric motifs. The interplay between cut layout and the bidirectional nature of in-plane shear enables a variegated bifurcation landscape that includes unusual inversions and sharp kinks in the bifurcation curves. The numerical analysis is complemented with experimental validation of the out-of-plane bifurcation curves of several cases of interest by means of shadow moiré and laser displacement sensing. Together with the experimental validation, a proof-of-concept demonstration of the applicability of Kirigami-shear metamaterials as multistate electro-mechanical switches is presented….

The mechanical properties and deformation mechanisms of single crystal magnesium under c-axis quasi-static and high-strain rate compressions are investigated through in situ scanning electron microscope (SEM) experiments and post-mortem transmission electron microscope (TEM) characterization. The findings revealed that ductility and high rates of hardening are preserved for pillars as large as 15 μm. Furthermore, rate effects result in a mild increase in flow stress with plastic deformations controlled primarily by the slip of <a+c> type dislocations. Importantly and in contrast to other literature reports, plastic deformation occurs in the absence of twining. As the strain increases and plastic deformation exceeds about 4%, crystal rotation activates basal slip, <a> type dislocations, resulting in a more rate independent flow stress. TEM observation on micropillars compressed at a strain rate of 250/s, revealed the activation of {112‾2‾}<1‾1‾23> slip systems and high mobility of screw dislocations as major contributors to plastic strains in excess of 10% without fracture….

Mechanical metamaterials, whose unique mechanical properties stem from their structural design rather than material constituents, are gaining popularity in engineering applications. In particular, recent advances in self-assembly techniques offer the potential to fabricate load-bearing mechanical metamaterials with unparalleled feature size control and scalability compared to those produced by additive manufacturing (AM). Yet, the field is still in its early stages. In this perspective, we first provide an overview of the state-of-the-art self-assembly techniques, with a focus on the copolymer and colloid crystal self-assembly processes. We then discuss current challenges and future opportunities in this research area, focusing on novel fabrication approaches, the need for high-throughput characterization methods, and the integration of Machine Learning (ML) and lab automation for inverse design…

Vacancy engineering of 2H-transition metal dichalcogenides (2H-TMDs) has recently attracted great attention due to its potential to fine-tune the phonon and opto-electric properties of these materials. From a mechanical perspective, this symmetry-breaking process typically reduces the overall crack resistance of the material and adversely affects its reliability. However, vacancies can trigger the formation of heterogeneous phases that synergistically improve fracture properties. In this study, using MoSe₂ as an example, we characterize the types and density of vacancies that can emerge under electron irradiation and quantify their effect on fracture. Molecular dynamic (MD) simulations, employing a re-parameterized Tersoff potential capable of accurately capturing bond dissociation and structural phase changes, reveal that isolated transition metal monovacancies or chalcogenide divacancies tend to arrest the crack tip and hence enhance the monolayer toughness…

Lattice-based constructs, often made by additive manufacturing, are attractive for many applications. Typically, such constructs are made from microscale or larger elements; however, smaller nanoscale components can lead to more unusual properties, including greater strength, lighter weight, and unprecedented resiliencies. Here, solid and hollow nanoparticles (nanoframes and nanocages; frame size: ~15 nanometers) were assembled into colloidal crystals using DNA, and their mechanical strengths were studied. Nanosolid, nanocage, and nanoframe lattices with identical crystal symmetries exhibit markedly different specific stiffnesses and strengths. Unexpectedly, the nanoframe lattice is approximately six times stronger than the nanosolid lattice. Nanomechanical experiments, electron microscopy, and finite element analysis show that this property results from the buckling, densification, and size-dependent strain hardening of nanoframe lattices…

For many decades, experimental solid mechanics has played a crucial role in characterizing and understanding the mechanical properties of natural and novel artificial materials. Recent advances in machine learning (ML) provide new opportunities for the field, including experimental design, data analysis, uncertainty quantification, and inverse problems. As the number of papers published in recent years in this emerging field is growing exponentially, it is timely to conduct a comprehensive and up-to-date review of recent ML applications in experimental solid mechanics. Here, we first provide an overview of common ML algorithms and terminologies that are pertinent to this review, with emphasis placed on physics-informed and physics-based ML methods. Then, we provide thorough coverage of recent ML applications in traditional and emerging areas of experimental mechanics, including fracture mechanics, biomechanics, nano- and micromechanics, architected materials, and two-dimensional materials… 

Membrane disruption using Bulk Electroporation (BEP) is a widely used non-viral method for delivering biomolecules into cells. Recently, its microfluidic counterpart, Localized Electroporation (LEP), has been successfully used for several applications ranging from reprogramming and engineering cells for therapeutic purposes to nondestructive sampling from live cells for temporal analysis. However, the side effects of these processes on gene expression, that can affect the physiology of sensitive stem cells are not well understood. Here, we use single cell RNA sequencing (scRNA-seq) to investigate the effects of BEP and LEP on murine neural stem cell (NSC) gene expression. Our results indicate that unlike BEP, LEP does not lead to extensive cell death or activation of cell stress response pathways that may affect their long-term physiology. Additionally, our demonstrations show that LEP is suitable for multi-day delivery protocols as it enables better preservation of cell viability and integrity as compared to BEP.

Delivery of proteins and protein–nucleic acid constructs into live cells enables a wide range of applications from gene editing to cell-based therapies and intracellular sensing. However, electroporation-based protein delivery remains challenging due to the large sizes of proteins, their low surface charge, and susceptibility to conformational changes that result in loss of function. Here, we use a nanochannel-based localized electroporation platform with multiplexing capabilities to optimize the intracellular delivery of large proteins (β-galactosidase, 472 kDa, 75.38% efficiency), protein–nucleic acid conjugates (protein spherical nucleic acids (ProSNA), 668 kDa, 80.25% efficiency), and Cas9-ribonucleoprotein complex (160 kDa, ∼60% knock-out and ∼24% knock-in) while retaining functionality post-delivery…

Multi-walled carbon nanotube (MWCNT) and carbon nanofiber (CNF) additions increase the elastic modulus, flexural strength, and toughness of Portland cement concrete. However, the interaction mechanism between cement constituents and these nanomaterials is not fully understood. A modified MWCNT-coated atomic force microscopy (AFM) probe is developed by coating a silica particle with oxidized MWCNT through layer-by-layer assembly and adhering it to a tipless AFM cantilever. The probe allows measurement of adhesion between MWCNT and the substrate with a force control procedure. SEM-EDS is acquired in the same region as AFM measurements through a benchmarking scheme to correlate chemistry with the measured adhesion. Statistical deconvolution shows C–S–H regions have lower adhesion to MWCNT than intermixed regions (C–S–H/Clinker)…

Quantifying the intrinsic mechanical properties of two-dimensional (2D) materials is essential to predict the long-term reliability of materials and systems in emerging applications ranging from energy to health to next-generation sensors and electronics. Currently, measurements of fracture toughness and identification of associated atomistic mechanisms remain challenging. Herein, we report an integrated experimental–computational framework in which in-situ high-resolution transmission electron microscopy (HRTEM) measurements of the intrinsic fracture energy of monolayer MoS₂ and MoSe₂ are in good agreement with atomistic model predictions based on an accurately parameterized interatomic potential. Changes in crystalline structures at the crack tip and crack edges, as observed in in-situ HRTEM crack extension tests, are properly predicted. Such a good agreement is the result of including large…

The emerging field of cell therapy offers the potential to treat and even cure a diverse array of diseases for which existing interventions are inadequate. Recent advances in micro and nanotechnology have added a multitude of single cell analysis methods to our research repertoire. At the same time, techniques have been developed for the precise engineering and manipulation of cells. Together, these methods have aided the understanding of disease pathophysiology, helped formulate corrective interventions at the cellular level, and expanded the spectrum of available cell therapeutic options. This review discusses how micro and nanotechnology have catalyzed the development of cell sorting, cellular engineering, and single cell analysis technologies, which have become essential workflow components in developing cell-based therapeutics. The review focuses on the technologies adopted in research studies and explores the opportunities and challenges in combining the various elements of cell engineering and single cell analysis into the next generation of integrated and automated platforms that can accelerate preclinical studies and translational research.

Kirigami-engineering has become an avenue for realizing multifunctional metamaterials that tap into the instability landscape of planar surfaces embedded with cuts. Recently, it has been shown that two-dimensional Kirigami motifs can unfurl a rich space of out-of-plane deformations, which are programmable and controllable across spatial scales. Notwithstanding Kirigami’s versatility, arriving at a cut layout that yields the desired functionality remains a challenge. Here, we introduce a comprehensive machine learning framework to shed light on the Kirigami design space and to rationally guide the design and control of Kirigami-based materials from the meta-atom to the metamaterial level. We employ a combination of clustering, tandem neural networks, and symbolic regression analyses to obtain Kirigami that fulfills specific design constraints and inform on their control and deployment…

Manipulation of cells for applications such as biomanufacturing and cell-based therapeutics involves introducing biomolecular cargoes into cells. However, successful delivery is a function of multiple experimental factors requiring several rounds of optimization. Here, we present a high-throughput multiwell-format localized electroporation device (LEPD) assisted by deep learning image analysis that enables quick optimization of experimental factors for efficient delivery. We showcase the versatility of the LEPD platform by successfully delivering biomolecules into different types of adherent and suspension cells. We also demonstrate multicargo delivery with tight dosage distribution…

Genome engineering of cells using CRISPR/Cas systems has opened new avenues for pharmacological screening and investigating the molecular mechanisms of disease. A critical step in many such studies is the intracellular delivery of the gene editing machinery and the subsequent manipulation of cells. However, these workflows often involve processes such as bulk electroporation for intracellular delivery and fluorescence activated cell sorting for cell isolation that can be harsh to sensitive cell types such as human-induced pluripotent stem cells (hiPSCs). This often leads to poor viability and low overall efficacy, requiring the use of large starting samples….

Nondestructive cell membrane permeabilization systems enable the intracellular delivery of exogenous biomolecules for cell engineering tasks as well as the temporal sampling of cytosolic contents from live cells for the analysis of dynamic processes. Here, we report a microwell array format live-cell analysis device (LCAD) that can perform localized-electroporation induced membrane permeabilization, for cellular delivery or sampling, and directly interfaces with surface-based biosensors for analyzing the extracted contents. We demonstrate the capabilities of the LCAD via an automated high-throughput workflow for multimodal analysis of live-cell dynamics, consisting of quantitative measurements of enzyme activity using self-assembled monolayers for MALDI mass spectrometry (SAMDI) and deep-learning enhanced imaging and analysis. By combining….

Soft biological tissues are natural biomaterials with structures that have evolved to perform physiological functions, for example, conferring elasticity while preserving the mechanical integrity of arteries. Furthermore, the mechanical properties of the tissue extracellular matrix (ECM) significantly affect cell behavior and organ function. ECM mechanical properties are strongly affected by collagen ultrastructure, and perturbations in collagen networks can cause tissue mechanical failure. It is thus crucial to understand the ultrastructural mechanical properties of soft tissues. Herein, the ultrastructural and nanomechanical properties of arterial tissues are reported…

Reliable transfer processes that enable manipulation of two-dimensional (2D) materials, e.g., transition metal dichalcogenides (TMDCs) and MXenes, from one substrate to another has been a necessity for successful device fabrication. With both mechanical exfoliation and chemical vapor deposition (CVD) widely used, a versatile, clean, deterministic, and yet simple transfer technique is highly needed. To address such need, we developed a transfer method that takes advantage of wettability contrast between interfaces without the use of sacrificial layers or chemical processes. More importantly, a setup was developed to carry out this transfer method with high sample selectivity and fine control of the position…

Annihilation of vacancy clusters in monolayer molybdenum diselenide (MoSe₂) under electron beam irradiation is reported. In situ high‐resolution transmission electron microscopy observation reveals that the annihilation is achieved by diffusion of vacancies to the free edge near the vacancy clusters. Monte Carlo simulations confirm that it is energetically favorable for the vacancies to locate at the free edge. By computing the minimum energy path for the annihilation of one vacancy cluster as a case study, it is further shown that electron beam irradiation and pre‐stress in the suspended MoSe₂ monolayer are necessary for the vacancies to overcome the energy barriers for diffusion.

Combining experimental and computational studies of nanocomposite interfaces is highly needed to gain insight into their performance. However, there are very few literature reports, combining well-controlled atomic force microscopy experiments with molecular dynamic simulations, which explore the role of polymer chemistry and assembly on interface adhesion and shear strength. In this work, we investigate graphene oxide (GO)-polymer interfaces prevalent in nanocomposites based on a nacre-like architectures. We examine the interfacial strength resulting from van der Waals and hydrogen bonding interactions by comparing the out-of-plane separation and in-plane shear deformations of GO-polyethylene glycol (PEG) and GO-polyvinyl alcohol (PVA). The investigation reveals an overall better mechanical performance for the anhydrous GO-PVA system in both out-of-plane and in-plane deformation modes, highlighting the benefits of the donor-acceptor hydrogen bond formation present in GO-PVA. 

This investigation presents a generally applicable framework for parameterizing interatomic potentials to accurately capture large deformation pathways. It incorporates a multi-objective genetic algorithm, training and screening property sets, and correlation and principal component analyses. The framework enables iterative definition of properties in the training and screening sets, guided by correlation relationships between properties, aiming to achieve optimal parametrizations for properties of interest. Specifically, the performance of increasingly complex potentials, Buckingham, Stillinger-Weber, Tersoff, and modified reactive empirical bond-order potentials are compared. Using MoSe₂ as a case study, we demonstrate good reproducibility of training/screening properties and superior transferability. 

Significant efforts have been devoted to enhancing the sensitivity and working range of flexible pressure sensors to improve the precise measurement of subtle variations in pressure over a wide detection spectrum. However, achieving sensitivities exceeding 1000 kPa− 1 while maintaining a pressure working range over 100 kPa is still challenging because of the limited intrinsic properties of soft matrix materials. Here, we report a magnetic field-induced porous elastomer with micropillar arrays (MPAs) as sensing materials and a well-patterned nickel fabric as an electrode. The developed sensor exhibits an ultrahigh sensitivity of 10,268 kPa− 1 (0.6–170 kPa) with a minimum detection pressure of 0.25 Pa and a fast response time of 3 ms because of the unique structure of the MPAs and the textured morphology of the electrode. 

Single-cell delivery platforms like microinjection and nanoprobe electroporation enable unparalleled control over cell manipulation tasks but are generally limited in throughput. Here, we present an automated single-cell electroporation system capable of automatically detecting cells with artificial intelligence (AI) software and delivering exogenous cargoes of different sizes with uniform dosage. We implemented a fully convolutional network (FCN) architecture to precisely locate the nuclei and cytosol of six cell types with various shapes and sizes, using phase contrast microscopy. Nuclear staining or reporter fluorescence was used along with phase contrast images of cells within the same field of view to facilitate the manual annotation process.

Dislocation dynamic simulations are intended as a tool to understand and predict the mechanical behavior of metallic materials, but its prediction has never been directly verified by experiments due to differences in specimen strain rate and size. In this work, a comprehensive experimental framework is proposed to attempt direct comparison between experiments and discrete dislocation dynamics (DDD) modelling. By integrating high-throughput sample fabrication and a customized testing apparatus, the sample size and strain rate typically employed in DDD simulations are explored experimentally. Constitutive properties such as stress-strain response are measured, and microstructural information is obtained from transmission electron microscopy (TEM) imaging, electron backscatter diffraction (EBSD), and TEM-based orientation mapping.

A Matter of Size? Material, Structural and Mechanical strategies for size adaptation in the elytra of Cetoniinae beetles

M. Asgari*, N.A. Alderete*, Z. Lin*, R. Benavides, H. D. Espinosa†

Acta Biomaterialia 122, 2021, pp. 236-248

The ability of Nature to adapt common structural motifs and leverage a limited pool of materials to respond to evolutionary pressures is unparalleled. Beetles, in particular, embody the finest expressions of Nature’s deftness, as evinced by their pervasiveness and the richness of their diversity. Here, material, structural and mechanical characteristics of the elytra of four different species of Cetoniinae beetles are compared to elucidate the natural strategies guiding adaptations to body size. Commensurate with the integral implications of body size on functionality, a variety of multimodal and multiscale characterization methods are used, revealing the presence of size-invariant and size-dependent features. As such, this work seeks to establish a roadmap for future systematic, comparative analyses of beetle biomechanics, scaling and phylogenetics.

Kirigami Engineering—Nanoscale Structures Exhibiting a Range of Controllable 3D Configurations

X. Zhang, L. Medina, H. Cai, V. Aksyuk, H.D. Espinosa†, D. Lopez†

Advanced Materials 33(5), 2021, pp. 2005275

Kirigami structures provide a promising approach to transform flat films into 3D complex structures that are difficult to achieve by conventional  fabrication approaches. By designing the cutting geometry, it is shown that distinct buckling-induced out-of-plane configurations can be obtained, separated by a sharp transition characterized by a critical geometric dimension of the structures. In situ electron microscopy experiments reveal the effect of the ratio between the in-plane cut size and film thickness on out-of-plane configurations. Moreover, geometrically nonlinear finite element analyses (FEA) accurately predict the out-of-plane modes measured experimentally, their transition as a function of cut geometry, and provide the stress–strain response of the kirigami structures. The combined computational–experimental approach and results reported here represent a step forward in the characterization of thin films experiencing buckling-induced out-of-plane shape transformations and provide a path to control 3D configurations of micro- and nanoscale kirigami structures.

Programmable 3D structures via Kirigami engineering and controlled stretching

N.A. Alderete, L. Medina, L. Lamberti, C. Sciammarella, H.D. Espinosa†

Extreme Mechanics Letters 43, 2021, pp. 101146

Kirigami with a variety of cut patterns have been recently investigated as means to transform 2D surfaces into 3D structures, offering a number of functionalities. In this work, we show that a single Kirigami motif, defining two inner panels connected by hinges, can generate a rich variety of out-of-plane symmetric and asymmetric deformation modes. The out-of-plane responses, caused by local instabilities when a far field tensile load is applied, manifest themselves at the inner plates, exhibiting a combination of one- and two-dimensional rotations (tilt and twist), effectively morphing the planar geometries into complex 3D surfaces. By conducting numerical analyses and experiments, the relationship between changes in the cut geometry, and out-of-plane responses is identified. In particular, two types of instabilities emerge, coincident bifurcation points for both inner plates, responsible for two-dimensional tilts, and sequential bifurcation points, which produce one-dimensional tilts with distinct buckling thresholds. 

High Throughput and Highly Controllable Methods for In Vitro Intracellular Delivery

J. Brooks, G. Minnick, P. Mukherjee, A. Jaberi, L. Chang†, H.D. Espinosa†, R. Yang†

Small 16(51), 2020, pp. 2004917

In vitro and ex vivo intracellular delivery methods hold the key for releasing the full potential of tissue engineering, drug development, and many other applications. In recent years, there has been significant progress in the design and implementation of intracellular delivery systems capable of delivery at the same scale as viral transfection and bulk electroporation but offering fewer adverse outcomes. This review strives to examine a variety of methods for in vitro and ex vivo intracellular delivery such as flow-through microfluidics, engineered substrates, and automated probe-based systems from the perspective of throughput and control. Special attention is paid to a particularly promising method of electroporation using micro/nanochannel based porous substrates, which expose small patches of cell membrane to permeabilizing electric field. Porous substrate electroporation parameters discussed include system design, cells and cargos used, transfection efficiency and cell viability, and the electric field and its effects on molecular transport. 

Nanofountain Probe Electroporation Enables Versatile Single‐Cell Intracellular Delivery and Investigation of Postpulse Electropore Dynamics

S. S. P. Nathamgari*, N. Pathak*, V. Lemaitre*, P. Mukherjee, J. J. Muldoon, C.-Y. Peng, T. McGuire, J. N. Leonard, J.A. Kessler, H.D. Espinosa†

Small 16(51), 2020, pp. 2004917

Introducing exogenous molecules into cells with high efficiency and dosage control is a crucial step in basic research as well as clinical applications. Here, the capability of the nanofountain probe electroporation (NFP‐E) system to deliver proteins and plasmids in a variety of continuous and primary cell types with appropriate dosage control is reported. It is shown that the NFP‐E can achieve fine control over the relative expression of two cotransfected plasmids. Finally, the dynamics of electropore closure after the pulsing ends with the NFP‐E is investigated. Localized electroporation has recently been utilized to demonstrate the converse process of delivery (sampling), in which a small volume of the cytosol is retrieved during electroporation without causing cell lysis. 

Modelling and Analyses of Fiber Fabric and Fabric-Reinforced Polymers under Hypervelocity Impact Using Smooth Particle Hydrodynamics

S. Zhao, Z. Song, H.D. Espinosa†

International Journal of Impact Engineering 144, 2020, pp. 103586

In a hypervelocity impact (HVI) event, fiber fabrics and the fabric-reinforced polymers (FRP) would undergo shock compression, large deformation and fragmentation. The smooth particle hydrodynamics (SPH) approach was applied to assess the shielding performance of the fabric and its composite structure in a Whipple shield. In the fabric model, a fiber is built by SPH particles to properly reproduce the spreading feature of fragmented fabric under HVI. The simulations display that an aluminum panel, serving as the bumper of a Whipple, has the better performance in debris spreading than fabric layers. In the stuffed layer of a Whipple, the widely used plain weave fabric has the similar performance as the 3D weave both in debris spreading and speed retarding. The fabric model is further developed and extended to FRP by building fiber and polymer materials separately based on specific geometries. 

Fiber reorientation in hybrid helicoidal composites

D. Wang, A. Zaheri, B. Russell, H. Espinosa†, P. Zavattieri†

Journal of the Mechanical Behavior of Biomedical Materials 110, 2020, pp. 103914

Naturally occurring biological materials with stiff fibers embedded in a ductile matrix are commonly known to achieve excellent balance between stiffness, strength and ductility. In particular, biological composite materials with helicoidal architecture have been shown to exhibit enhanced damage tolerance and increased impact energy absorption. However, the role of fiber reorientation inside the flexible matrix of helicoid composites on their mechanical behaviors have not yet been extensively investigated. In the present work, we introduce a Discontinuous Fiber Helicoid (DFH) composite inspired by both the helicoid microstructure in the cuticle of mantis shrimp and the nacreous architecture of the red abalone shell. We employ 3D printed specimens, analytical models and finite element models to analyze and quantify in-plane fiber reorientation in helicoid architectures with different geometrical features. 

Folding at the Microscale: Enabling Multifunctional 3D Origami‐Architected Metamaterials

Z. Lin*, L.S. Novelino*, H. Wei*, N.A. Alderete*, G.H. Paulino†, H.D. Espinosa†, S. Krishnaswamy†

Small 16(35), 2020, pp. 2002229

Mechanical metamaterials inspired by the Japanese art of paper folding have gained considerable attention because of their potential to yield deployable and highly tunable assemblies. The inherent foldability of origami structures enlarges the material design space with remarkable properties such as auxeticity and high deformation recoverability and deployability, the latter being key in applications where spatial constraints are pivotal. This work integrates the results of the design, 3D direct laser writing fabrication, and in situ scanning electron microscopic mechanical characterization of microscale origami metamaterials, based on the multimodal assembly of Miura‐Ori tubes. The origami‐architected metamaterials, achieved by means of microfabrication, display remarkable mechanical properties: stiffness and Poisson’s ratio tunable anisotropy, large degree of shape recoverability, multistability, and even reversible auxeticity whereby the metamaterial switches Poisson’s ratio sign during deformation. 

Temporal Sampling of Enzymes from Live Cells by Localized  Electroporation and Quantification of Activity by SAMDI Mass Spectrometry

P. Mukherjee*, E.J. Berns*, C.A. Patino, E.H. Moully, L. Chang, S.S.P. Nathamgari, J.A. Kessler, M. Mrksich†, H.D. Espinosa†

Small 16(26), 2020, pp. 2000584

Measuring changes in enzymatic activity over time from small numbers of cells remains a significant technical challenge. In this work, a method for sampling the cytoplasm of cells is introduced to extract enzymes and measure their activity at multiple time points. A microfluidic device, termed the live cell analysis device (LCAD), is designed, where cells are cultured in microwell arrays fabricated on polymer membranes containing nanochannels. Localized electroporation of the cells opens transient pores in the cell membrane at the interface with the nanochannels, enabling extraction of enzymes into nanoliter‐volume chambers. In the extraction chambers, the enzymes modify immobilized substrates, and their activity is quantified by self‐assembled monolayers for matrix‐assisted laser desorption/ionization (SAMDI) mass spectrometry. By employing the LCAD‐SAMDI platform, protein delivery into cells is demonstrated. 

Scaling up single-cell mechanics to multicellular tissues–the role of the intermediate filament–desmosome network

J.A. Broussard†, A. Jaiganesh, H. Zarkoob, D.E. Conway, A.R. Dunn, H.D. Espinosa, P.A. Janmey, K.J. Green†

Journal of cell science 133(6), 2020, pp. 228031

Cells and tissues sense, respond to and translate mechanical forces into biochemical signals through mechanotransduction, which governs individual cell responses that drive gene expression, metabolic pathways and cell motility, and determines how cells work together in tissues. Mechanotransduction often depends on cytoskeletal networks and their attachment sites that physically couple cells to each other and to the extracellular matrix. One way that cells associate with each other is through Ca2+-dependent adhesion molecules called cadherins, which mediate cell–cell interactions through adherens junctions, thereby anchoring and organizing the cortical actin cytoskeleton. This actin-based network confers dynamic properties to cell sheets and developing organisms. However, these contractile networks do not work alone but in concert with other cytoarchitectural elements, including a diverse network of intermediate filaments. This Review takes a close look at the intermediate filament network and its associated intercellular junctions.

Nanofountain Probe Electroporation for Monoclonal Cell Line Generation

H.D. Espinosa, P. Mukherjee, C. Patino

Electroporation Protocols, 2020, pp. 59-68

In the field of genetic engineering, the modification of genes to produce stable cell lines has a variety of applications ranging from the development of novel therapeutics to patient specific treatments. To successfully generate a cell line, the gene of interest must be delivered into the cell and integrated into the genome. The efficiency of cell line generation systems therefore depends on the efficiency of delivery of genetically modifying molecules such as plasmids and CRISPR/CAS9 complexes. In this work, we describe a localized electroporation-based system to generate stable monoclonal cell lines. By employing the nanofountain probe electroporation (NFP-E) system, single cells in patterned cultures are selectively transfected with plasmids, grown, and harvested to obtain stably expressing cell lines. Methods for microcontact printing, cell culture, electroporation, and harvesting are detailed in this chapter.

Advancements in Optical Methods & Digital Image Correlation in Experimental Mechanics, Volume 3

M.-T. Lin, C. Sciammarella, H.D. Espinosa, C. Furlong, L. Lamberti, P. Reu, M. Sutton, C.-H. Hwang

Proceedings of the 2019 Annual Conference on Experimental and Applied Mechanics

Advancement of Optical Methods & Digital Image Correlation in Experimental Mechanics, Volume 3 of the Proceedings of the 2019 SEM Annual Conference & Exposition on Experimental and Applied Mechanics, the third volume of six from the Conference, brings together contributions to this important area of research and engineering.  The collection presents early findings and case studies on a wide range of optical methods ranging from traditional photoelasticity and interferometry to more recent DIC and DVC techniques, and includes papers in the following general technical research areas: DIC Methods & Its Applications, Photoelasticity and Interferometry Applications, Micro-Optics and Microscopic Systems, Multiscale and New Developments in Optical Methods DIC and its Applications for Inverse Problems.

Localized electroporation with track-etched membranes

S.S.P. Nathamgari, P. Mukherjee, J.A. Kessler, H.D. Espinosa

Proceedings of the National Academy of Sciences 116(46), 2019, pp. 22909-22910

In situ wear study reveals role of microstructure on self-sharpening mechanism in sea urchin teeth

H.D. Espinosa*†, A. Zaheri, H. Nguyen, D. Restrepo, M. Daly, M. Frank, J. McKittrick

Matter 1(5), 2019, pp. 1246-1261

The teeth of animals play a crucial role in their survival, and, like other body parts, they adapted to the host’s habitat to maximize their functionality. Superior performance in the sea urchin dentition system was hypothesized to emerge from sharpness preservation during the organism’s life span. In this work, a novel in situ scanning electron microscopy experimental methodology was employed to visualize a mechanism for sharpness preservation and to quantify conditions for its activation. Nonlinear finite-element modeling, incorporating experimentally measured nanoscale properties of constituents and interfaces, provided insight into synergistic effects between tooth architecture and material properties leading to sharpness preservation. The reported findings have the potential to influence the design of tools for mining, boring, and machining operations, e.g., cutting and grinding.

Stiffening of graphene oxide films by soft porous sheets

L. Mao, H. Park, R.A. Soler-Crespo, H.D. Espinosa†, T.H. Han†, S.B.T. Nguyen†, J. Huang†

Nature Communications 10(1), 2019, pp. 1-7

Graphene oxide (GO) sheets have been used as a model system to study how the mechanical properties of two-dimensional building blocks scale to their bulk form, such as paper-like, lamellar-structured thin films. Here, we report that the modulus of multilayer GO films can be significantly enhanced if some of the sheets are drastically weakened by introducing in-plane porosity. Nanometer-sized pores are introduced in GO sheets by chemical etching. Membrane-deflection measurements at the single-layer level show that the sheets are drastically weakened as the in-plane porosity increases. However, the mechanical properties of the corresponding multilayer films are much less sensitive to porosity. Surprisingly, the co-assembly of pristine and etched GO sheets yields even stiffer films than those made from pristine sheets alone. This is attributed to the more compliant nature of the soft porous sheets, which act as a binder to improve interlayer packing and load transfer in the multilayer films.

Atomically thin polymer layer enhances toughness of graphene oxide monolayers

R.A. Soler-Crespo, L. Mao, J. Wen, H.T. Nguyen, X. Zhang, X. Wei, J. Huang†, S.B.T. Nguyen†, H.D. Espinosa†

Matter 1(2), 2019, pp. 369-388

Two-dimensional (2D) materials with unconventional properties have emerged as promising candidates for addressing societal needs for advanced electronic devices and strong lightweight composites. However, their intrinsic brittle behavior and the associated risk for catastrophic failure have thus far limited their adoption. Here, we demonstrate a strategy for extrinsically toughening these materials through engineering the surface chemistry of a graphene oxide ultrathin polymer model system. Our combined experimental and computational explorations reveal a hierarchy of interactions that lead to an impressive 2-fold enhancement in graphene oxide toughness without modulus degradation. Such an extrinsic toughening strategy should be applicable to enhance the failure resistance of a variety of 2D materials, in their pristine state or with surface functionalization, which will in turn inspire the design of next-generation electronics and structural materials.

Load sensor instability and optimization of MEMS-based tensile testing devices

M.F. Pantano†, B. Calusi, B. Mazzolai, H.D. Espinosa, N.M. Pugno

Frontiers in Materials 6, 2019, pp. 161

MEMS-based tensile testing platforms are very powerful tools for the mechanical characterization of nanoscale materials, as they allow for testing of micro/nano-sized components in situ electron microscopes. In a typical configuration, they consist of an actuator, to deliver force/displacement, and a load sensor, which is connected to the sample like springs in series. Such configuration, while providing a high resolution force measurement, can cause the onset of instability phenomena, which can later compromise the test validity. In the present paper such phenomena are quantitatively discussed through the development of an analytical model, which allows to find a relationship between the rise of instability and the sensor stiffness, which is the key parameter to be optimized.

How Water Can Affect Keratin: Hydration‐Driven Recovery of Bighorn Sheep (Ovis Canadensis) Horns

W. Huang, A. Zaheri, W. Yang, D. Kisailus, R.O. Ritchie, H. Espinosa†, J. McKittrick†

Advanced Functional Materials 29(27), 2019, pp. 1901077

Keratin is one of the most common structural biopolymers exhibiting high strength, toughness, and low density. It is found in various tissues such as hairs, feathers, horns, and hooves with various functionalities. For instance, horn keratin absorbs a large amount of energy during intraspecific fights. Keratinized tissues are permanent tissues because of their basic composition consisting of dead keratinized cells that are not able to remodel or regrow once broken or damaged. The lack of a self‐healing mechanism presents a problem for horns, as they are under continued high risk from mechanical damage. In the present work, it is shown for the first time that a combination of material architecture and a water‐assisted recovery mechanism, in the horn of bighorn sheep, endows them with shape and mechanical property recoverability after being subjected to severe compressive loading. 

A Novel In Situ Experiment to Investigate Wear Mechanisms in Biomaterials

N. Alderete*, A. Zaheri*, H.D. Espinosa†

Experimental Mechanics 59(5), 2019, pp. 659-667

A number of experimental techniques have been used to characterize the mechanical properties and wear of biomaterials, from nanoindentation to scratch to atomic force microscopy testing. While all these experiments provide valuable information on the mechanics and functionality of biomaterials (e.g., animals’ teeth), they lack the ability to combine the measurement of force and sliding velocities with high resolution imaging of the processes taking place at the biomaterial-substrate interface. Here we present an experiment for the in situ scanning electron microscopy characterization of the mechanics of friction and wear of biomaterials with simultaneous control of mechanical and kinematic variables. To illustrate the experimental methodology, we report the wear of the sea urchin tooth, which exhibits a unique combination of architecture and material properties tailored to withstand abrasion loads in different directions. 

Advanced microelectromechanical systems-based nanomechanical testing: beyond stress and strain measurements

S. Bhowmick†, H. Espinosa, K. Jungjohann, T. Pardoen, O. Pierron

MRS Bulletin 44(6), 2019, pp. 487-493

The field of in situ nanomechanics is greatly benefiting from microelectromechanical systems (MEMS) technology and integrated microscale testing machines that can measure a wide range of mechanical properties at nanometer scales, while characterizing the damage or microstructure evolution in electron microscopes. This article focuses on the latest advances in MEMS-based nanomechanical testing techniques that go beyond stress and strain measurements under typical monotonic loadings. Specifically, recent advances in MEMS testing machines now enable probing key mechanical properties of nanomaterials related to fracture, fatigue, and wear. Tensile properties can be measured without instabilities or at high strain rates, and signature parameters such as activation volume can be obtained. Opportunities for environmental in situ nanomechanics enabled by MEMS technology are also discussed.

Nonlinear Mode Coupling and One-to-One Internal Resonances in a Monolayer WS2 Nanoresonator

S.S.P. Nathamgari, S. Dong, L. Medina, N. Moldovan, D. Rosenmann, R. Divan, D. Lopez, L.J. Lauhon, H.D. Espinosa†

NanoLetters 19(6), 2019, pp. 4052-4059

Nanomechanical resonators make exquisite force sensors due to their small footprint, low dissipation, and high frequencies. Because the lowest resolvable force is limited by ambient thermal noise, resonators are either operated at cryogenic temperatures or coupled to a high-finesse optical or microwave cavity to reach sub aN Hz–1/2 sensitivity. Here, we show that operating a monolayer WS2 nanoresonator in the strongly nonlinear regime can lead to comparable force sensitivities at room temperature. Cavity interferometry was used to transduce the nonlinear response of the nanoresonator, which was characterized by multiple pairs of 1:1 internal resonance. Some of the modes exhibited exotic line shapes due to the appearance of Hopf bifurcations, where the bifurcation frequency varied linearly with the driving force and forms the basis of the advanced sensing modality. 

An experimental setup for combined in-vacuo Raman spectroscopy and cavity-interferometry measurements on TMDC nano-resonators

S.S.P. Nathamgari*, S. Dong*, E. Hosseinian, L.J. Lauhon, H.D. Espinosa†

Experimental Mechanics 59(3), 2019, pp. 349-359

Nanoelectromechanical (NEMS) systems fabricated using atomically thin materials have low mass and high stiffness and are thus ideal candidates for force and mass sensing applications. Transition metal dichalcogenides (TMDCs) offer certain unique properties in their few-layered form – such as piezoelectricity and a direct band gap (in some cases) – and are an interesting alternative to graphene based NEMS. Among the demonstrated methods for displacement transduction in NEMS, cavity-interferometry provides exquisite displacement sensitivity. Typically, interferometric measurements are complemented with Raman spectroscopy to characterize the number of layers in 2D materials, and the measurements necessitate high vacuum conditions to eliminate viscous damping. Here, we report an experimental setup that facilitates both Raman spectroscopy and interferometric measurements on few-layered Tungsten Disulfide (WS2) resonators in high vacuum (<10−5 Torr) conditions.

Nanoscale toughening of ultrathin graphene oxide-polymer composites: mechano-chemical insights into hydrogen-bonding/van der Waals interactions, polymer chain alignment, and steric parameters

X. Zhang*, H. Nguyen*, M. Daly, S.B. T. Nguyen†, Horacio D Espinosa†

Nanoscale 11, 2019, pp. 12305-12316

This paper describes a systematic study on the nanoscale toughening of monolayer graphene oxide (GO) by an ultra-thin polymer adlayer, which impedes the propagation of cracks during intraplanar fracture. Using molecular dynamics simulations, the crack-bridging capabilities of a library of five hydrogen-bonding-capable polymers are explored against an epoxide-rich GO substrate. The best crack-bridging effect is found in polymers with functional groups that can both donate/accept hydrogen atoms and have better capability to form cooperative hydrogen bonds. Aligning the chains of poly(acrylic acid) orthogonally to the crack propagation direction significantly enhances the fracture toughness of monolayer GO (by 310%) in comparison to that for an adlayer with randomly arranged chains (180% enhancement). 

Lessons from the ocean: whale baleen fracture resistance

B. Wang†, T.N. Sullivan, A.Pissarenko, A. Zaheri, H.D. Espinosa, M.A. Meyers†

Advanced Materials 31(3), 2019, pp. 1804574

Whale baleen is a keratin‐based biological material; it provides life‐long (40–100 years) filter‐feeding for baleen whales in place of teeth. This study reveals new aspects of the contribution of the baleen’s hierarchical structure to its fracture toughness and connects it to the unique performance requirements, which require anisotropy of fracture resistance. Baleen plates are subjected to competing external effects of hydration and varying loading rates and demonstrate a high fracture toughness in transverse loading, which is the most important direction in the filtering function; in the longitudinal direction, the toughness is much lower since delamination and controlled flexure are expected and desirable. The compressive strength is also established and results support the fracture toughness measurements: it is also highly anisotropic, and exhibits a ductile‐to‐brittle transition with increasing strain rate in the dry condition, which is absent in the hydrated condition, conferring impact resistance to the baleen.

Identification of deformation mechanisms in biomaterials through AFM and digital image correlation

H.D. Espinosa†

Advancement of Optical Methods & Digital Image Correlation in Experimental Mechanics, Volume 3, 2019, pp. 89-93

Most biological composite materials achieve higher toughness without sacrificing stiffness and strength. Interrogating how Nature employs these strategies and decoding the structure-function relationship of these materials is a challenging task that requires knowledge about the actual loading and environmental conditions of the material in their natural habitat, as well as a complete characterization of their constituents and hierarchical ultrastructure. In this work, we present an experimental framework that combines in situ and ex situ fracture testing with digital image correlation to allow the identification and quantification of toughening mechanisms involved during fracture of natural systems. We present this methodology in two case studies: (1) pangolin scales, and (2) nacre from seashells. We envision that the outcome of this research will pave the way for more bio-inspired design systems that can subsequently shed light on how Nature has evolved materials to optimize mechanical properties.

In situ electron microscopy tensile testing of constrained carbon nanofibers

R. Ramachandramoorthy, A. Beese, H. Espinosa†

International Journal of Mechanical Sciences 149, 2018, pp. 452-458.

Electrospun carbon nanofibers, produced from polyacrylonitrile (PAN) nanofiber precursors, with their superior mechanical properties, are promising candidates for manufacturing advanced polymer composites. Here, we report a series of tensile tests performed in situ under scanning electron microscope (SEM)/transmission electron microscope (TEM) observation, which show that the modulus and strength of electrospun carbon nanofibers can be enhanced through a simple mechanical constraint during the carbonization step in the electrospinning process. The constrained carbon nanofibers of diameter less than 150 nm were nanomanipulated inside the SEM onto a specialized microelectromechanical systems (MEMS) based testing platform and subsequently tested in uniaxial tension until failure. It was identified that both the strength and modulus of the constrained carbon nanofibers with sub-150 nm diameters are on average higher compared to their unconstrained counterparts by ∼22% and ∼31% respectively.

Combined numerical and experimental investigation of localized electroporation-based cell transfection and sampling

P. Mukherjee*, S.S.P. Nathamgari*, J.A. Kessler, H.D. Espinosa†

ACS Nano 12(12), 2018, pp. 12118–12128

Localized electroporation has evolved as an effective technology for the delivery of foreign molecules into cells while preserving their viability. Consequently, this technique has potential applications in sampling the contents of live cells and the temporal assessment of cellular states at the single-cell level. Although there have been numerous experimental reports on localized electroporation-based delivery, a lack of a mechanistic understanding of the process hinders its implementation in sampling. In this work, we develop a multiphysics model that predicts the transport of molecules into and out of the cell during localized electroporation. Based on the model predictions, we optimize experimental parameters such as buffer conditions, electric field strength, cell confluency, and density of nanochannels in the substrate for successful delivery and sampling via localized electroporation. We also identify that cell membrane tension plays a crucial role in enhancing both the amount and the uniformity of molecular transport, particularly for macromolecules. 

Revealing the mechanics of helicoidal composites through additive manufacturing and beetle developmental stage analysis

A. Zaheri, J.S. Fenner, B.P. Russell, D. Restrepo, M. Daly, D. Wang, C. Hayashi, M.A. Meyers, P.D. Zavattieri, H.D. Espinosa†

Advanced Functional Materials 28(33), 2018, pp. 1803073

Investigation into the microstructure of high performance natural materials has revealed common patterns that are pervasive across animal species. For example, the helicoid motif has gained significant interest in the biomaterials community, where recent studies have highlighted its role in enabling damage tolerance in a diverse set of animals. Moreover, the helicoid motif corresponds to a highly adaptable architecture where the control of the pitch rotation angle between fibrous structures produces large changes in its mechanical response. Nature, takes advantage of this special feature enabling an active response to particular biological needs occurring during an animal’s ontogeny. In this work, we demonstrate this adaptive behavior in helicoidal architectures by performing a mechanistic analysis of the changes occurring in the cuticle of the figeater beetle (Cotinis mutabilis) during its life cycle. 

The role of water in mediating interfacial adhesion and shear strength in graphene oxide

R.A. Soler-Crespo, W. Gao, L. Mao, H.T. Nguyen, M.R. Roenbeck, J.T. Paci, J. Huang†, S.B.T. Nguyen†, H.D. Espinosa†

ACS Nano 12(6), 2018, pp. 6089-6099

Graphene oxide (GO), whose highly tunable surface chemistry enables the formation of strong interfacial hydrogen-bond networks, has garnered increasing interest in the design of devices that operate in the presence of water. For instance, previous studies have suggested that controlling GO’s surface chemistry leads to enhancements in interfacial shear strength, allowing engineers to manage deformation pathways and control failure mechanisms. However, these previous reports have not explored the role of ambient humidity and only offer extensive chemical modifications to GO’s surface as the main pathway to control GO’s interfacial properties. Herein, through atomic force microscopy experiments on GO–GO interfaces, the adhesion energy and interfacial shear strength of GO were measured as a function of ambient humidity. Experimental evidence shows that adhesion energy and interfacial shear strength can be improved by a factor of 2–3.

Design of piezoMEMS for high strain rate nanomechanical experiments

R. Ramachandramoorthy, M. Milan, Z. Lin, S. Trolier-McKinstry, A. Corigliano, H. Espinosa†

Extreme Mechanics Letters 20, 2018, pp. 14-20

Nanomechanical experiments on 1-D and 2-D materials are typically conducted at quasi-static strain rates of 10−4/s, while their analysis using molecular dynamic (MD) simulations are conducted at ultra-high strain rates of 106/s and above. This large order of magnitude difference in the strain rates prevents a direct one-on-one comparison between experiments and simulations. In order to close this gap in strain rates, nanoscale actuation/sensing options were explored to increase the experimental strain rates. Using a combination of COMSOL multiphysics finite element simulations and experiments, it is shown that thermal actuation, which uses structural expansion due to Joule heating, is capable of executing uniaxial nanomechanical testing up to a strain rate of 100/s. The limitation arises from system inertia and thermal transients. In contrast, piezoelectric actuation can respond in the GHz frequency range. However, given that the piezoelectric displacement is limited in range, a sagittal displacement amplification scheme is examined in the actuator design, which imposes a lower frequency limit for operation. 

Techniques to stimulate and interrogate cell–cell adhesion mechanics

R. Yang, J.A. Broussard, K.J. Green, H.D. Espinosa†

Extreme Mechanics Letters 20, 2018, pp. 125-139

Cell–cell adhesions maintain the mechanical integrity of multicellular tissues and have recently been found to act as mechanotransducers, translating mechanical cues into biochemical signals. Mechanotransduction studies have primarily focused on focal adhesions, sites of cell-substrate attachment. These studies leverage technical advances in devices and systems interfacing with living cells through cell–extracellular matrix adhesions. As reports of aberrant signal transduction originating from mutations in cell–cell adhesion molecules are being increasingly associated with disease states, growing attention is being paid to this intercellular signaling hub. Along with this renewed focus, new requirements arise for the interrogation and stimulation of cell–cell adhesive junctions. This review covers established experimental techniques for stimulation and interrogation of cell–cell adhesion from cell pairs to monolayers.

Monoclonal cell line generation and CRISPR/Cas9 manipulation via single‐cell electroporation

R. Yang, V. Lemaître, C. Huang, A. Haddadi, R. McNaughton, H.D. Espinosa†

Small 14(12), 2018, pp. 1702495

Stably transfected cell lines are widely used in drug discovery and biological research to produce recombinant proteins. Generation of these cell lines requires the isolation of multiple clones, using time‐consuming dilution methods, to evaluate the expression levels of the gene of interest. A new and efficient method is described for the generation of monoclonal cell lines, without the need for dilution cloning. In this new method, arrays of patterned cell colonies and single cell transfection are employed to deliver a plasmid coding for a reporter gene and conferring resistance to an antibiotic. Using a nanofountain probe electroporation system, probe positioning is achieved through a micromanipulator with sub‐micron resolution and resistance‐based feedback control. The array of patterned cell colonies allows for rapid selection of numerous stably transfected clonal cell lines located on the same culture well, conferring a significant advantage over slower and labor‐intensive traditional methods. 

Formulation and validation of a reduced order model of 2D materials exhibiting a two-phase microstructure as applied to graphene oxide

I. Benedetti*, H. Nguyen*, R.A. Soler-Crespo*, W. Gao, L. Mao, A. Ghasemi, J. Wen, S.B. Nguyen, H.D. Espinosa†

Journal of the Mechanics and Physics of Solids 112, 2018, pp. 66-88

Novel 2D materials, e.g., graphene oxide (GO), are attractive building blocks in the design of advanced materials due to their reactive chemistry, which can enhance interfacial interactions while providing good in-plane mechanical properties. Recent studies have hypothesized that the randomly distributed two-phase microstructure of GO, which arises due to its oxidized chemistry, leads to differences in nano- vs meso‑scale mechanical responses. However, this effect has not been carefully studied using molecular dynamics due to computational limitations. Herein, a continuum mechanics model, formulated based on density functional based tight binding (DFTB) constitutive results for GO nano-flakes, is establish for capturing the effect of oxidation patterns on the material mechanical properties. GO is idealized as a continuum heterogeneous two-phase material, where the mechanical response of each phase, graphitic and oxidized, is informed from DFTB simulations. 

Hierarchical structure and compressive deformation mechanisms of bighorn sheep (Ovis canadensis) horn

W. Huang*, A. Zaheri*, J.-Y. Jung, H.D. Espinosa†, J. Mckittrick†

Acta biomaterialia 61, 2017, pp. 1-14

Bighorn sheep (Ovis canadensis) horns show remarkable impact resistance and energy absorption when undergoing high speed impact during the intraspecific fights. The present work illustrates the hierarchical structure of bighorn sheep horn at different length scales and investigates the energy dissipation mechanisms under different strain rates, loading orientations and hydration states. These results demonstrate how horn dissipates large amounts of energy, thus provide a new path to fabricate energy absorbent and crashworthiness engineering materials.

Lamellae spatial distribution modulates fracture behavior and toughness of african pangolin scales

M.J. Chon, M. Daly, B. Wang, X. Xiao, A. Zaheri, M.A. Meyers, H.D. Espinosa†

Journal of the Mechanical Behavior of Biomedical Materials 76, 2017, pp. 30-37

Pangolin scales form a durable armor whose hierarchical structure offers an avenue towards high performance bio-inspired materials design. In this study, the fracture resistance of African pangolin scales is examined using single edge crack three-point bend fracture testing in order to understand toughening mechanisms arising from the structures of natural mammalian armors. In these mechanical tests, the influence of material orientation and hydration level are examined. The fracture experiments reveal an exceptional fracture resistance due to crack deflection induced by the internal spatial orientation of lamellae. An order of magnitude increase in the measured fracture resistance due to scale hydration, reaching up to ~ 25 kJ/m2 was measured. Post-mortem analysis of the fracture samples was performed using a combination of optical and electron microscopy, and X-ray computerized tomography. Interestingly, the crack profile morphologies are observed to follow paths outlined by the keratinous lamellae structure of the pangolin scale. 

The desmoplakin–intermediate filament linkage regulates cell mechanics

J.A. Broussard, R. Yang, C. Huang, S.S.P. Nathamgari, A.M. Beese, L.M. Godsel, M.H. Hegazy, S. Lee, F. Z., N.J. Sniadecki, K.J. Green, H.D. Espinosa†

Molecular biology of the cell 28(23), 2017, pp. 3156-3164

The translation of mechanical forces into biochemical signals plays a central role in guiding normal physiological processes during tissue development and homeostasis. Interfering with this process contributes to cardiovascular disease, cancer progression, and inherited disorders. The actin-based cytoskeleton and its associated adherens junctions are well-established contributors to mechanosensing and transduction machinery; however, the role of the desmosome–intermediate filament (DSM–IF) network is poorly understood in this context. Because a force balance among different cytoskeletal systems is important to maintain normal tissue function, knowing the relative contributions of these structurally integrated systems to cell mechanics is critical. Here we modulated the interaction between DSMs and IFs using mutant forms of desmoplakin, the protein bridging these structures. Using micropillar arrays and atomic force microscopy, we demonstrate that strengthening the DSM–IF interaction increases cell–substrate and cell–cell forces and cell stiffness both in cell pairs and sheets of cells. 

Reversible attachment with tailored permeability: The feather vane and bioinspired designs

T.N. Sullivan, M. Chon, R. Ramachandramoorthy, M.R. Roenbeck, T.‐T. Hung, H.D. Espinosa, M.A. Meyers†

Advanced Functional Materials 27(39), 2017, pp. 1702954

In bird flight, the majority of the wing surface consists of highly refined and hierarchically organized feathers. They are composed of barbs that stem from the feather shaft and barbules that branch from barbs, forming a rigid feather vane. Barbules provide adhesion within the vane through an interlocking hook‐and‐groove mechanism to allow for the effective capture of air. This functional adhesive can reattach if structures unfasten from one another, preventing catastrophic damage of the vane. Here, using pelican primary feathers as a model material, we investigate the in‐plane adhesion and stiffness of barbules. With guineafowl, pelican, and dove feathers, we determine the effect of barbules on the feather vane’s ability to capture air. The vane is found to have directional permeability, and the effect of detaching barbules on the feather’s competency is determined to be a function of barb dimensions. Interestingly, barbule spacing is found to vary within a narrow 8–16 µm range for birds weighing from 4–11 000 g (hummingbird to condor).

Extreme lightweight structures: avian feathers and bones

T.N. Sullivan, B. Wang, H.D. Espinosa, M.A. Meyers†

Materials Today 20(7), 2017, pp. 377-391

Flight is not the exclusive domain of birds; mammals (bats), insects, and some fish have independently developed this ability by the process of convergent evolution. Birds, however, greatly outperform other flying animals in efficiency and duration; for example the common swift (Apus apus) has recently been reported to regularly fly for periods of 10 months during migration. Birds owe this extraordinary capability to feathers and bones, which are extreme lightweight biological materials. They achieve this crucial function through their efficient design spanning multiple length scales. Both feathers and bones have unusual combinations of structural features organized hierarchically from nano- to macroscale and enable a balance between lightweight and bending/torsional stiffness and strength. The complementary features between the avian bone and feather are reviewed here, for the first time, and provide insights into nature’s approach at creating structures optimized for flight. 

A coarse-grained model for the mechanical behavior of graphene oxide

Z. Meng*, R.A. Soler-Crespo*, W. Xia, W. Gao, L. Ruiz, H.D. Espinosa†, S. Keten†

Carbon 117, 2017, pp. 476-487

Graphene oxide (GO) shows promise as a nanocomposite building block due to its exceptional mechanical properties. While atomistic simulations have become central to investigating its mechanical properties, the method remains prohibitively expensive for large deformations and mesoscale failure mechanisms. To overcome this, we establish a coarse-grained (CG) model that captures key mechanical and interfacial properties, and the non-homogeneous effect of oxidation in GO sheets. The CG model consists of three types of CG beads, representing groups of pristine sp2 carbon atoms, and hydroxyl and epoxide functionalized regions. The CG force field is parameterized based on density functional-based tight binding simulations on three extreme cases. It accurately quantifies deterioration of tensile modulus and strength at the expense of improving interlayer adhesion with increasing oxidation of varying chemical compositions. We demonstrate the applicability of the model to study mesoscale phenomena by reproducing different force vs. indentation curves in silico, corroborating recent experimental observations.

Reliability of single crystal silver nanowire-based systems: stress assisted instabilities

R. Ramachandramoorthy*, Y. Wang*, A. Aghaei, G. Richter, W. Cai†, H.D. Espinosa†

ACS Nano 11(5), 2017, pp. 4768-4776

Time-dependent mechanical characterization of nanowires is critical to understand their long-term reliability in applications, such as flexible-electronics and touch screens. It is also of great importance to develop a theoretical framework for experimentation and analysis on the mechanics of nanowires under time-dependent loading conditions, such as stress-relaxation and fatigue. Here, we combine in situ scanning electron microscope (SEM)/transmission electron microscope (TEM) tests with atomistic and phase-field simulations to understand the deformation mechanisms of single crystal silver nanowires held under constant strain. We observe that the nanowires initially undergo stress-relaxation, where the stress reduces with time and saturates after some time period. The stress-relaxation process occurs due to the formation of few dislocations and stacking faults. Remarkably, after a few hours the nanowires rupture suddenly. The reason for this abrupt failure of the nanowire was identified as stress-assisted diffusion, using phase-field simulations.

Plasticity resulted from phase transformation for monolayer molybdenum disulfide film during nanoindentation simulations

W. Wang, L. Li, C. Yang, R.A. Soler-Crespo, Z. Meng, M. Li, X. Zhang, S. Keten†, H.D. Espinosa†

Nanotechnology 28(16), 2017, pp. 164005

Molecular dynamics simulations on nanoindentation of circular monolayer molybdenum disulfide (MoS₂) film are carried out to elucidate the deformation and failure mechanisms. Typical force–deflection curves are obtained, and in-plane stiffness of MoS₂ is extracted according to a continuum mechanics model. The measured in-plane stiffness of monolayer MoS₂ is about 182 ± 14 N m⁻¹, corresponding to an effective Young’s modulus of 280 ± 21 GPa. More interestingly, at a critical indentation depth, the loading force decreases sharply and then increases again. The loading–unloading–reloading processes at different initial unloading deflections are also conducted to explain the phenomenon. It is found that prior to the critical depth, the monolayer MoS₂ film can return to the original state after completely unloading, while there is hysteresis when unloading after the critical depth and residual deformation exists after indenter fully retracted, indicating plasticity. 

Lessons from tooth enamel

H.D. Espinosa†, R.A. Soler-Crespo

Nature 543(7643), 2017, pp. 42-43

A remarkable composite material has been made that mimics the structure of tooth enamel. This achievement opens up the exploration of new composite materials and of computational methods that reliably predict their properties. 

AFM identification of beetle exocuticle: Bouligand structure and nanofiber anisotropic elastic properties

R. Yang, A. Zaheri, W. Gao, C. Hayashi, H.D. Espinosa†

Advanced Functional Materials 27(6), 2017, pp. 1603993

One of the common architectures in natural materials is the helicoidal (Bouligand) structure, where fiber layers twist around a helical screw. Despite the many studies that have shown the existence of Bouligand structures, methods for nanoscale structural characterization and identification of fiber mechanical properties remain to be developed. In this study, we used the exocuticle of Cotinis mutabilis (a beetle in the Cetoniinae) as a model material to develop a new experimental‐theoretical methodology that combines atomic force microscopy‐based nanoindentation and anisotropic contact mechanics analysis. Using such methodology, we studied both the helicoidal structure and the mechanical properties of its constituent fibers. The twist angle between the layers was found to be in the range of 12°–18° with a pitch size of 220 nm for the helicoidal pattern. In addition, the constituent fiber diameter was measured to be approximately 20 nm, which is consistent with the fiber diameters found in helicoids of other arthropod species. 

Editorial for the focus issue on ‘‘Mechanics in Extreme Manufacturing’’ in Extreme Mechanics Letters

B. Xu, X. Li and H. D. Espinosa

Extreme Mechanics Letters 7, 2016, pp. 42-43

It is our great pleasure to present the Extreme Mechanics Letters themed issue on Mechanics in Extreme Manufacturing. Manufacturing, which appeared as a labor-intensive handicraft exercise centuries ago, has shifted to an information-rich digital technique over the past few decades, where mechanics, as a discipline, has demonstrated a foundational role in the process of material choice and manufacturing optimization through the probing of deformation and failure in materials. As further evolution of manufacturing toward smart, flexible, and customizable, mechanics is expected to play a leading role in the refinement of the existing manufacturing techniques and the guidance of emerging new approaches.

Micro-and nanoscale technologies for delivery into adherent cells

W. Kang, R.L. McNaughton, H.D. Espinosa†

Trends in biotechnology 34(8), 2016, pp. 665-678

Several recent micro- and nanotechnologies have provided novel methods for biological studies of adherent cells because the small features of these new biotools provide unique capabilities for accessing cells without the need for suspension or lysis. These novel approaches have enabled gentle but effective delivery of molecules into specific adhered target cells, with unprecedented spatial resolution. We review here recent progress in the development of these technologies with an emphasis on in vitro delivery into adherent cells utilizing mechanical penetration or electroporation. We discuss the major advantages and limitations of these approaches and propose possible strategies for improvements. Finally, we discuss the impact of these technologies on biological research concerning cell-specific temporal studies, for example non-destructive sampling and analysis of intracellular molecules.

Engineering the mechanical properties of monolayer graphene oxide at the atomic level

R.A. Soler-Crespo*, W. Gao*, P. Xiao, X. Wei, J.T. Paci, G. Henkelman, H.D. Espinosa†

The Journal of Physical Chemistry Letters 7(14), 2016, pp. 2702-2707

The mechanical properties of graphene oxide (GO) are of great importance for applications in materials engineering. Previous mechanochemical studies of GO typically focused on the influence of the degree of oxidation on the mechanical behavior. In this study, using density functional-based tight binding simulations, validated using density functional theory simulations, we reveal that the deformation and failure of GO are strongly dependent on the relative concentrations of epoxide (−O−) and hydroxyl (−OH) functional groups. Hydroxyl groups cause GO to behave as a brittle material; by contrast, epoxide groups enhance material ductility through a mechanically driven epoxide-to-ether functional group transformation. Moreover, with increasing epoxide group concentration, the strain to failure and toughness of GO significantly increases without sacrificing material strength and stiffness. These findings demonstrate that GO should be treated as a versatile, tunable material that may be engineered by controlling chemical composition, rather than as a single, archetypical material.

Recoverable slippage mechanism in multilayer graphene leads to repeatable energy dissipation

X. Wei*, Z. Meng*, L. Ruiz, W. Xia, C. Lee, J.W. Kysar, J.C. Hone, S. Keten†, H.D. Espinosa†

ACS Nano 10(2), 2016, pp. 1820-1828

Understanding the deformation mechanisms in multilayer graphene (MLG), an attractive material used in nanodevices as well as in the reinforcement of nanocomposites, is critical yet challenging due to difficulties in experimental characterization and the spatiotemporal limitations of atomistic modeling. In this study, we combine nanomechanical experiments with coarse-grained molecular dynamics (CG-MD) simulations to elucidate the mechanisms of deformation and failure of MLG sheets. Elastic properties of graphene sheets with one to three layers are measured using film deflection tests. A nonlinear behavior in the force vs deflection curves for MLGs is observed in both experiments and simulations: during loading/unloading cycles, MLGs dissipate energy through a “recoverable slippage” mechanism. The CG-MD simulations further reveal an atomic level interlayer slippage process and suggest that the dissipated energy scales with film perimeter. Moreover, our study demonstrates that the finite shear strength between individual layers could explain the experimentally measured size-dependent strength with thickness scaling in MLG sheets.

High strain rate tensile testing of silver nanowires: rate-dependent brittle-to-ductile transition

R. Ramachandramoorthy*, W. Gao*, R. Bernal, H. Espinosa†

Nano Letters 16(1), 2016, pp. 255-263

The characterization of nanomaterials under high strain rates is critical to understand their suitability for dynamic applications such as nanoresonators and nanoswitches. It is also of great theoretical importance to explore nanomechanics with dynamic and rate effects. Here, we report in situ scanning electron microscope (SEM) tensile testing of bicrystalline silver nanowires at strain rates up to 2/s, which is 2 orders of magnitude higher than previously reported in the literature. The experiments are enabled by a microelectromechanical system (MEMS) with fast response time. It was identified that the nanowire plastic deformation has a small activation volume (<10b³), suggesting dislocation nucleation as the rate controlling mechanism. Also, a remarkable brittle-to-ductile failure mode transition was observed at a threshold strain rate of 0.2/s. Transmission electron microscopy (TEM) revealed that along the nanowire, dislocation density and spatial distribution of plastic regions increase with increasing strain rate. 

Isolating single cells in a neurosphere assay using inertial microfluidics

S.S.P. Nathamgari, B. Dong, F. Zhou, W. Kang, J.P. Giraldo-Vela, T. McGuire, R.L. McNaughton, C. Sun, J.A. Kessler† and  H.D. Espinosa†

Lab on a Chip 24(15), 2015, pp. 4591-4597

Sphere forming assays are routinely used for in vitro propagation and differentiation of stem cells. Because the stem cell clusters can become heterogeneous and polyclonal, they must first be dissociated into a single cell suspension for further clonal analysis or differentiation studies. The dissociated population is marred by the presence of doublets, triplets and semi-cleaved/intact clusters which makes identification and further analysis of differentiation pathways difficult. In this work, we use inertial microfluidics to separate the single cells and clusters in a population of chemically dissociated neurospheres. In contrast to previous microfluidic sorting technologies which operated at high flow rates, we implement the spiral microfluidic channel in a novel focusing regime that occurs at lower flow rates. In this regime, the curvature-induced Dean’s force focuses the smaller, single cells towards the inner wall and the larger clusters towards the center. 

A new Monte Carlo model for predicting the mechanical properties of fiber yarns

X. Wei, M. Ford, R.A. Soler-Crespo, H.D. Espinosa†

Journal of the Mechanics and Physics of Solids 84, 2015, pp. 325-335

Understanding the complicated failure mechanisms of hierarchical composites such as fiber yarns is essential for advanced materials design. In this study, we developed a new Monte Carlo model for predicting the mechanical properties of fiber yarns that includes statistical variation in fiber strength. Furthermore, a statistical shear load transfer law based on the shear lag analysis was derived and implemented to simulate the interactions between adjacent fibers and provide a more accurate tensile stress distribution along the overlap distance. Simulations on two types of yarns, made from different raw materials and based on distinct processing approaches, predict yarn strength values that compare favorably with experimental measurements. Furthermore, the model identified very distinct dominant failure mechanisms for the two materials, providing important insights into design features that can improve yarn strength.

Double-tilt in situ TEM holder with multiple electrical contacts and its application in MEMS-based mechanical testing of nanomaterials

R.A. Bernal, R. Ramachandramoorthy, H.D. Espinosa†

Ultramicroscopy 156, 2015, pp. 23-28

MEMS and other lab-on-a-chip systems are emerging as attractive alternatives to carry out experiments in situ the electron microscope. However, several electrical connections are usually required for operating these setups. Such connectivity is challenging inside the limited space of the TEM side-entry holder. Here, we design, implement and demonstrate a double-tilt TEM holder with capabilities for up to 9 electrical connections, operating in a high-resolution TEM. We describe the operating principle of the tilting and connection mechanisms and the physical implementation of the holder. To demonstrate the holder capabilities, we calibrate the tilting action, which has limits of ±15°, and establish the insulation resistance of the electronics to be 36 GΩ, appropriate for measurements of currents down to the nano-amp (nA) regime. Furthermore, we demonstrate tensile testing of silver nanowires using a previously developed MEMS device for mechanical testing, using the implemented holder as the platform for electronic operation and sensing. 

Plasticity and ductility in graphene oxide through a mechanochemically induced damage tolerance mechanism

X. Wei, L. Mao, R.A. Soler-Crespo, J.T. Paci, J. Huang†, S.B.T. Nguyen†, H.D. Espinosa†

Nature Communications 6(1), 2015, pp. 1-9

The ability to bias chemical reaction pathways is a fundamental goal for chemists and material scientists to produce innovative materials. Recently, two-dimensional materials have emerged as potential platforms for exploring novel mechanically activated chemical reactions. Here we report a mechanochemical phenomenon in graphene oxide membranes, covalent epoxide-to-ether functional group transformations that deviate from epoxide ring-opening reactions, discovered through nanomechanical experiments and density functional-based tight binding calculations. These mechanochemical transformations in a two-dimensional system are directionally dependent, and confer pronounced plasticity and damage tolerance to graphene oxide monolayers. Additional experiments on chemically modified graphene oxide membranes, with ring-opened epoxide groups, verify this unique deformation mechanism. 

Sustaining dry surfaces under water

P.R. Jones, X. Hao, E.R. Cruz-Chu, K. Rykaczewski, K. Nandy, T.M. Schutzius, K.K. Varanasi, C.M. Megaridis, J.H. Walther, P.Koumoutsakos, H.D. Espinosa†, Neelesh A Patankar†

Scientific Reports 5(1), 2015, pp. 1-10

Rough surfaces immersed under water remain practically dry if the liquid-solid contact is on roughness peaks, while the roughness valleys are filled with gas. Mechanisms that prevent water from invading the valleys are well studied. However, to remain practically dry under water, additional mechanisms need consideration. This is because trapped gas (e.g. air) in the roughness valleys can dissolve into the water pool, leading to invasion. Additionally, water vapor can also occupy the roughness valleys of immersed surfaces. If water vapor condenses, that too leads to invasion. These effects have not been investigated and are critically important to maintain surfaces dry under water. In this work, we identify the critical roughness scale, below which it is possible to sustain the vapor phase of water and/or trapped gases in roughness valleys – thus keeping the immersed surface dry. Theoretical predictions are consistent with molecular dynamics simulations and experiments.

Molecular-level engineering of adhesion in carbon nanomaterial interfaces

M.R. Roenbeck, A. Furmanchuk, Z. An, J.T. Paci, X. Wei, S.B.T. Nguyen, G.C. Schatz, H.D. Espinosa†

Nano Letters 15(7), 2015, pp. 4504-4516

Weak interfilament van der Waals interactions are potentially a significant roadblock in the development of carbon nanotube- (CNT-) and graphene-based nanocomposites. Chemical functionalization is envisioned as a means of introducing stronger intermolecular interactions at nanoscale interfaces, which in turn could enhance composite strength. This paper reports measurements of the adhesive energy of CNT–graphite interfaces functionalized with various coverages of arylpropionic acid. Peeling experiments conducted in situ in a scanning electron microscope show significantly larger adhesive energies compared to previously obtained measurements for unfunctionalized surfaces (Roenbeck et al. ACS Nano2014, 8 (1), 124–138). Surprisingly, however, the adhesive energies are significantly higher when both surfaces have intermediate coverages than when one surface is densely functionalized. 

Pushing the Envelope of In Situ Transmission Electron Microscopy

R. Ramachandramoorthy, R. Bernal, H.D. Espinosa†

ACS Nano 9(5), 2015, pp. 4675-4685

Recent major improvements to the transmission electron microscope (TEM) including aberration-corrected electron optics, light-element-sensitive analytical instrumentation, sample environmental control, and high-speed and sensitive direct electron detectors are becoming more widely available. When these advances are combined with in situ TEM tools, such as multimodal testing based on microelectromechanical systems, key measurements and insights on nanoscale material phenomena become possible. In particular, these advances enable metrology that allows for unprecedented correlation to quantum mechanics and the predictions of atomistic models. In this Perspective, we provide a summary of recent in situ TEM research that has leveraged these new TEM capabilities as well as an outlook of the opportunities that exist in the different areas of in situ TEM experimentation. 

Single‐Cell Detection of mRNA Expression Using Nanofountain‐Probe Electroporated Molecular Beacons

J.P. Giraldo‐Vela, W. Kang, R.L. McNaughton, X. Zhang, B.M. Wile, A. Tsourkas, G. Bao, H.D. Espinosa†

Small 11(20), 2015, pp. 2345-2345

New techniques for single‐cell analysis enable new discoveries in gene expression and systems biology. Time‐dependent measurements on individual cells are necessary, yet the common single‐cell analysis techniques used today require lysing the cell, suspending the cell, or long incubation times for transfection, thereby interfering with the ability to track an individual cell over time. Here a method for detecting mRNA expression in live single cells using molecular beacons that are transfected into single cells by means of nanofountain probe electroporation (NFP‐E) is presented. Molecular beacons are oligonucleotides that emit fluorescence upon binding to an mRNA target, rendering them useful for spatial and temporal studies of live cells. The NFP‐E is used to transfect a DNA‐based beacon that detects glyceraldehyde 3‐phosphate dehydrogenase and an RNA‐based beacon that detects a sequence cloned in the green fluorescence protein mRNA. 

Statistical shear lag model–Unraveling the size effect in hierarchical composites

X. Wei, T. Filleter, H.D. Espinosa†

Acta Biomaterialia 18, 2015, pp. 206-212

Numerous experimental and computational studies have established that the hierarchical structures encountered in natural materials, such as the brick-and-mortar structure observed in sea shells, are essential for achieving defect tolerance. Due to this hierarchy, the mechanical properties of natural materials have a different size dependence compared to that of typical engineered materials. This study aimed to explore size effects on the strength of bio-inspired staggered hierarchical composites and to define the influence of the geometry of constituents in their outstanding defect tolerance capability. A statistical shear lag model is derived by extending the classical shear lag model to account for the statistics of the constituents’ strength. A general solution emerges from rigorous mathematical derivations, unifying the various empirical formulations for the fundamental link length used in previous statistical models. The model shows that the staggered arrangement of constituents grants composites a unique size effect on mechanical strength in contrast to homogenous continuous materials. 

Multiphysics design and implementation of a microsystem for displacement-controlled tensile testing of nanomaterials

M.F. Pantano*, R.A. Bernal*, L. Pagnotta, H.D. Espinosa†

Meccanica 50(2), 2014, pp. 549-560

MEMS-based tensile testing devices are powerful tools for mechanical characterization of nanoscale materials. In a typical configuration, their design includes an actuator to deliver loads/displacements to a sample, and a sensing unit for load measurement. The sensing unit consists of a flexible structure, which deforms in response to the force imposed to the sample. Such deformation, while being necessary for the sensing function, may become a source of instability. When the sample experiences a load drop, as it may result from yield, necking or phase transitions, the elastic energy accumulated by the sensor can be released, thus leading to loss of the displacement-controlled condition and dynamic failure. Here, we report a newly-developed MEMS testing system where the sensor is designed to constantly keep its equilibrium position through an electrostatic feedback-control. We show design, implementation, and calibration of the system, as well as validation by tensile testing of silver nanowires. 

Microfluidic Parallel Patterning and Cellular Delivery of Molecules with a Nanofountain Probe

W. Kang, R.L. McNaughton, F. Yavari, M. Minary-Jolandan, A. Safi and H.D. Espinosa†

Journal of Laboratory Automation 19(1), 2014, pp. 100-109

This brief report describes a novel tool for microfluidic patterning of biomolecules and delivery of molecules into cells. The microdevice is based on integration of nanofountain probe (NFP) chips with packaging that creates a closed system and enables operation in liquid. The packaged NFP can be easily coupled to a micro/nano manipulator or atomic force microscope for precise position and force control. We demonstrate here the functionality of the device for continuous direct-write parallel patterning on a surface in air and in liquid. Because of the small volume of the probes (~3 pL), we can achieve flow rates as low as 1 fL/s and have dispensed liquid drops with submicron to 10 µm diameters in a liquid environment. Furthermore, we demonstrate that this microdevice can be used for delivery of molecules into single cells by transient permeabilization of the cell membrane (i.e., electroporation). The significant advantage of NFP-based electroporation compared with bulk electroporation and other transfection techniques is that it allows for precise and targeted delivery while minimizing stress to the cell. 

Intrinsic Bauschinger effect and recoverable plasticity in pentatwinned silver nanowires tested in tension

R.A. Bernal*, A. Aghaei*, S. Lee, S. Ryu, K. Sohn, J. Huang, W. Cai†, H. Espinosa†

Nano Letters 15(1), 2014, pp. 139-146

Silver nanowires are promising components of flexible electronics such as interconnects and touch displays. Despite the expected cyclic loading in these applications, characterization of the cyclic mechanical behavior of chemically synthesized high-quality nanowires has not been reported. Here, we combine in situ TEM tensile tests and atomistic simulations to characterize the cyclic stress–strain behavior and plasticity mechanisms of pentatwinned silver nanowires with diameters thinner than 120 nm. The experimental measurements were enabled by a novel system allowing displacement-controlled tensile testing of nanowires, which also affords higher resolution for capturing stress–strain curves. We observe the Bauschinger effect, that is, asymmetric plastic flow, and partial recovery of the plastic deformation upon unloading. TEM observations and atomistic simulations reveal that these processes occur due to the pentatwinned structure and emerge from reversible dislocation activity. 

Key factors limiting carbon nanotube yarn strength: exploring processing-structure-property relationships

A.M. Beese, X. Wei, S. Sarkar, R. Ramachandramoorthy, M.R. Roenbeck, A. Moravsky, M. Ford, F. Yavari, D.T. Keane, R.O. Loutfy, S.B.T. Nguyen, H.D. Espinosa†

ACS Nano 8(11), 2014, pp. 11454-11466

Studies of carbon nanotube (CNT) based composites have been unable to translate the extraordinary load-bearing capabilities of individual CNTs to macroscale composites such as yarns. A key challenge lies in the lack of understanding of how properties of filaments and interfaces across yarn hierarchical levels govern the properties of macroscale yarns. To provide insight required to enable the development of superior CNT yarns, we investigate the fabrication–structure–mechanical property relationships among CNT yarns prepared by different techniques and employ a Monte Carlo based model to predict upper bounds on their mechanical properties. We study the correlations between different levels of alignment and porosity and yarn strengths up to 2.4 GPa. The uniqueness of this experimentally informed modeling approach is the model’s ability to predict when filament rupture or interface sliding dominates yarn failure based on constituent mechanical properties and structural organization observed experimentally. 

Inherent carbonaceous impurities on arc-discharge multiwalled carbon nanotubes and their implications for nanoscale interfaces

Z. An*, A. Furmanchuk*, R. Ramachandramoorthy, T. Filleter, M.R. Roenbeck, H.D. Espinosa†, G.C. Schatz†, S.B.T. Nguyen†

Carbon 80, 2014, pp. 1-11

This paper presents evidence that strongly adhered carbonaceous surface impurities, intrinsic impurities that accompany multiwall carbon nanotubes (MWCNTs) synthesized by arc-discharge, are a component that cannot be ignored in experiments involving single nanotubes and their interfaces with a second surface. At the interface that forms between a carbon nanotube and a graphitic surface, these impurities can significantly alter the adhesion properties of the underlying nanotube and can cause over 30% scatter in computed interaction energies, similar in magnitude to the scatter reported in experimental measurements involving individual CNTs. Also presented is high-resolution TEM evidence that commonly used purification techniques that are effective at removing larger impurity particles from as-produced arc-discharge MWCNT samples do not remove these strongly adhered carbonaceous surface impurities.

Shear and friction between carbon nanotubes in bundles and yarns

J.T. Paci†, A. Furmanchuk, H.D. Espinosa, G.C. Schatz†

Nano Letters 14(11), 2014, pp. 6138-6147

We perform a detailed density functional theory assessment of the factors that determine shear interactions between carbon nanotubes (CNTs) within bundles and in related CNT and graphene structures including yarns, providing an explanation for the shear force measured in recent experiments (Filleter, T.etal. Nano Lett. 2012, 12, 732). The potential energy barriers separating AB stacked structures are found to be irrelevant to the shear analysis for bundles and yarns due to turbostratic stacking, and as a result, the tube–tube shear strength for pristine CNTs is estimated to be <0.24 MPa, that is, extremely small. Instead, it is pinning due to the presence of defects and functional groups at the tube ends that primarily cause resistance to shear when bundles are fractured in weak vacuum (∼10–5 Torr). Such defects and groups are estimated to involve 0.55 eV interaction energies on average, which is much larger than single-atom vacancy defects (approximately 0.039 eV). 

Microfluidic device for stem cell differentiation and localized electroporation of postmitotic neurons

W. Kang, J.P. Giraldo-Vela, S.S.P. Nathamgari, T. McGuire, R.L. McNaughton, J.A. Kessler, H.D. Espinosa†

Lab on a Chip 14(23), 2014, pp. 4486-4495

New techniques to deliver nucleic acids and other molecules for gene editing and gene expression profiling, which can be performed with minimal perturbation to cell growth or differentiation, are essential for advancing biological research. Studying cells in their natural state, with temporal control, is particularly important for primary cells that are derived by differentiation from stem cells and are adherent, e.g., neurons. Existing high-throughput transfection methods either require cells to be in suspension or are highly toxic and limited to a single transfection per experiment. Here we present a microfluidic device that couples on-chip culture of adherent cells and transfection by localized electroporation. Integrated microchannels allow long-term cell culture on the device and repeated temporal transfection. The microfluidic device was validated by first performing electroporation of HeLa and HT1080 cells, with transfection efficiencies of ~95% for propidium iodide and up to 50% for plasmids. 

USNCTAM perspectives on mechanics in medicine

G. Bao, Y. Bazilevs, J.-H. Chung, P. Decuzzi, H.D. Espinosa, M. Ferrari, H. Gao, S.S. Hossain, T.J.R. Hughes, R.D. Kamm, W.K. Liu, A. Marsden, B. Schrefler

Journal of The Royal Society Interface 11(97), 2014, pp. 20140301

Over decades, the theoretical and applied mechanics community has developed sophisticated approaches for analysing the behaviour of complex engineering systems. Most of these approaches have targeted systems in the transportation, materials, defence and energy industries. Applying and further developing engineering approaches for understanding, predicting and modulating the response of complicated biomedical processes not only holds great promise in meeting societal needs, but also poses serious challenges. This report, prepared for the US National Committee on Theoretical and Applied Mechanics, aims to identify the most pressing challenges in biological sciences and medicine that can be tackled within the broad field of mechanics. This echoes and complements a number of national and international initiatives aiming at fostering interdisciplinary biomedical research. This report also comments on cultural/educational challenges.

Defect‐Tolerant Nanocomposites through Bio‐Inspired Stiffness Modulation

A.M. Beese, Z. An, S. Sarkar, S.S.P. Nathamgari, H.D. Espinosa†, S.B.T. Nguyen†

Advanced Functional Materials 24(19), 2014, pp. 2883-2891

A biologically inspired, multilayer laminate structural design is deployed into nanocomposite films of graphene oxide‐poly(methyl methacrylate) (GO‐PMMA). The resulting multilayer GO‐PMMA films show greatly enhanced mechanical properties compared to pure‐graphene‐oxide films, with up to 100% increases in stiffness and strength when optimized. Notably, a new morphology is observed at fracture surfaces: whereas pure‐graphene‐oxide films show clean fracture surfaces consistent with crack initiation and propagation perpendicular to the applied tensile load, the GO‐PMMA multilayer laminates show terracing consistent with crack stopping and deflection mechanisms. As a consequence, these macroscopic GO‐PMMA films become defect‐tolerant and can maintain their tensile strengths as their sample volumes increase. Linear elastic fracture analysis supports these observations by showing that the stiffness modulation introduced by including PMMA layers within a graphene oxide film can act to shield or deflect cracks, thereby delaying failure and allowing the material to access more of its inherent strength. Together, these data clearly demonstrate that desirable defect‐tolerant traits of structural biomaterials can indeed be incorporated into graphene‐ oxide‐based nanocomposites.

In Situ Electron Microscopy Four‐Point Electromechanical Characterization of Freestanding Metallic and Semiconducting Nanowires

R.A. Bernal, T. Filleter, J.G. Connell, K. Sohn, J. Huang, L.J. Lauhon, H.D. Espinosa†

Small 10(4), 2014, pp. 725-733

Electromechanical coupling is a topic of current interest in nanostructures, such as metallic and semiconducting nanowires, for a variety of electronic and energy applications. As a result, the determination of structure‐property relations that dictate the electromechanical coupling requires the development of experimental tools to perform accurate metrology. Here, a novel micro‐electro‐mechanical system (MEMS) that allows integrated four‐point, uniaxial, electromechanical measurements of freestanding nanostructures in‐situ electron microscopy, is reported. Coupled mechanical and electrical measurements are carried out for penta‐twinned silver nanowires, their resistance is identified as a function of strain, and it is shown that resistance variations are the result of nanowire dimensional changes. Furthermore, in situ SEM piezoresistive measurements on n‐type, [111]‐oriented silicon nanowires up to unprecedented levels of ∼7% strain are demonstrated. The piezoresistance coefficients are found to be similar to bulk values. 

In Situ Scanning Electron Microscope Peeling To Quantify Surface Energy between Multiwalled Carbon Nanotubes and Graphene

M.R. Roenbeck, X. Wei, A.M. Beese, M. Naraghi, A. Furmanchuk, J.T. Paci, G.C. Schatz, H.D. Espinosa†

ACS Nano 8(1), 2014, pp. 124-138

Understanding atomic interactions between constituents is critical to the design of high-performance nanocomposites. Here, we report an experimental–computational approach to investigate the adhesion energy between as-produced arc discharge multiwalled carbon nanotubes (MWCNTs) and graphene. An in situ scanning electron microscope (SEM) experiment is used to peel MWCNTs from graphene grown on copper foils. The force during peeling is obtained by monitoring the deflection of a cantilever. Finite element and molecular mechanics simulations are performed to assist the data analysis and interpretation of the results. A finite element analysis of the experimental configuration is employed to confirm the applicability of Kendall’s peeling model to obtain the adhesion energy. Molecular mechanics simulations are used to estimate the effective contact width at the MWCNT–graphene interface. 

Tailoring the mechanical properties of carbon nanotube fibers

T. Filleter, A.M. Beese, M.R. Roenbeck, X. Wei, H.D. Espinosa†

Nanotube Superfiber Materials Ch. 3, 2014, pp. 61-85

Performance and efficiency demands in industrial applications are pushing a need for carbon fibers that can outperform existing technologies. Fibers that incorporate carbon nanotubes (CNTs) to enhance specific mechanical properties are a promising route to addressing this need. Some of the major roadblocks to unlocking the full potential of macroscopic fibers based on CNTs are controlling and optimizing the shear interactions within and between CNTs, geometrical organization of the CNTs, and structural properties of the individual CNTs. Several approaches have been pursued in order to optimize the mechanical behavior of CNT fibers, including irradiation-induced covalent cross-linking, reformable or rehealable bonding, and optimized geometrical and structural fiber designs. These approaches are inspired by nature, which uses hierarchical bonding schemes in optimized orientations to tailor the mechanical properties of its materials to the needs and environment of specific organisms. In this chapter, these approaches for developing high-performance CNT fibers will be reviewed, and an outlook of their potential impact will be discussed.

Microfluidics & nanotechnology: towards fully integrated analytical devices for the detection of cancer biomarkers

G. Perozziello, P. Candeloro, F. Gentile, A. Nicastri, A. Perri, M. L. Coluccio, A. Adamo, F. Pardeo, R. Catalano, E. Parrotta, H.D. Espinosa, G. Cuda, E. Di Fabrizio

RSC Advances 4(98), 2014, pp. 55590-55598

In this paper, we describe an innovative modular microfluidic platform allowing filtering, concentration and analysis of peptides from a complex mixture. The platform is composed of a microfluidic filtering device and a superhydrophobic surface integrating surface enhanced Raman scattering (SERS) sensors. The microfluidic device was used to filter specific peptides (MW 1553.73 D) derived from the BRCA1 protein, a tumor-suppressor molecule which plays a pivotal role in the development of breast cancers, from albumin (66.5 KD), the most represented protein in human plasma. The filtering process consisted of driving the complex mixture through a porous membrane having a cut-off of 12–14 kD by hydrodynamic flow. The filtered samples coming out of the microfluidic device were subsequently deposited on a superhydrophobic surface formed by micro pillars on top of which nanograins were fabricated. 

Experimental and Theoretical Studies of Fiber-Reinforced Composite Panels Subjected to Underwater Blast Loading

X. Wei, H.D. Espinosa†

Blast Mitigation Ch. 4, 2014, pp. 91-122

Fluid–structure interaction (FSI) experiments on monolithic and sandwich composite panels were performed to identify key failure mechanisms resulting from underwater blast loading. Panel performance was compared in terms of impulse deflection. Various failure mechanisms such as delamination between laminas, matrix damage and fiber rupture in laminas, and foam crushing were identified. A 3-D rate-dependent numerical model was developed to understand the experimentally observed failure mechanisms. A new failure criterion that includes strain-rate effects was formulated and implemented to simulate different damage modes in unidirectional composite plies. This rate-dependent numerical model predicted the responses of composite panels subjected to underwater blast loading with more correlated material damage patterns with the experimental observation than previously developed models. 

Optimization of nanofountain probe microfabrication enables large-scale nanopatterning

A. Safi, W. Kang, D. Czapleski, R. Divan, N. Moldovan, H.D. Espinosa†

Journal of Micromechanics and Microengineering 23(12), 2013, pp. 125014

A technological gap in nanomanufacturing has prevented the translation of many nanomaterial discoveries into real-world commercialized products. Bridging this gap requires a paradigm shift in methods for fabricating nanoscale devices in a reliable and repeatable fashion. Here we present the optimized fabrication of a robust and scalable nanoscale delivery platform, the nanofountain probe (NFP), for parallel direct-write of functional materials. Microfabrication of a new generation of NFP was realized with the aim of increasing the uniformity of the device structure. Optimized probe geometry was integrated into the design and fabrication process by modifying the precursor mask dimensions and by using an isotropic selective dry etching of the outer shell that defines the protrusion area. Probes with well-conserved sharp tips and controlled protrusion lengths were obtained. Sealing effectiveness of the channels was optimized. 

In situ transmission electron microscope tensile testing reveals structure–property relationships in carbon nanofibers

A.M. Beese, D. Papkov, S. Li, Y. Dzenis†, H.D. Espinosa†

Carbon 60(12), 2013, pp. 246-253

Tensile tests were performed on carbon nanofibers in situ a transmission electron microscope (TEM) using a microelectromechanical system (MEMS) tensile testing device. The carbon nanofibers tested in this study were produced via the electrospinning of polyacrylonitrile (PAN) into fibers, which are subsequently stabilized in an oxygen environment at 270 °C and carbonized in nitrogen at 800 °C. To investigate the relationship between the fiber molecular structure, diameter, and mechanical properties, nanofibers with diameters ranging from ∼100 to 300 nm were mounted onto a MEMS device using nanomanipulation inside the chamber of a Scanning Electron Microscope, and subsequently tested in tension in situ a TEM. The results show the dependence of strength and modulus on diameter, with a maximum modulus of 262 GPa and strength of 7.3 GPa measured for a 108 nm diameter fiber. 

Nanofountain probe electroporation (NFP-E) of single cells

W. Kang*, F. Yavari*, M. Minary-Jolandan, J.P. Giraldo-Vela, A. Safi, R.L. McNaughton, V. Parpoil, H.D. Espinosa†

Nano Letters 13(6), 2013, pp. 2448-2457

Tensile tests were performed on carbon nanofibers in situ a transmission electron microscope (TEM) using a microelectromechanical system (MEMS) tensile testing device. The carbon nanofibers tested in this study were produced via the electrospinning of polyacrylonitrile (PAN) into fibers, which are subsequently stabilized in an oxygen environment at 270 °C and carbonized in nitrogen at 800 °C. To investigate the relationship between the fiber molecular structure, diameter, and mechanical properties, nanofibers with diameters ranging from ∼100 to 300 nm were mounted onto a MEMS device using nanomanipulation inside the chamber of a Scanning Electron Microscope, and subsequently tested in tension in situ a TEM. The results show the dependence of strength and modulus on diameter, with a maximum modulus of 262 GPa and strength of 7.3 GPa measured for a 108 nm diameter fiber. 

A new rate-dependent unidirectional composite model–application to panels subjected to underwater blast

X. Wei, A. De Vaucorbeil, P. Tran, H.D. Espinosa†

Journal of the Mechanics and Physics of Solids 61(6), 2013, pp. 1305-1318

In this study, we developed a finite element fluid–structure interaction model to understand the deformation and failure mechanisms of both monolithic and sandwich composite panels. A new failure criterion that includes strain-rate effects was formulated and implemented to simulate different damage modes in unidirectional glass fiber/matrix composites. The laminate model uses Hashin’s fiber failure criterion and a modified Tsai–Wu matrix failure criterion. The composite moduli are degraded using five damage variables, which are updated in the post-failure regime by means of a linear softening law governed by an energy release criterion. A key feature in the formulation is the distinction between fiber rupture and pull-out by introducing a modified fracture toughness, which varies from a fiber tensile toughness to a matrix tensile toughness as a function of the ratio of longitudinal normal stress to effective shear stress. The delamination between laminas is modeled by a strain-rate sensitive cohesive law. 

Three-dimensional numerical modeling of composite panels subjected to underwater blast

X. Wei, P. Tran, A. De Vaucorbeil, R.B. Ramaswamy, F. Latourte, H.D. Espinosa†

Journal of the Mechanics and Physics of Solids 61(6), 2013, pp. 1319-1336

Designing lightweight high-performance materials that can sustain high impulsive loadings is of great interest for marine applications. In this study, a finite element fluid–structure interaction model was developed to understand the deformation and failure mechanisms of both monolithic and sandwich composite panels. Fiber (E-glass fiber) and matrix (vinylester resin) damage and degradation in individual unidirectional composite laminas were modeled using Hashin failure model. The delamination between laminas was modeled by a strain-rate sensitive cohesive law. In sandwich panels, core compaction (H250 PVC foam) is modeled by a crushable foam plasticity model with volumetric hardening and strain-rate sensitivity. The model-predicted deformation histories, fiber/matrix damage patterns, and inter-lamina delamination, in both monolithic and sandwich composite panels, were compared with experimental observations. 

Multi-scale mechanical improvement produced in carbon nanotube fibers by irradiation cross-linking

T. Filleter, H.D. Espinosa†

Carbon 56, 2013, pp. 1-11

Fibers and yarns based on carbon nanotubes (CNT) are emerging as a possible improvement over more traditional high strength carbon fibers used as reinforcement elements in composite materials. This is driven by a desire to translate the exceptional mechanical properties of individual CNT shells to achieve high performance macroscopic fibers and yarns. One of the central limitations in this approach is the weak shear interactions between adjacent CNT shells and tubes within macroscopic fibers and yarns. Furthermore, the multiple levels of interaction, e.g., between tubes within a multi-walled CNT or between bundles within a fiber, compound the problem. One promising direction to overcome this limitation is the introduction of strong and stiff cross-linking bonds between adjacent carbon shells. A great deal of research has been devoted to studying such cross-linking by the irradiation of CNT based materials using either high energy particles, such as electrons, to directly covalently cross-link CNTs, or electromagnetic irradiation, such as gamma rays to strengthen polymer cross-links between CNT shells and tubes. 

Bio-inspired carbon nanotube–polymer composite yarns with hydrogen bond-mediated lateral interactions

A.M. Beese, S. Sarkar, A. Nair, M. Naraghi, Z. An, A. Moravsky, R.O. Loutfy, M.J. Buehler†, S.B.T. Nguyen†, H.D. Espinosa†

ACS Nano 7(4), 2013, pp. 3434-3446

Polymer composite yarns containing a high loading of double-walled carbon nanotubes (DWNTs) have been developed in which the inherent acrylate-based organic coating on the surface of the DWNT bundles interacts strongly with poly(vinyl alcohol) (PVA) through an extensive hydrogen-bond network. This design takes advantage of a toughening mechanism seen in spider silk and collagen, which contain an abundance of hydrogen bonds that can break and reform, allowing for large deformation while maintaining structural stability. Similar to that observed in natural materials, unfolding of the polymeric matrix at large deformations increases ductility without sacrificing stiffness. As the PVA content in the composite increases, the stiffness and energy to failure of the composite also increases up to an optimal point, beyond which mechanical performance in tension decreases. Molecular dynamics (MD) simulations confirm this trend, showing the dominance of nonproductive hydrogen bonding between PVA molecules at high PVA contents, which lubricates the interface between DWNTs.

Atomistic Investigation of Load Transfer Between DWNT Bundles “Crosslinked” by PMMA Oligomers

M. Naraghi*, G.H. Bratzel*, T. Filleter, Z. An, X. Wei, S.B.T. Nguyen, M.J. Buehler†, H.D. Espinosa†

Advanced Functional Materials 23(15), 2013, pp. 1976-1976

The production of carbon nanotube (CNT) yarns possessing high strength and toughness remains a major challenge due to the intrinsically weak interactions between “bare” CNTs. To this end, nanomechanical shear experiments between functionalized bundles of CNTs are combined with multiscale simulations to reveal the mechanistic and quantitative role of nanotube surface functionalization on CNT‐CNT interactions. Notably, the in situ chemical vapor deposition (CVD) functionalization of CNT bundles by poly(methyl methacrylate) (PMMA)‐like oligomers is found to enhance the shear strength of bundle junctions by about an order of magnitude compared with “bare” van der Waals interactions between pristine CNTs. Through multiscale simulations, the enhancement of the shear strength can be attributed to an interlocking mechanism of polymer chains in the bundles, dominated by van der Waals interactions, and stretching and alignment of chains during shearing. Unlike covalent bonds, such synergistic weak interactions can re‐form upon failure, resulting in strong, yet robust fibers. 

Extraordinary improvement of the graphitic structure of continuous carbon nanofibers templated with double wall carbon nanotubes

D. Papkov, A.M. Beese, A. Goponenko, Y. Zou, M. Naraghi, H.D. Espinosa, B. Saha, G.C. Schatz, A. Moravsky, R. Loutfy, S.B.T. Nguyen, Y. Dzenis†

ACS Nano 7(1), 2013, pp. 126-142

Carbon nanotubes are being widely studied as a reinforcing element in high-performance composites and fibers at high volume fractions. However, problems with nanotube processing, alignment, and non-optimal stress transfer between the nanotubes and surrounding matrix have so far prevented full utilization of their superb mechanical properties in composites. Here, we present an alternative use of carbon nanotubes, at a very small concentration, as a templating agent for the formation of graphitic structure in fibers. Continuous carbon nanofibers (CNF) were manufactured by electrospinning from polyacrylonitrile (PAN) with 1.2% of double wall nanotubes (DWNT). Nanofibers were oxidized and carbonized at temperatures from 600 °C to 1850 °C. Structural analyses revealed significant improvements in graphitic structure and crystal orientation in the templated CNFs, with the largest improvements observed at lower carbonization temperatures. 

Nano and Cell Mechanics: Fundamentals and Frontiers

G. Bao, H.D. Espinosa (Editors)

John Wiley & Sons  2013

Research in nano and cell mechanics has received much attention from the scientific community as a result of society needs and government initiatives to accelerate developments in materials, manufacturing, electronics, medicine and healthcare, energy, and the environment. Engineers and scientists are currently engaging in increasingly complex scientific problems that require interdisciplinary approaches. In this regard, studies in this field draw from fundamentals in atomistic scale phenomena, biology, statistical and continuum mechanics, and multiscale modeling and experimentation. As a result, contributions in these areas are spread over a large number of specialized journals, which prompted the Editors to assemble this book. Nano and Cell Mechanics: Fundamentals and Frontiers brings together many of the new developments in the field for the first time, and covers fundamentals and frontiers in mechanics to accelerate developments in nano- and bio-technologies.

In situ TEM electromechanical testing of nanowires and nanotubes

H.D. Espinosa†, R.A. Bernal, T. Filleter

Small 8(21), 2012, pp. 3233-3252

The emergence of one‐dimensional nanostructures as fundamental constituents of advanced materials and next‐generation electronic and electromechanical devices has increased the need for their atomic‐scale characterization. Given its spatial and temporal resolution, coupled with analytical capabilities, transmission electron microscopy (TEM) has been the technique of choice in performing atomic structure and defect characterization. A number of approaches have been recently developed to combine these capabilities with in‐situ mechanical deformation and electrical characterization in the emerging field of in‐situ TEM electromechanical testing. This has enabled researchers to establish unambiguous synthesis‐structure‐property relations for one‐dimensional nanostructures. In this article, the development and latest advances of several in‐situ TEM techniques to carry out mechanical and electromechanical testing of nanowires and nanotubes are reviewed. 

Nucleation‐controlled distributed plasticity in penta‐twinned silver nanowires

T. Filleter*, S. Ryu*, K. Kang, J. Yin, R.A. Bernal, K. Sohn, S. Li, J. Huang, W. Cai, H.D. Espinosa†

Small 8(19), 2012, pp. 2986-2993

A unique size‐dependent strain hardening mechanism, that achieves both high strength and ductility, is demonstrated for penta‐twinned Ag nanowires (NWs) through a combined experimental‐computational approach. Thin Ag NWs are found to deform via the surface nucleation of stacking fault decahedrons (SFDs) in multiple plastic zones distributed along the NW. Twin boundaries lead to the formation of SFD chains that locally harden the NW and promote subsequent nucleation of SFDs at other locations. Due to surface undulations, chain reactions of SFD arrays are activated at stress concentrations and terminated as local stress decreases, revealing insensitivity to defects imparted by the twin structures. Thick NWs exhibit lower flow stress and number of distributed plastic zones due to the onset of necking accompanied by more complex dislocation structures.

A review of mechanical and electromechanical properties of piezoelectric nanowires

H.D. Espinosa†, R.A. Bernal, M. Minary‐Jolandan

Advanced Materials 24(34), 2012, pp. 4656-4675

Piezoelectric nanowires are promising building blocks in nanoelectronic, sensing, actuation and nanogenerator systems. In spite of great progress in synthesis methods, quantitative mechanical and electromechanical characterization of these nanostructures is still limited. In this article, the state‐of‐the art in experimental and computational studies of mechanical and electromechanical properties of piezoelectric nanowires is reviewed with an emphasis on size effects. The review covers existing characterization and analysis methods and summarizes data reported in the literature. It also provides an assessment of research needs and opportunities. Throughout the discussion, the importance of coupling experimental and computational studies is highlighted. This is crucial for obtaining unambiguous size effects of nanowire properties, which truly reflect the effect of scaling rather than a particular synthesis route. We show that such a combined approach is critical to establish synthesis‐structure‐property relations that will pave the way for optimal usage of piezoelectric nanowires.

Design and identification of high performance steel alloys for structures subjected to underwater impulsive loading

F. Latourte, X. Wei, Z.D. Feinberg, A. De Vaucorbeil, P. Tran, G.B. Olson, H.D. Espinosa†

International Journal of Solids and Structures 49(13), 2012, pp. 1573-1587

Martensitic and austenitic steel alloys were designed to optimize the performance of structures subjected to impulsive loads. The deformation and fracture characteristics of the designed steel alloys were investigated experimentally and computationally. The experiments were based on an instrumented fluid–structure interaction apparatus, in which deflection profiles are recorded using a shadow Moiré technique combined with high speed imaging. Fractographic analysis and post-mortem thickness reduction measurements were also conducted in order to identify deformation and fracture modes. The computational study was based on a modified Gurson damage model able to accurately describe ductile failure under various loading paths. The model was calibrated for two high performance martensitic steels (HSLA-100 and BA-160) and an austenitic steel (TRIP-120). The martensitic steel (BA-160) was designed to maximize strength and fracture toughness while the austenitic steel (TRIP-120) was designed to maximize uniform ductility, in other words, to delay necking instability. 

Multiscale experimental mechanics of hierarchical carbon‐based materials

H.D. Espinosa†, T. Filleter, M. Naraghi

Advanced Materials 24(21), 2012, pp. 2805-2823

Investigation of the mechanics of natural materials, such as spider silk, abalone shells, and bone, has provided great insight into the design of materials that can simultaneously achieve high specific strength and toughness. Research has shown that their emergent mechanical properties are owed in part to their specific self‐organization in hierarchical molecular structures, from nanoscale to macroscale, as well as their mixing and bonding. To apply these findings to manmade materials, researchers have devoted significant efforts in developing a fundamental understanding of multiscale mechanics of materials and its application to the design of novel materials with superior mechanical performance. These efforts included the utilization of some of the most promising carbon‐based nanomaterials, such as carbon nanotubes, carbon nanofibers, and graphene, together with a variety of matrix materials. At the core of these efforts lies the need to characterize material mechanical behavior across multiple length scales starting from nanoscale characterization of constituents and their interactions to emerging micro‐ and macroscale properties.

Atom Probe Tomography of a-Axis GaN Nanowires: Analysis of Nonstoichiometric Evaporation Behavior

J.R. Riley, R.A. Bernal, Q. Li, H.D. Espinosa, G.T. Wang, L.J. Lauhon†

ACS Nano 6(5), 2012, pp. 3898-3906

GaN nanowires oriented along the nonpolar a-axis were analyzed using pulsed laser atom probe tomography (APT). Stoichiometric mass spectra were achieved by optimizing the temperature, applied dc voltage, and laser pulse energy. Local variations in the measured stoichiometry were observed and correlated with facet polarity using scanning electron microscopy. Fewer N atoms were detected from nonpolar and Ga-polar surfaces due to uncorrelated evaporation of N₂ ions following N adatom diffusion. The observed differences in Ga and N ion evaporation behaviors are considered in detail to understand the influence of intrinsic materials characteristics on the reliability of atom probe tomography analysis. We find that while reliable analysis of III–N alloys is possible, the standard APT procedure of empirically adjusting analysis conditions to obtain stoichiometric detection of Ga and N is not necessarily the best approach for this materials system.

Carbon‐carbon contacts for robust nanoelectromechanical switches

O. Loh, X. Wei, J. Sullivan, L.E. Ocola, R. Divan, H.D. Espinosa†

Advanced Materials 24(18), 2012, pp. 2463-2468

Nanoelectromechanical devices exhibiting dramatically improved robustness through novel material selection are demonstrated. A unique combination of carbon nanotube active elements and conductive diamond‐like carbon contact electrodes results in reliable switching performance not found in devices with ubiquitously‐used metal thin film electrodes. This in turn represents a viable means to improve the reliability of a diverse, and widely‐pursued class of nanoscale devices ranging from single‐nanostructure switches to massively parallel arrays.

Nanoelectromechanical contact switches

O.Y. Loh, H.D. Espinosa†

Nature Nanotechnology 7(5), 2012, pp. 283-295

Nanoelectromechanical (NEM) switches are similar to conventional semiconductor switches in that they can be used as relays, transistors, logic devices and sensors. However, the operating principles of NEM switches and semiconductor switches are fundamentally different. These differences give NEM switches an advantage over semiconductor switches in some applications — for example, NEM switches perform much better in extreme environments — but semiconductor switches benefit from a much superior manufacturing infrastructure. Here we review the potential of NEM-switch technologies to complement or selectively replace conventional complementary metal-oxide semiconductor technology, and identify the challenges involved in the large-scale manufacture of a representative set of NEM-based devices.

Optimal length scales emerging from shear load transfer in natural materials: application to carbon-based nanocomposite design

X. Wei, M. Naraghi, H.D. Espinosa†

ACS Nano 6(3), 2012, pp. 2333-2344

Numerous theoretical and experimental studies on various species of natural composites, such as nacre in abalone shells, collagen fibrils in tendon, and spider silk fibers, have been pursued to provide insight into the synthesis of novel bioinspired high-performance composites. However, a direct link between the mechanical properties of the constituents and the various geometric features and hierarchies remains to be fully established. In this paper, we explore a common denominator leading to the outstanding balance between strength and toughness in natural composite materials. We present an analytical model to link the mechanical properties of constituents, their geometric arrangement, and the chemistries used in their lateral interactions. Key critical overlap length scales between adjacent reinforcement constituents, which directly control strength and toughness of composite materials, emerge from the analysis. 

Experimental-computational study of shear interactions within double-walled carbon nanotube bundles

T. Filleter, S. Yockel, M. Naraghi, J.T. Paci, O.C. Compton, M.L. Mayes, S.B.T. Nguyen, G.C. Schatz, H.D. Espinosa†

Nano Letters 12(2), 2012, pp. 732-742

The mechanical behavior of carbon nanotube (CNT)-based fibers and nanocomposites depends intimately on the shear interactions between adjacent tubes. We have applied an experimental-computational approach to investigate the shear interactions between adjacent CNTs within individual double-walled nanotube (DWNT) bundles. The force required to pull out an inner bundle of DWNTs from an outer shell of DWNTs was measured using in situ scanning electron microscopy methods. The normalized force per CNT–CNT interaction (1.7 ± 1.0 nN) was found to be considerably higher than molecular mechanics (MM)-based predictions for bare CNTs (0.3 nN). This MM result is similar to the force that results from exposure of newly formed CNT surfaces, indicating that the observed pullout force arises from factors beyond what arise from potential energy effects associated with bare CNTs. 

Individual GaN nanowires exhibit strong piezoelectricity in 3D

M. Minary-Jolandan, R.A. Bernal, I. Kuljanishvili, V. Parpoil, H.D. Espinosa†

Nano Letters 12(2), 2012, pp. 970-976

Semiconductor GaN NWs are promising components in next generation nano- and optoelectronic systems. In addition to their direct band gap, they exhibit piezoelectricity, which renders them particularly attractive in energy harvesting applications for self-powered devices. Nanowires are often considered as one-dimensional nanostructures; however, the electromechanical coupling leads to a third rank tensor that for wurtzite crystals (GaN NWs) possesses three independent coefficients, d₃₃, d₁₃, and d₁₅. Therefore, the full piezoelectric characterization of individual GaN NWs requires application of electric fields in different directions and measurements of associated displacements on the order of several picometers. In this Letter, we present an experimental approach based on scanning probe microscopy to directly quantify the three-dimensional piezoelectric response of individual GaN NWs. Experimental results reveal that GaN NWs exhibit strong piezoelectricity in three dimensions, with up to six times the effect in bulk. 

Mechanical characterization of materials at small length scales

M.F. Pantano, H.D. Espinosa, L. Pagnotta†

Journal of Mechanical Science and Technology 26(2), 2012, pp. 545-561

A review on the mechanical characterization of materials at small length scale is presented. The focus is on the different micro- and nanoscale testing techniques, the variety of materials investigated by the scientific and industrial communities and the mechanical quantities identified by such methodologies. The interest on materials behavior at small length scales has gained considerable attention in the last two decades, as a consequence of the increasing application, production, and commercialization of various kinds of micro- and nano- electro-mechanical systems (MEMS/NEMS). Due to their small size, short time response, high performance and low energy requirements, these devices are currently used in a variety of industrial, consumer and biomedical applications.

Strong piezoelectricity in individual GaN nanowires

M. Minary-Jolandan†, R.A. Bernal, H.D. Espinosa

MRS Communications 1(1), 2011, pp. 45-48

GaN nanowires are promising building blocks for future nanoelectronics, optoelectronic devices, and nanogenerators. Here, we report on strong piezoelectricity in individual single-crystal GaN nanowires revealed by direct measurement of the piezoelectric constant using piezo-response force microscopy. Our experimental results show that individual c-axis GaN nanowires, with a characteristic dimension as small as 65 nm, show a shear piezoelectric constant of tid₁₅~ 10 pm/V, which is several times that measured in bulk. The revealed strong piezoelectricity could open promising opportunities for application of GaN nanowires in nanowire-based sensors and generators for self-powered nanodevices.

Characterizing atomic composition and dopant distribution in wide band gap semiconductor nanowires using laser-assisted atom probe tomography

R. Agrawal, R.A. Bernal, D. Isheim, H.D. Espinosa†

The Journal of Physical Chemistry C 115(36), 2011, pp. 17688-17694

Characterization of atomic composition and spatially resolved dopant distribution in wide band gap semiconducting nanowires is critical for their applications in next-generation nanoelectronic and optoelectronic devices. We have applied laser-assisted atom probe tomography to measure the spatially resolved composition of wide band gap semiconducting undoped GaN nanowires and Mg-doped GaN nanowires. Stoichiometric evaporation of individual GaN nanowires was achieved, and optimal experimental conditions to characterize the concentration and spatial distribution of the dopant in the Mg:GaN nanowire samples were established. Extremely mild operating conditions, with laser pulse energy as low as 3 pJ, are required to avoid preferential loss of nitrogen and achieve stoichiometric evaporation. The role of nanowire morphology in the selection of optimal experimental conditions is discussed in the context of thermal transport within the nanowire under a heat load imposed by the pulsing laser. 

Failure mechanisms in composite panels subjected to underwater impulsive loads

F. Latourte*, D. Gregoire*, D. Zenkert, X. Wei, H.D. Espinosa†

Journal of the Mechanics and Physics of Solids 59(8), 2011, pp. 1623-1646

This work examines the performance of composite panels when subjected to underwater impulsive loads. The scaled fluid–structure experimental methodology developed by Espinosa and co-workers was employed. Failure modes, damage mechanisms and their distributions were identified and quantified for composite monolithic and sandwich panels subjected to typical blast loadings. The temporal evolutions of panel deflection and center deflection histories were obtained from shadow Moiré fringes acquired in real time by means of high speed photography. A linear relationship of zero intercept between peak center deflections versus applied impulse per areal mass was obtained for composite monolithic panels. For composite sandwich panels, the relationship between maximum center deflection versus applied impulse per areal mass was found to be approximately bilinear but with a higher slope. Performance improvement of sandwich versus monolithic composite panels was, therefore, established specially at sufficiently high impulses per areal mass (I₀/M¯>170 m s⁻¹). 

Ultrahigh strength and stiffness in cross‐linked hierarchical carbon nanotube bundles

T. Filleter, R. Bernal, S. Li, H.D. Espinosa†

Advanced Materials 23(25), 2011, pp. 2855-2860

Electron irradiation induced covalent cross‐linking at multiple length scales within double‐walled nanotube bundles is demonstrated to lead to ultrahigh effective strength and stiffness. In situ transmission electron microscopy tensile testing reveals both order of magnitude enhancements in the mechanical properties as well as distinct failure mechanisms of cross‐linked versus un‐crosslinked bundles.

In-situ AFM experiments with discontinuous DIC applied to damage identification in biomaterials

D. Grégoire, O. Loh, A. Juster, H.D. Espinosa†

Experimental Mechanics 51(4), 2011, pp. 591-607

Natural materials (e.g. nacre, bone, and spider silk) exhibit unique and outstanding mechanical properties. This performance is due to highly evolved hierarchical designs. Building a comprehensive understanding of the multi-scale mechanisms that enable this performance represents a critical step toward realizing strong and tough bio-inspired materials. This paper details a multi-scale experimental investigation into the toughening mechanisms in natural nacre. By applying extended digital image correlation and other image processing techniques, quantitative information is extracted from otherwise prodominantly qualitative experiments. In situ three point bending fracture tests are performed to identify and quantify the toughening mechanisms involved during the fracture of natural nacre across multiple length scales. At the macro and micro scales, fracture tests performed in situ with a macro lens and optical microscope enable observation of spreading of damage outward from the crack tip. 

Effect of growth orientation and diameter on the elasticity of GaN nanowires. A combined in situ TEM and atomistic modeling investigation

R.A. Bernal, R. Agrawal, B. Peng, K.A. Bertness, N.A. Sanford, A.V. Davydov, H.D. Espinosa†

Nano Letters 11(2), 2011, pp. 548-555

We characterized the elastic properties of GaN nanowires grown along different crystallographic orientations. In situ transmission electron microscopy tensile tests were conducted using a MEMS-based nanoscale testing system. Complementary atomistic simulations were performed using density functional theory and molecular dynamics. Our work establishes that elasticity size dependence is limited to nanowires with diameters smaller than 20 nm. For larger diameters, the elastic modulus converges to the bulk values of 300 GPa for c-axis and 267 GPa for a– and m-axis.

Giant piezoelectric size effects in zinc oxide and gallium nitride nanowires. A first principles investigation

R. Agrawal, H.D. Espinosa†

Nano Letters 11(2), 2011, pp. 786-790

Nanowires made of materials with noncentrosymmetric crystal structure are under investigation for their piezoelectric properties and suitability as building blocks for next-generation self-powered nanodevices. In this work, we investigate the size dependence of piezoelectric coefficients in nanowires of two such materials − zinc oxide and gallium nitride. Nanowires, oriented along their polar axis, ranging from 0.6 to 2.4 nm in diameter were modeled quantum mechanically. A giant piezoelectric size effect is identified for both GaN and ZnO nanowires. However, GaN exhibits a larger and more extended size dependence than ZnO. The observed size effect is discussed in the context of charge redistribution near the free surfaces leading to changes in local polarization. The study reveals that local changes in polarization and reduction of unit cell volume with respect to bulk values lead to the observed size effect. 

Dimensional analysis and parametric studies for designing artificial nacre

J.E. Rim, P. Zavattieri, A. Juster, H.D. Espinosa†

Journal of the Mechanical Behavior of Biomedical Materials 4(2), 2011, pp. 190-211

Nacre, the iridescent material found in Abalone shells, exhibits remarkable strength and toughness despite its composition of over 95% brittle ceramic. Its hierarchical structure over multiple length scales gives rise to its increase in toughness despite its material composition. In this work we develop a computational model of composites incorporating key morphological features of nacre’s microstructure. By conducting a parametric analysis we are able to determine an optimal geometry that increases energy dissipation over 70 times. We discuss the contribution of varying ceramic strengths and size effect to see how this affects the overall performance of the composite. We then compare our simulations to experiments performed on a material possessing the same microstructure investigated computationally. For both simulations and experiments we show that our optimal geometry corresponds to that of natural nacre indicating the importance of specifically incorporating nacre’s key morphological and constituent features. 

Tablet-level origin of toughening in abalone shells and translation to synthetic composite materials

H.D. Espinosa†, A.L. Juster, F.J. Latourte, O.Y. Loh, D. Gregoire, P.D. Zavattieri

Nature Communications 2(1), 2011, pp. 1-9

Nacre, the iridescent material in seashells, is one of many natural materials employing hierarchical structures to achieve high strength and toughness from relatively weak constituents. Incorporating these structures into composites is appealing as conventional engineering materials often sacrifice strength to improve toughness. Researchers hypothesize that nacre’s toughness originates within its brick-and-mortar-like microstructure. Under loading, bricks slide relative to each other, propagating inelastic deformation over millimeter length scales. This leads to orders-of-magnitude increase in toughness. Here, we use in situ atomic force microscopy fracture experiments and digital image correlation to quantitatively prove that brick morphology (waviness) leads to transverse dilation and subsequent interfacial hardening during sliding, a previously hypothesized dominant toughening mechanism in nacre.

Substrate stiffness regulates extracellular matrix deposition by alveolar epithelial cells

J.L. Eisenberg, A. Safi, X. Wei, H.D. Espinosa, G.R.S. Budinger, D. Takawira, S.B. Hopkinson, J.C.R. Jones†

Research and Reports in Biology 2(1), 2011, pp. 1-9

The aim of the study was to address whether a stiff substrate, a model for pulmonary fibrosis, is responsible for inducing changes in the phenotype of alveolar epithelial cells (AEC) in the lung, including their deposition and organization of extracellular matrix (ECM) proteins. Freshly isolated lung AEC from male Sprague Dawley rats were seeded onto polyacrylamide gel substrates of varying stiffness and analyzed for expression and organization of adhesion, cytoskeletal, differentiation, and ECM components by Western immunoblotting and confocal immunofluorescence microscopy. We observed that substrate stiffness influences cell morphology and the organization of focal adhesions and the actin cytoskeleton. Surprisingly, however, we found that substrate stiffness has no influence on the differentiation of type II into type I AEC, nor does increased substrate stiffness lead to an epithelial mesenchymal transition. 

Robust Carbon‐Nanotube‐Based Nano‐electromechanical Devices: Understanding and Eliminating Prevalent Failure Modes Using Alternative Electrode Materials

O. Loh, X. Wei, C. Ke, J. Sullivan, H.D. Espinosa†

Small 7(1), 2011, pp. 79-86

The failure modes common to widely pursued carbon‐nanotube‐based nano‐electromechanical systems are investigated. A fundamental understanding of the underlying mechanisms for failure and their relation to the device design space is developed through complementary in situ electromechanical characterization and dynamic multiphysics models. It is then found that the facile replacement of commonly used metal thin‐film electrodes with diamondlike carbon structures leads to a dramatic improvement in reliability. This enables experimental demonstration of numerous actuation cycles without failure, and reliable application to volatile memory operations.

The evolving role of experimental mechanics in 1-D nanostructure-based device development

R. Agrawal*, O. Loh*, H.D. Espinosa†

Experimental Mechanics 51(1), 2011, pp. 1-9

Future generations of transistors, sensors, and other devices maybe revolutionized through the use of one-dimensional nanostructures such as nanowires, nanotubes, and nanorods. The unique properties of these nanostructures will set new benchmarks for speed, sensitivity, functionality, and integration. These devices may even be self-powered, harvesting energy directly from their surrounding environment. However, as their critical dimensions continue to decrease and performance demands grow, classical mechanics and associated experimental techniques no longer fully characterize the observed behavior. This perspective examines the evolving role of experimental mechanics in driving the development of these new devices.

Comparison of the Ewald and Wolf methods for modeling electrostatic interactions in nanowires

E.E. Gdoutos, R. Agrawal, H.D. Espinosa†

International Journal for Numerical Methods in Engineering 84(13), 2011, pp. 1541-1551

Ionic compounds pose extra challenges with the appropriate modeling of long‐range coulombic interactions. Here, we study the mechanical properties of zinc oxide (ZnO) nanowires using molecular dynamic simulations with Buckingham potential and determine the suitability of the Ewald (Ann. Phys. 1921; 19) and Wolf (J. Chem. Phys. 1999; 110(17):8254–8282) summation methods to account for the long‐range Coulombic forces. A comparative study shows that both the summation methods are suitable for modeling bulk structures with periodic boundary conditions imposed on all sides; however, significant differences are observed when nanowires with free surfaces are modeled. As opposed to Wolf’s prediction of a linear stress–strain response in the elastic regime, Ewald’s method predicts an erroneous behavior. This is attributed to the Ewald method’s inability to account for surface effects properly. Additionally, Wolf’s method offers highly improved computational performance as the model size is increased. This gain in computational time allows for modeling realistic nanowires, which can be directly compared with the existing experimental results.

A multiscale study of high performance double-walled nanotube− polymer fibers

M. Naraghi*, T. Filleter*, A. Moravsky, M. Locascio, R.O. Loutfy, H.D. Espinosa†

ACS Nano, 4(11), 2010, pp. 6463-6476

The superior mechanical behavior of carbon nanotubes (CNT) and their electrical and thermal functionalities has motivated researchers to exploit them as building blocks to develop advanced materials. Here, we demonstrate high performance double-walled nanotube (DWNT)−polymer composite yarns formed by twisting and stretching of ribbons of randomly oriented bundles of DWNTs thinly coated with polymeric organic compounds. A multiscale in situ scanning electron microscopy experimental approach was implemented to investigate the mechanical performance of yarns and isolated DWNT bundles with and without polymer coatings. DWNT−polymer yarns exhibited significant ductility of ∼20%, with energy-to-failure of as high as ∼100 J g⁻¹, superior to previously reported CNT-based yarns. The enhanced ductility is not at the expense of strength, as yarns exhibited strength as high as ∼1.4 GPa. In addition, the significance of twisting on the densification of yarns and corresponding enhancement in the lateral interactions between bundles is identified. 

Micro and nanotechnology for biological and biomedical applications

C.T. Lim†, J. Han, J. Guck, H. Espinosa

Medical & Biological Engineering & Computing, 48(10), 2010, pp. 941-943

This special issue contains some of the current state-of-the-art development and use of micro and nanotechnological tools, devices and techniques for both biological and biomedical research and applications. These include nanoparticles for bioimaging and biosensing, optical and biophotonic techniques for probing diseases at the nanoscale, micro and nano-fabricated tools for elucidating molecular mechanisms of mechanotransduction in cell and molecular biology and cell separation microdevices and techniques for isolating and enriching targeted cells for disease detection and diagnosis. Although some of these works are still at the research stage, there is no doubt that some of the important outcomes will eventually see actual biomedical applications in the not too distant future.

Large-scale density functional theory investigation of failure modes in ZnO nanowires

R. Agrawal, J.T. Paci, H.D. Espinosa†

Nano Letters 10(9), 2010, pp. 3432-3438

Electromechanical and photonic properties of semiconducting nanowires depend on their strain states and are limited by their extent of deformation. A fundamental understanding of the mechanical response of individual nanowires is therefore essential to assess system reliability and to define the design space of future nanowire-based devices. Here we perform a large-scale density functional theory (DFT) investigation of failure modes in zinc oxide (ZnO) nanowires. Nanowires as large as 3.6 nm in diameter with 864 atoms were investigated. The study reveals that pristine nanowires can be elastically deformed to strains as high as 20%, prior to a phase transition leading to fracture. The current study suggests that the phase transition predicted at ∼10% strain in pristine nanowires by the Buckingham pairwise potential (BP) is an artifact of approximations inherent in the BP. 

MEMS for in situ testing—Handling, actuation, loading, and displacement measurements

M.A. Haque, H.D. Espinosa, H.J. Lee

MRS Bulletin 35(5), 2010, pp. 375-381

Mechanical testing of micro- and nanoscale materials is challenging due to the intricate nature of specimen preparation and handling and the required load and displacement resolution. In addition, in Situ testing requires the entire experimental setup to be drastically miniaturized, because conventional high-resolution microscopes or analytical tools usually have very small chambers. These challenges are increasingly being addressed using microelectromechanical systems (MEMS)-based sensors and actuators. Because of their very small size, MEMS-based experimental setups are the natural choice for materials characterization under virtually all forms of in Situ electron, optical, and probe microscopy. The unique advantage of such in Situ studies is the simultaneous acquisition of qualitative (up to near atomic visualization of microstructures and deformation mechanisms) and quantitative (load, displacement, flaw size) information of fundamental materials behavior. 

Shear and tensile plastic behavior of austenitic steel TRIP-120 compared with martensitic steel HSLA-100

F. Latourte, Z. Feinberg, L.F. Mori, G.B. Olson, H.D. Espinosa†

International Journal of Fracture 162(1), 2010, pp. 187-204

The mechanical performance of TRIP-120 a novel transformation induced plasticity steel alloy, is evaluated for different loading cases and strain rates. The performance is compared with HSLA-100, a low-alloy steel developed by the United States Navy and currently used in naval hulls. The response of these materials under uniaxial tension and shear was investigated to the point of fracture at isothermal and adiabatic conditions. TRIP-120 shows a significant improvement in dissipated energy at fracture compared to HSLA-100. SEM images of ductile fracture surfaces for tensile and torsion samples of both TRIP-120 and HSLA-100 are compared, and the presence of transformed martensite in the TRIP-120 dynamic torsion specimens is confirmed with optical microscopy and magnetometry.

Nanofountain Probes for direct-write nanomanufacturing and in vitro single cell studies

O.Y. Loh, H.D. Espinosa†

2010 3rd International Nanoelectronics Conference (INEC), 2010, pp. 423-424

We present a broadly-applicable nanodeposition tool, the Nanofountain Probe, for direct-write fabrication of functional nanostructures using liquid molecular inks. Examples of nanopatterning of biomolecules, catalysts for subsequent nanostructure growth, and functional nanoparticle arrays for nanosystems fabrication and single cell studies will be presented. Recent developments in the use of the Nanofountain Probe as an in vitro single cell injection tool are also discussed.

Microfluidic Platforms for Nanoparticle Delivery and Nanomanufacturing in Biology and Medicine

O. Loh, R. Lam, M. Chen, D. Ho†, H. Espinosa†

Nanodiamonds, Springer 2010, pp. 225-234

Nanoparticles are rapidly emerging as promising vehicles for next-generation therapeutic delivery. These highly mobile nanomaterials exhibit large carrier capacity and excellent stability which, when combined with innate biocompatibility, have captured the focus of numerous research efforts. As such, the ability to deliver well-controlled subcellular doses of these functional nanoparticles, both for fundamental research at the single cell level and in related device manufacturing, remains a challenge. Patterning these nanomaterials on biologically compatible substrates enables both novel biological studies and nanomanufacturing avenues through precise spatial control of dosing. Delivering them directly to live cells enables further studies where transfection remains a challenge. This chapter describes a unique tool for functional nanoparticle delivery, called the Nanofountain Probe. 

Celebrating 50 Years of Experimental Mechanics

H. Espinosa†

Experimental Mechanics 50(1), 2010, pp. 1-2

This year, Experimental Mechanics celebrates its 50th anniversary. The Journal was launched in January of 1961 by the then Society for Experimental Stress Analysis (SESA) now the Society for Experimental Mechanics (SEM). Benjamin J. Lazan, who is credited as the individual most responsible for the creation of the Journal, wrote an editorial in the first issue of Experimental Mechanics where he articulated the reasons for the creation of the journal. He pointed out that the new journal was the result of the society growth, the broader realm of experimental mechanics, and the role of experimental studies in verification and improvement of theory. In his visionary editorial, Lazan wrote “the solution of the increasingly complex engineering problems must rely more and more on experimental mechanics studies to indicate limitations in current theories, to formulate more realistic and general…

Bioinspired noncovalently crosslinked “fuzzy” carbon nanotube bundles with superior toughness and strength

G.H. Bratzel, S.W. Cranford, H. Espinosa, M.J. Buehler†

Journal of Materials Chemistry 20(46), 2010, pp. 10465-10474

Carbon nanotubes (CNTs) constitute a prominent example of structural nanomaterials, with many potential applications that could take advantage of their unique mechanical properties. Utilizing the inherent strength of CNTs at larger length-scales is, however, hindered by the inherently weak inter-tube bonding interactions, allowing slippage of nanotubes within a bundle before large macroscopic stresses are reached. Many lamellar biological materials crosslink stiff fibrous components via the introduction of a soft binding matrix to achieve a combination of high strength and toughness, as seen in cellulosic wood, silk, or collagenous bone fibrils. Here we present atomistic-based multi-scale simulation studies of bundles of carbon nanotubes with the inclusion of a binding polymer (polyethylene chains with functional end groups) to demonstrate the control of mechanical properties via variations of polymer structure, content and fiber geometry.

Experimental-computational investigation of ZnO nanowires strength and fracture

R. Agrawal*, B. Peng*, H.D. Espinosa†

Nano Letters 9(12), 2009, pp. 4177-4183

An experimental and computational approach is pursued to investigate the fracture mechanism of [0001] oriented zinc oxide nanowires under uniaxial tensile loading. A MEMS-based nanoscale material testing stage is used in situ a transmission electron microscope to perform tensile tests. Experiments revealed brittle fracture along (0001) cleavage plane at strains as high as 5%. The measured fracture strengths ranged from 3.33 to 9.53 GPa for 25 different nanowires with diameters varying from 20 to 512 nm. Molecular dynamic simulations, using the Buckingham potential, were used to examine failure mechanisms in nanowires with diameters up to 20 nm. Simulations revealed a stress-induced phase transformation from wurtzite phase to a body-centered tetragonal phase at ∼6% strain, also reported earlier by Wang et al.

Merger of structure and material in nacre and bone–Perspectives on de novo biomimetic materials

H.D. Espinosa†, J.E. Rim, F. Barthelat, M.J. Buehler

Progress in Materials Science 54(8), 2009, pp. 1059-1100

In contrast to synthetic materials, evolutionary developments in biology have resulted in materials with remarkable structural properties, made out of relatively weak constituents, arranged in complex hierarchical patterns. For instance, nacre from seashells is primarily made of a fragile ceramic, yet it exhibits superior levels of strength and toughness. Structural features leading to this performance consist of a microstructure organized in a hierarchical fashion, and the addition of a small volume fraction of biopolymers. A key to this mechanical performance is the cohesion and sliding of wavy ceramic tablets. Another example is bone, a structural biological material made of a collagen protein phase and nanoscopic mineral platelets, reaching high levels of toughness and strength per weight.

Multiscale experiments: state of the art and remaining challenges

R. Agrawal, H.D. Espinosa†

Journal of Engineering Materials and Technology 131(4), 2009, pp. 041208

In this article we review recent advances in experimental techniques for the mechanical characterization of materials and structures at various length scales with an emphasis in the submicron- and nanoregime. Advantages and disadvantages of various approaches are discussed to highlight the need for carefully designed experiments and rigorous analysis of experimentally obtained data to yield unambiguous findings. By examining in depth a few case studies we demonstrate that the development of robust and innovative experimentation is crucial for the advancement of theoretical frameworks, assessment of model predictive capabilities, and discovery of new physical phenomena.

An energy-based model to predict wear in nanocrystalline diamond atomic force microscopy tips

R. Agrawal, N. Moldovan, H.D. Espinosa†

Journal of Applied Physics 106(6), 2009, pp. 064311

Atomic force microscopy (AFM) is one of the most powerful techniques to probe surfaces and material properties at the nanoscale, and pattern organic and inorganic molecules. In all cases, knowledge of the tip geometry and its evolution with continued use is essential. In this work, a broadly applicable energy model for the evolution of scanning probe tip radii during use is presented based on quantitative wear experiments. Experiments were conducted using AFM probes made of both undoped and nitrogen-doped diamond. Undoped diamond probes were found to be nearly ten times more wear resistant than commercially available silicon nitride probes. For a constant applied force, a linear relationship between wear volume and total dissipation energy is identified. 

Special Issue on Advances in Impact Engineering

A. Vaziri, Z. Xue, V.S. Deshpande, H.D. Espinosa

Journal of Applied Mechanics 76(5), 2009, pp. 050601

It is now established that computational tools are indispensable to augment experimental techniques for the analysis of complex structures under dynamic loading. Many new computational techniques are currently being developed and new applications in the fields of impact and shock loadings are emerging. In this special issue of Journal of Applied Mechanics, we have assembled a number of recent studies in the field of impact engineering. The present issue attempts to provide a glimpse into the wide range of engineering problems in the field of Impact Engineering that mainly can be dealt with by employing computational techniques. A brief overview of each article published in this special issue is provided here.

Nanofountain‐probe‐based high‐resolution patterning and single‐cell injection of functionalized nanodiamonds

O. Loh*, R. Lam*, M. Chen, N. Moldovan, H. Huang, D. Ho†, H.D. Espinosa†

Small 5(14), 2009, pp. 1667-1674

Nanodiamonds are rapidly emerging as promising carriers for next‐generation therapeutics and drug delivery. However, developing future nanoscale devices and arrays that harness these nanoparticles will require unrealized spatial control. Furthermore, single‐cell in vitro transfection methods lack an instrument that simultaneously offers the advantages of having nanoscale dimensions and control and continuous delivery via microfluidic components. To address this, two modes of controlled delivery of functionalized diamond nanoparticles are demonstrated using a broadly applicable nanofountain probe, a tool for direct‐write nanopatterning with sub‐100‐nm resolution and direct in vitro single‐cell injection. This study demonstrates the versatility of the nanofountain probe as a tool for high‐fidelity delivery of functionalized nanodiamonds and other agents in nanomanufacturing and single‐cell biological studies. 

Rodney James Clifton

H.D. Espinosa†, K.T. Ramesh, G. Ravichandran

Experimental Mechanics 49(2), 2009, pp. 165-168

Mechanics of crystalline nanowires

H.S. Park, W. Cai, H.D. Espinosa, H. Huang

MRS Bulletin 34(3), 2009, pp. 178-183

Tailoring the load carrying capacity of MWCNTs through inter-shell atomic bridging

M. Locascio*, B. Peng*, P. Zapol, Y. Zhu, S. Li, T. Belytschko, H.D. Espinosa†

Experimental Mechanics 49(2), 2009, pp. 169-182

Deformation and failure modes of I-core sandwich structures subjected to underwater impulsive loads

L.F. Mori, D.T. Queheillalt, H.N.G. Wadley, H.D. Espinosa†

Experimental Mechanics 49(2), 2009, pp. 257-275

Modeling and experiments in cell and biomolecular mechanics

G. Bao, A. Vaziri, H.D. Espinosa†

Experimental Mechanics 49(1), 2009, pp. 1-2

Nanowear of atomic force microscope tips: Modeling and experiments

N.M. Pugno†, R. Agrawal, H.D. Espinosa

Reviews on Advanced Materials Science 19(1), 2009, pp. 73-77

The potential of MEMS for advancing experiments and modeling in cell mechanics

O. Loh, A. Vaziri, H.D. Espinosa†

Experimental Mechanics 49(1), 2009, pp. 105-124

A review on the structure and mechanical properties of mollusk shells–perspectives on synthetic biomimetic materials

F. Barthelat, J.E. Rim, H.D. Espinosa†

Applied scanning probe methods XIII, Springer, 2009, pp. 17-44

Elasticity size effects in ZnO nanowires− a combined experimental-computational approach

R. Agrawal*, B. Peng*, E.E. Gdoutos, H.D. Espinosa†

Nano Letters 8(11), 2009, pp. 3668-3674

Electric field-induced direct delivery of proteins by a nanofountain probe

O.Y. Loh*, A.M. Ho*, J.E. Rim, P. Kohli, N.A. Patankar, H.D. Espinosa†

Proceedings of the National Academy of Sciences 105(43), 2008, pp. 16438-16443

Measurements of near-ultimate strength for multiwalled carbon nanotubes and irradiation-induced crosslinking improvements

B. Peng*, M. Locascio*, P. Zapol, S. Li, S.L. Mielke, G.C. Schatz, H.D. Espinosa†

Nature Nanotechnology 3(10), 2008, pp. 626-631

Dislocation-source shutdown and the plastic behavior of single-crystal micropillars

H. Tang, K.W. Schwarz, H.D. Espinosa†

Physical Review Letters 100(18), 2008, pp. 185503

Comment on “Effects of focused ion beam milling on the nanomechanical behavior of a molybdenum-alloy single crystal” Appl. Phys. Lett. 91, 111915 (2007)

J.R. Greer†, H. Espinosa, K.T. Ramesh, E. Nadgorny

Applied Physics Letters 92(9), 2008, pp. 111915

A microelectromechanical system for nano-scale testing of one dimensional nanostructures

B. Peng, Y. Zhu, I. Petrov, H.D. Espinosa†

Sensor Letters 6(1), 2008, pp. 76-87

Direct delivery and submicrometer patterning of DNA by a nanofountain probe

K‐H. Kim, R.G. Sanedrin, A.M. Ho, S.W. Lee, N. Moldovan, C.A. Mirkin, H.D. Espinosa†

Advanced Materials 20(2), 2008, pp. 330-334

Scanning Probes for the Life Sciences

A.M. Ho, H.D. Espinosa†

Applied Scanning Probe Methods VIII, Springer, 2008, pp. 183-217

Nano-Scale Testing of Nanowires and Carbon Nanotubes Using a Micro-Electro-Mechanical System

H.D. Espinosa, Y. Zhu, B. Peng, O. Loh

Advances In Multiphysics Simulation And Experimental Testing Of Mems, World Scientific, 2008, pp. 455-489

Nanoscale testing of one-dimensional nanostructures

B. Peng†, Y. Sun, Y. Zhu, H-H. Wang, H. Espinosa

Micro and Nano Mechanical Testing of Materials and Devices, Springer, 2008, pp. 280-304

Deformation and fracture modes of sandwich structures subjected to underwater impulsive loads

L. Mori, S. Lee, Z. Xue, A. Vaziri, D. Queheillalt, K. Dharmasena, H. Wadley, J. Hutchinson, H. Espinosa

Journal of Mechanics of Materials and Structures 2(10), 2007, pp. 1981-2006

Design and operation of a MEMS-based material testing system for nanomechanical characterization

H.D. Espinosa†, Y. Zhu, N. Moldovan

Journal of Microelectromechanical Systems 16(5), 2007, pp. 1219-1231

This paper addresses the design and optimization of thermal actuators employed in a novel MEMS-based material testing system. The testing system is designed to measure the mechanical properties of a variety of materials/structures from thin films to one-dimensional structures, e.g. carbon nanotubes (CNTs) and nanowires (NWs). It includes a thermal actuator and a capacitive load sensor with a specimen in-between. The thermal actuator consists of a number of V-shaped beams anchored at both ends. It is capable of generating tens of milli-Newton force and a few micrometers displacement depending on the beams’ angle and their number. Analytical expressions of the actuator thermomechanical response are derived and discussed. From these expressions, a number of design criteria are drawn and used to optimize the device response. The analytical predictions are compared with both finite element multiphysics analysis (FEA) and experiments. To demonstrate the actuator performance, polysilicon freestanding specimens cofabricated with the testing system are tested.

Direct deposition and assembly of gold colloidal particles using a nanofountain probe

B. Wu, A. Ho, N. Moldovan, H.D. Espinosa†

Langmuir 23(17), 2007, pp. 9120-9123

Study of the size effects and friction conditions in microextrusion—part II: size effect in dynamic friction for brass-steel pairs

L.F. Mori, N. Krishnan, J. Cao, H.D. Espinosa†

Journal of Manufacturing Science and Engineering 129(4), 2007, pp. 677-689

An elasto-viscoplastic interface model for investigating the constitutive behavior of nacre

H. Tang, F. Barthelat, H.D. Espinosa†

Journal of the Mechanics and Physics of Solids 55(7), 2007, pp. 1410-1438

An experimental investigation of deformation and fracture of nacre–mother of pearl

F. Barthelat, H.D. Espinosa†

Experimental Mechanics 47(3), 2007, pp. 311-324

Fracture size effect in ultrananocrystalline diamond: Applicability of Weibull theory

B. Peng, C. Li, N. Moldovan, H.D. Espinosa†, X. Xiao, O. Auciello, J.A. Carlisle

Journal of Materials Research 22(4), 2007, pp. 913-925

Dislocation escape-related size effects in single-crystal micropillars under uniaxial compression

H. Tang, K.W. Schwarz, H.D. Espinosa†

Acta Materialia 55(5), 2007, pp. 1607-1616

Experimental techniques for the mechanical characterization of one-dimensional nanostructures

Y. Zhu, C. Ke, H.D. Espinosa†

Experimental Mechanics 47(1), 2007, pp. 7-24

On the mechanics of mother-of-pearl: a key feature in the material hierarchical structure

F. Barthelat†, H. Tang, P.D. Zavattieri, C-M. Li, H.D. Espinosa†

Journal of the Mechanics and Physics of Solids 55(2), 2007, pp. 306-337

Novel AFM nanoprobes

H.D. Espinosa, N. Moldovan, K-H. Kim

Applied Scanning Probe Methods VII, Springer 2007, pp. 77-134

Nanoelectromechanical systems—experiments and modeling

H.D. Espinosa, C. Ke

Applied Scanning Probe Methods VII, Springer 2007, pp. 135-196

A novel fluid structure interaction experiment to investigate deformation of structural elements subjected to impulsive loading

H.D. Espinosa†, S. Lee, N. Moldovan

Experimental Mechanics 46(6), 2006, pp. 805-824

In situ electron microscopy electromechanical characterization of a bistable NEMS device

C. Ke, H.D. Espinosa†

Small 12(2), 2006, pp. 1484-1489

Dynamic failure of metallic pyramidal truss core materials–experiments and modeling

S. Lee, F. Barthelat, J.W. Hutchinson, H.D. Espinosa†

International Journal of Plasticity 22(11), 2006, pp. 2118-2145

Discrete dislocation dynamics simulations to interpret plasticity size and surface effects in freestanding FCC thin films

H.D. Espinosa†, M. Panico, S. Berbenni, K.W. Schwarz

International Journal of Plasticity 22(11), 2006, pp. 2091-2117

Elasticity, strength, and toughness of single crystal silicon carbide, ultrananocrystalline diamond, and hydrogen-free tetrahedral amorphous carbon

H.D. Espinosa†, B. Peng, N. Moldovan, T.A. Friedmann, X. Xiao, D.C. Mancini, O. Auciello, J. Carlisle, C.A. Zorman, M. Merhegany

Applied Physics Letters 89(7), 2006, pp. 073111

A multi-ink linear array of nanofountain probes

N. Moldovan, K.H. Kim, H.D. Espinosa†

Journal of Micromechanics and Microengineering 16(10), 2006, pp. 1935-1942

Effect of strain rate and temperature on mechanical properties and fracture mode of high strength precipitation hardened ferritic steels

S. Vaynman†, M.E. Fine, S. Lee, H.D. Espinosa

Scripta Materialia 55(4), 2006, pp. 351-354

Mechanical properties of nacre constituents and their impact on mechanical performance

F. Barthelat, C-M. Li, C. Comi, H.D. Espinosa†

Journal of Materials Research 21(8), 2006, pp. 1977-1986

An analysis of the membrane deflection experiment used in the investigation of mechanical properties of freestanding submicron thin films

B. Peng, N. Pugno, H.D. Espinosa†

International Journal of Solids and Structures 43(11-12), 2006, pp. 3292-3305

Design and fabrication of a novel microfluidic nanoprobe

N. Moldovan, K-H. Kim, H.D. Espinosa†

Journal of Microelectromechanical Systems 15(1), 2006, pp. 204-213

Microcantilever-based Nanodevices in the Life Sciences

H.D. Espinosa, K-H. Kim, N. Moldovan

Nanotechnologies for the Life Sciences: Vol. 4: Nanodevices for the Life sciences, Willey-VCH Publishers, 2006, pp. 199-144

Nanoelectromechanical Systems (NEMS): Device and Modeling

H.D. Espinosa, C-H. Ke, N. Pugno

Encyclopedia of Materials: Science and Technology, Elsevier, 2006, pp. 1-9

Nanoelectromechanical systems (NEMSs) are systems with characteristic dimensions of a few nanometers. By exploiting nanoscale effects, NEMSs present interesting and unique characteristics, which deviate greatly from their predecessor microelectromechanical systems (MEMSs). For instance, NEMSbased devices can have fundamental frequencies in the microwave range (B100 GH) (Rueckes et al. 2000); mechanical quality factors in the tens of thousands (ultralow energy dissipation); active mass in the femtogram range; force sensitivity at the attonewton level; mass sensitivity up to attogram (Ilic et al. 2000) and subattogram (Davis et al. 2000) levels; heat capacities far below a ‘‘yoctocalorie’’ (Roukes 1999); power consumption in the order of 10 aw (Roukes 2004); and extreme high integration level, approaching 1012 elements per cm2 (Rueckes et al. 2000). All these distinguishing properties of NEMS devices pave the way to applications such as force sensors, chemical sensors, biological sensors, and ultrahigh frequency resonators.

MEMS Based Material Testing Systems: In-situ electron microscopy testing of nano objects

H.D. Espinosa, Y. Zhu, N. Moldovan

Encyclopedia of Materials: Science and Technology, Elsevier, 2006, pp. 1-10

In the early 1990s, Iijima (1991) discovered a onedimensional (1D) nanostructure, the carbon nanotube (CNT), which sparked an entirely new avenue within nanoscience and nanotechnology. Nanowires (NWs) (Cui and Lieber 2001), nanorods (Li and Alivisatos 2003), nanotubes (Wang and Li 2003), and nanobelts (Pan et al. 2001) of various materials have been successfully synthesized shortly afterwards. These 1D nanostructures demonstrate novel mechanical (Yu et al. 2000), electronic (Wildoer et al. 1998), and optical properties (Duan et al. 2003). Potential applications for these structures range from nanoelectromechanical systems (NEMSs) (Fennimore et al. 2003) to nanoelectronics (Cui et al. 2001) to nanophotonics (Law et al. 2004).

On-chip Testing and Properties of MEMS Materials: Size Scale Effects

H.D. Espinosa, S. Berbinni, M. Panico, B. Peng

Encyclopedia of Materials: Science and Technology, Elsevier, 2006, pp. 1-8

Market needs in the micro-electronics industry require the production of devices with increasing performance based on the continuous reduction of the characteristic dimensions of the main components. Therefore, in the last decade many efforts in the scientific community have focused on understanding how the material mechanical behavior changes at small scales, particularly at structural dimensions less than a micrometer. At this small scale the geometric features of the samples become comparable to the characteristic length scales of the material microstructure, and classical plasticity, which is essentially size independent, cannot be applied. This is especially the case for metallic thin films, which are of particular interest because of their wide use in electronic components.

Mechanical properties of undoped and doped ultrananocrystalline diamond: elasticity, strength, and toughness

H.D. Espinosa, B. Peng, N. Moldovan, X. Xiao, O. Auciello, J. Carlisle

Ultrananocrystalline Diamond, William Andrew Publishing, 2006, pp. 303-331

This chapter investigates the elasticity, toughness, and strength of Ultrananocrystalline Diamond (UNCD) and the validity of the Weibull statistical analysis. The fracture strength of UNCD thin films is obtained by testing submicron free-standing films by means of the membrane deflection experiment. The Weibull modulus, m, and the scale parameter, σ₀, are obtained by analyzing the tensile data. The applicability of the Weibull statistics in prediction of the strength of doped and undoped UNCD thin films is examined. A particular emphasis is placed on assessing the role of volume vs. surface in the prediction of the material strength. The chapter begins with a description of the investigated materials and a short description of the testing methodologies, followed by the reporting of experimental results including fractographic observations. A statistical analysis of the reported data based on maximum likelihood estimation is used to identify Weibull parameters. The discussion of results and their implication in the design of micro-electromechanical systems MEMS/NEMS based on UNCD are also discussed in the chapter.

Deformation rate effects on failure modes of open-cell Al foams and textile cellular materials

S. Lee, F. Barthelat, N. Moldovan, H.D. Espinosa†, H.N.G. Wadley

International Journal of Solids and Structures 43(1), 2006, pp. 53-73

The compressive behavior of open-cell aluminum alloy foam and stainless steel woven textile core materials have been investigated at three different deformation rate regimes. Quasi-static compressive tests were performed using a miniature loading frame, intermediate rates were achieved using a stored energy Kolsky bar, and high strain rate tests were performed using a light gas gun. In agreement with previous studies on foam materials, the strain rate was not found to have a significant effect on the plateau stress of metallic foams. For all the tests, real time imaging of the specimen combined with digital image correlation analysis allowed the determination of local deformation fields and failure modes. For the Kolsky bar tests, the deformations in the foam specimen were found to be more distributed than for the quasi-static test, which is attributed to moderate inertia effects. 

Dynamic failure of metallic cellular materials

S. Lee, H.D. Espinosa†

Proceedings of the 11th International Conference on Fracture 2005, ICF11, 2005, pp. 3545-3551

The quasi-static and dynamic compressive behavior of open-cell foams, textile cores, and pyramidal truss cores were investigated using a combination of experimental apparatus. Quasi-static tests were performed using a miniature loading stage and a Kolsky bar apparatus was used for intermediate deformation rates. For high deformation rates, a gas gun was employed. Optical observations of the sample deformation were performed in real time by means of high-speed photography. The deformation modes were investigated in detail from acquired images and digital image correlation. For the open cell foams, comparison between deformation fields under quasi-static and Kolsky bar loading revealed a moderate micro-inertia effect, where the inertia associated to the bending and buckling of ligaments delayed strain localization. Gas gun experiments performed on the same samples revealed a totally different deformation mode.

A comparison of mechanical properties of three MEMS materials-silicon carbide, ultrananocrystalline diamond, and hydrogen-free tetrahedral amorphous carbon (Ta-C)

H.D. Espinosa, B. Peng, N. Moldovan, T.A. Friedmann, X. Xiao, D.C. Mancini, O. Auciello, J. Carlisle, C.A. Zorman

Proceedings of the 11th International Conference on Fracture 2005, ICF11, 2005, pp. 20-25

Many MEMS devices are based on polysilicon because of the current availability of surface micromachining technology. However, polysilicon is not the best choice for devices where extensive sliding and/or thermal fields are applied due to its chemical, mechanical and tribological properties. In this work, we investigated the mechanical properties of three new materials for MEMS/NEMS devices: silicon carbide (SiC) from Case Western Reserve University (CWRU), ultrananocrystalline diamond (UNCD) from Argonne National Laboratory (ANL), and hydrogen-free tetrahedral amorphous carbon (ta-C) from Sandia National Laboratories (SNL). Young’s modulus, characteristic strength, fracture toughness, and theoretical strength were measured for these three materials using only one testing methodology–the Membrane Deflection Experiment (MDE) developed at Northwestern University

An interpretation of size-scale plasticity in geometrically confined systems

H.D. Espinosa†, S. Berbenni, M. Panico, K.W. Schwarz

Proceedings of the National Academy of Sciences 102(47), 2005, pp. 16933-16938

The mesoscopic constitutive behavior of face-centered cubic metals as a function of the system characteristic dimension recently has been investigated experimentally. Strong size effects have been identified in both polycrystalline submicron thin films and single crystal micro pillars. The size effect is manifested as an increase in strength and hardening rate as the system dimensions are decreased. In this article, we provide a mechanistic interpretation for the observed mesoscopic behavior. By performing 3D discrete dislocation dynamics simulations of grains representative of the system microstructure and associated characteristic dimensions, we show that the experimentally observed size effects can be qualitatively described. In these simulations, a constant density of dislocation sources per unit of grain boundary area is modeled by sources randomly distributed at grain boundaries. The source length (strength) is modeled by a Gaussian distribution, in which average and standard deviation is independent of the system characteristic dimension.

An electromechanical material testing system for in situ electron microscopy and applications

Y. Zhu, H.D. Espinosa†

Proceedings of the National Academy of Sciences 102(47), 2005, pp. 14503-14508

We report the development of a material testing system for in situ electron microscopy (EM) mechanical testing of nanostructures. The testing system consists of an actuator and a load sensor fabricated by means of surface micromachining. This previously undescribed nanoscale material testing system makes possible continuous observation of the specimen deformation and failure with subnanometer resolution, while simultaneously measuring the applied load electronically with nanonewton resolution. This achievement was made possible by the integration of electromechanical and thermomechanical components based on microelectromechanical system technology. The system capabilities are demonstrated by the in situ EM testing of free-standing polysilicon films, metallic nanowires, and carbon nanotubes. In particular, a previously undescribed real-time instrumented in situ transmission EM observation of carbon nanotubes failure under tensile load is presented here.

An atomistic investigation of elastic and plastic properties of Au nanowires

B. Hyde, H.D. Espinosa†, D. Farkas

Journal of Minerals, Metals and Materials 57(9), 2005, pp. 62-66

In this study, mechanical properties of cylindrical gold nanowires with diameters ranging from 5 nm to 17.5 nm were investigated using atomistic computer simulations. Displacement-controlled tensile tests were carried out to investigate the role of defects such as surface steps and twin boundaries commonly found in electrodeposited wires. The high surface-to-volume ratio of nanowires plays a critical role in the mechanical properties. Yield stress was found to be significantly affected by any disturbance in surface morphology. Twin boundaries were not more favorable as a source for dislocation nucleation but disturbed the surface in the neighborhood of the twin, resulting in lower yield strength. Twin boundaries are seen as obstacles for the propagation of dislocations leading to some hardening effects.

Numerical analysis of nanotube-based NEMS devices—Part I: Electrostatic charge distribution on multiwalled nanotubes

C. Ke, H.D. Espinosa†

Journal of Applied Mechanics 72(5), 2005, pp. 721-725

The charge distribution on the surface of a biased conductive, finite-length, cylindrical nanotube, free standing above an infinite grounded plane, is investigated. The diameter range of the cylinder tube under study is 20–60 nm, which is much larger than the screening length, meaning the quantum and statistical effects on the charge distribution are negligible. The relationship between the charge distribution and the geometry of the nanotube is examined in detail by classical electrostatics using full three-dimensional numerical simulations based on the boundary element method. A model of the concentrated charge at the end of nanotubes is proposed. The charge distribution for a clamped cantilever nanotube is also computed and discussed. The findings here reported are of particular usefulness in the design and modeling of electrostatic actuated nanotube/nanowire based nano-electromechanical systems.

Numerical analysis of nanotube based NEMS devices—Part II: Role of finite kinematics, stretching and charge concentrations

C. Ke, H.D. Espinosa†, N. Pugno

Journal of Applied Mechanics 72(5), 2005, pp. 726-731

In this paper a nonlinear analysis of nanotube based nano-electromechanical systems is reported. Assuming continuum mechanics, the complete nonlinear equation of the elastic line of the nanotube is derived and then numerically solved. In particular, we study singly and doubly clamped nanotubes under electrostatic actuation. The analysis emphasizes the importance of nonlinear kinematics effects in the prediction of the pull-in voltage of the device, a key design parameter. Moreover, the nonlinear behavior associated with finite kinematics (i.e., large deformations), neglected in previous studies, as well as charge concentrations at the tip of singly clamped nanotubes, are investigated in detail. We show that nonlinear kinematics results in an important increase in the pull-in voltage of doubly clamped nanotube devices, but that it is negligible in the case of singly clamped devices.

Novel Ultrananocrystalline Diamond Probes for High‐Resolution Low‐Wear Nanolithographic Techniques

K‐H. Kim, N. Moldovan, C. Ke, H.D. Espinosa†, X. Xiao, J.A. Carlisle, O. Auciello

Small 1(8-9), 2005, pp. 866-874

A hard, low‐wear probe for contact‐mode writing techniques, such as dip‐pen nanolithography (DPN), was fabricated using ultrananocrystalline diamond (UNCD). Molding within anisotropically etched and oxidized pyramidal pits in silicon was used to obtain diamond tips with radii down to 30 nm through growth of UNCD films followed by selective etching of the silicon template substrate. The probes were monolithically integrated with diamond cantilevers and subsequently integrated into a chip body obtained by metal electroforming. The probes were characterized in terms of their mechanical properties, wear, and atomic force microscopy imaging capabilities. The developed probes performed exceptionally well in DPN molecular writing/imaging mode. Furthermore, the integration of UNCD films with appropriate substrates and the use of directed microfabrication techniques are particularly suitable for fabrication of one‐ and two‐dimensional arrays of probes that can be used for massive parallel fabrication of nanostructures by the DPN method.

A nanofountain probe with sub‐100 nm molecular writing resolution

K‐H. Kim, N. Moldovan, H.D. Espinosa†

Small 1(6), 2005, pp. 632-635

Just the tip of the iceberg? The Nanofountain Probe (NFP) has been developed to produce molecular patterning at the sub‐100 nm scale. In conjunction with an atomic force microscope probe, the volcano‐shaped tip (shown in the SEM image) ensures controlled ink delivery from an on‐chip reservoir to provide high‐resolution patterning for manifold potential applications.

Experiments and modeling of carbon nanotube-based NEMS devices

C-H. Ke*, N. Pugno*, B. Peng, H.D. Espinosa†

Journal of the Mechanics and Physics of Solids 53(6), 2005, pp. 1314-1333

In this paper, carbon nanotube-based nanoelectromechanical systems (NEMS) are nanofabricated and tested. In-situ scanning electron microscopy measurements of the deflection of the cantilever under electrostatic actuation are reported. In particular, a cantilever nanotube suspended over an electrode (nanoswitch), or two symmetric cantilever nanotubes (nanotweezers), from which a differential in electrical potential is imposed, are studied. The finite deformation regime investigated here is the first of its kind. An analytical model based on the energy method in both small deformation and finite kinematics (large deformation) regimes is used to interpret the measurements. The theory overcomes limitations of prior analysis reported in the literature towards the prediction of the structural behavior of NEMS. Some of the simplifying hypotheses have been removed. Furthermore, the theory takes into account the cylindrical shape of the deflected nanotube in the evaluation of its electrical capacitance, the influence of the van der Waals forces as well as finite kinematics. 

Analysis of doubly clamped nanotube devices in the finite deformation regime

N. Pugno, C-H. Ke, H.D. Espinosa†

Journal of the Applied Mechanics 72(3), 2005, pp. 445-449

In this paper, a nonlinear theory applicable to the design of nanotube based devices is presented. The role of finite kinematics for a doubly clamped nanotube device is investigated. In particular, we analyze the continuous deformation and instability (pull in) of a clamped-clamped nanotube suspended over an electrode from which a potential differential is imposed. The transformation of an applied voltage into a nanomechanical deformation indeed represents a key step toward the design of innovative nanodevices. Likewise, accurate prediction of pull-in/pull-out voltages is highly needed. We show that an energy-based method can be conveniently used to predict the structural behavior and instability corresponding to the ON/OFF states of the device at the so-called pull-in voltage. The analysis reveals that finite kinematics effects can result in a significant increase of the pull-in voltage.

Epitaxially influenced boundary layer model for size effect in thin metallic films

Z.P. Bažant†, Z. Guo, H.D. Espinosa, Y. Zhu, B. Peng

Journal of Applied Physics 97(7), 2005, pp. 073506

It is shown that the size effect recently observed by Espinosa et al., [J. Mech. Phys. Solids51, 47 (2003)] in pure tension tests on free thin metallic films can be explained by the existence of a boundary layer of fixed thickness, located at the surface of the film that was attached onto the substrate during deposition. The boundary layer is influenced by the epitaxial effects of crystal growth on the dislocation density and texture (manifested by prevalent crystal plane orientations). This influence is assumed to cause significantly elevated yield strength. Furthermore, the observed gradual postpeak softening, along with its size independence, which is observed in short film strips subjected to pure tension, is explained by slip localization, originating at notch-like defects, and by damage, which can propagate in a stable manner when the film strip under pure tension is sufficiently thin and short. 

A new methodology to investigate fracture toughness of freestanding MEMS and advanced materials in thin film form

H.D. Espinosa†, B. Peng

Journal of Microelectromechanical Systems 14(1), 2005, pp. 153-159

This work presents a novel membrane deflection fracture experiment (MDFE) to investigate the fracture toughness of microelectromechanical systems (MEMS) and other advanced materials in thin film form. It involves the stretching of freestanding thin-film membranes, in a fixed-fixed configuration, containing preexisting cracks. The fracture behavior of ultrananocrystalline diamond (UNCD), a material developed at Argonne National Laboratory, is investigated to illustrate the methodology. When the fracture initiates from sharp cracks, produced by indentation, the fracture toughness was found to be 4.5/spl plusmn/0.25 MP m/sup 1/2/. When the fracture initiates from blunt notches with radii about 100 nm, machined by focused ion beam (FIB), the mean value of the apparent fracture toughness was found to be 6.9 MPa m/sup 1/2/. Comparison of these two values, using the model proposed by Drory et al., provides a correction factor of two-thirds, which corresponds to a mean value of /spl rho//2x=1/2.

New directions in mechanics

M.E. Kassner, S. Nemat-Nasser†, Z. Suo, G. Bao, J. C. Barbour, L. C. Brinson, H. Espinosa, H. Gao, S. Granick, P. Gumbsch, K-S. Kim, W. Knauss, L. Kubin, J. Langer, B.C. Larson, L. Mahadevan, A. Majumdar, S. Torquato, F. Van Swol

Mechanics of Materials 37(2-3), 2005, pp. 231-259

The Division of Materials Sciences and Engineering of the US Department of Energy (DOE) sponsored a workshop to identify cutting-edge research needs and opportunities, enabled by the application of theoretical and applied mechanics. The workshop also included input from biochemical, surface science, and computational disciplines, on approaching scientific issues at the nanoscale, and the linkage of atomistic-scale with nano-, meso-, and continuum-scale mechanics. This paper is a summary of the outcome of the workshop, consisting of three main sections, each put together by a team of workshop participants. 

A microelectromechanical load sensor for in situ electron and x-ray microscopy tensile testing of nanostructures

Y. Zhu, N. Moldovan, H.D. Espinosa†

Applied Physics Letters 86(1), 2005, pp. 013506

We report on the performance of a microelectromechanical system (MEMS) designed for the in situ electron and x-ray microscopy tensile testing of nanostructures, e.g., carbon nanotubes and nanowires. The device consists of an actuator and a load sensor with a gap in between, across which nanostructures can be placed, nanowelded, and mechanically tested. The load sensor is based on differential capacitance measurements, from which its displacement history is recorded. By determining the sensor stiffness, the load history during the testing is obtained. We calibrated the device and examined its resolution in the context of various applications of interest. The device is the first true MEMS in which the load is electronically measured. It is designed to be placed in scanning and transmission electron microscopes and on x-ray synchrotron stages.

Nanoelectromechanical systems and modeling

C. Ke, H.D. Espinosa†

Handbook of Theoretical and Computational Nanotechnology, American Scientific Publishers, 2005, pp. 1-38

We report on the performance of a microelectromechanical system (MEMS) designed for the in situ electron and x-ray microscopy tensile testing of nanostructures, e.g., carbon nanotubes and nanowires. The device consists of an actuator and a load sensor with a gap in between, across which nanostructures can be placed, nanowelded, and mechanically tested. The load sensor is based on differential capacitance measurements, from which its displacement history is recorded. By determining the sensor stiffness, the load history during the testing is obtained. We calibrated the device and examined its resolution in the context of various applications of interest. The device is the first true MEMS in which the load is electronically measured. It is designed to be placed in scanning and transmission electron microscopes and on x-ray synchrotron stages.

Predictions of strength in MEMS components with defects––a novel experimental–theoretical approach

N. Pugno, B. Peng, H.D. Espinosa†

International Journal of Solids and Structures 42(2), 2005, pp. 647-661

This paper presents a novel experimental–theoretical method to investigate the strength of structures having complex geometries, which are commonly used in microelectromechanical systems (MEMS). It involves the stretching to failure of freestanding thin-film membranes, in a fixed–fixed configuration, containing micro-fabricated sharp cracks, blunt notches and re-entrant corners. The defects, made by nanoindentation and focused ion beam milling, are characterized by scanning electron microscopy (SEM). MEMS structures made of ultra-nano-crystalline-diamond (UNCD), a material developed at Argonne National Laboratory, were investigated using this methodology. A theory to predict the strength of microstructures with defects is proposed and compared with experimental results. It is shown that fracture mechanics general concepts can be applied with confidence in the design of MEMS. An experimental methodology and formulas to predict strength of MEMS structures possessing defects of various geometries are provided.

A feedback controlled carbon nanotube based NEMS device

C. Ke, N. Pugno, H.D. Espinosa†

Proceedings of the 12th International Conference on Experimental Mechanics, 2004

A switchable carbon nanotube based nano-electromechanical systems (NEMS) device with close-loop feedback is examined. The device is made of a multi-walled carbon nanotube (MWNT) placed as a cantilever over a microfabricated step. A bottom electrode, power supply and a resistor are also parts of the device circuit. The pullin/pull-out and tunneling characteristics of the device are investigated by means of an electro-mechanical analysis. The model includes the concentration of electrical charge, at the end of the nanocantilever, and the van der Waals force. Finite kinematics accounting for large deformations of the cantilever is also included in the modeling. The result shows that the device has two well-defined stable equilibrium positions as a result of the tunneling and the incorporation of a feedback resistor to the circuit. The potential applications of the device include NEMS switches, random-access memory (RAM) elements, logic devices and electron-counters

Reliability of capacitive RF MEMS switches at high and low temperatures

Y. Zhu, H.D. Espinosa†

International Journal of RF and Microwave Computer‐Aided Engineering 14(4), 2004, pp. 317-328

Some applications of RF MEMS switches, such as aircraft condition monitoring and distributed satellite communication, present a unique challenge for device design and reliability. This article examines these switches when operational temperatures in the range −60°C to 100°C are envisioned. The basic operation of a capacitive MEMS switch is described and two tools for examining device reliability, modeling, and on‐chip experimentation, are discussed in the case of capacitive MEMS switches. 1D, 2D, and 3D models are presented with emphasis on 3D coupled‐field finite‐element analysis, including temperature effects. Results and findings from the 3D simulations are reported. In particular, the advantages of employing corrugated membranes in the design of RF MEMS switches are assessed. Their performance in terms of reliability as a function of temperature is quantified. The effects of corrugation on the geometric parameters are discussed in the context of device‐design optimization. In order to assess reliability experimentally, the M‐test and the membrane deflection experiment (MDE) are reviewed due to their on‐chip characteristic and simplicity. 

Effect of temperature on capacitive RF MEMS switch performance—a coupled-field analysis

Y. Zhu, H.D. Espinosa†

Journal of Micromechanics and Microengineering 14(8), 2004, pp. 1270-1279

Three-dimensional multiphysics finite element analysis (FEA) was performed to investigate the reliability of RF MEMS switches at various operational temperatures. The investigated MEMS capacitive switch consists of a freestanding metal membrane actuated by a bottom electrode coated by a dielectric film. Coupled-field simulations between thermal, structural and electrostatic domains were performed. The simulations show that temperature significantly changes both the membrane stress state and out-of-plane geometry. In particular, the membrane buckles when temperature increase, from room temperature, takes place. The buckling temperature, i.e. the upper bound to the operational temperature, is a function of manufacturing residual stress state, membrane initial out-of-plane profile and a mismatch in materials coefficient of thermal expansion. 

Materials science and fabrication processes for a new MEMS technology based on ultrananocrystalline diamond thin films

O. Auciello, J. Birrell, J.A. Carlisle, J.E. Gerbi, X. Xiao, B. Peng, H.D. Espinosa

Journal of Physics: Condensed Matter 16(16), 2004, pp. 539-552

Most MEMS devices are currently based on silicon because of the available surface machining technology. However, Si has poor mechanical and tribological properties which makes it difficult to produce high performance Si based MEMS devices that could work reliably, particularly in harsh environments; diamond, as a superhard material with high mechanical strength, exceptional chemical inertness, outstanding thermal stability and superior tribological performance, could be an ideal material for MEMS. A key challenge for diamond MEMS is the integration of diamond films with other materials. Conventional CVD thin film deposition methods produce diamond films with large grains, high internal stress, poor intergranular adhesion and very rough surfaces, and are consequently ill-suited for MEMS applications. 

Micro-and Nanomechanics

B.C. Prorok, Y. Zhu, H.D. Espinosa†, Z. Guo, Z. Bazant, Y. Zhao, B.I. Yakobson

Encyclopedia of Nanoscience and Nanotechnology Ch. 5, American Scientific Publishers, 2004, pp. 561-606

It has been known for quite some time that materials and structures with small-scale dimensions do not behave in the same manner as their bulk counterparts. This aspect was first observed in thin films where certain defect structures were found to have deleterious effects on the film’s structural integrity and reliability. This became a significant concern because thin films are routinely employed as components in microelectronics and microelectromechanical systems (MEMS). Their properties frequently allow essential device functions and therefore accurate identification of these properties is key to the development of new technologies. Unfortunately, most of our knowledge is based on bulk material behavior, which many times fails to describe material response in small-scale dimensions because of the dominance of surface and interface effects, finite number

Plasticity size effects in free-standing submicron polycrystalline FCC films subjected to pure tension

H.D. Espinosa†, B.C. Prorok, B. Peng

Journal of the Mechanics and Physics of Solids 52(3), 2004, pp. 667-689

The membrane deflection experiment developed by Espinosa and co-workers was used to examine size effects on mechanical properties of free-standing polycrystalline FCC thin films. We present stress–strain curves obtained on films 0.2, 0.3, 0.5 and 1.0 μm thick including specimen widths of 2.5, 5.0, 10.0 and 20.0 μm for each thickness. Elastic modulus was consistently measured in the range of 53–55 GPa for Au, 125–129 GPa for Cu and 65–70 GPa for Al. Several size effects were observed including yield stress variations with membrane width and film thickness in pure tension. The yield stress of the membranes was found to increase as membrane width and thickness decreased. It was also observed that thickness plays a major role in deformation behavior and fracture of polycrystalline FCC metals. A strengthening size scale of one over film thickness was identified. 

Mechanical Properties of Nacre Constituents: An Inverse Method Approach

F. Barthelat, H.D. Espinosa†

MRS Online Proceedings Library (OPL) 884, 2004, pp. 539-542

Nacre, also known as mother-of-pearl, is the iridescent layer found inside some mollusk species such as oyster or abalone. It is made of relatively weak materials, but its hierarchical microstructure is so well optimized that its macroscopic mechanical properties are far superior to those of its constituents. For this reason there is a great interest in nacre as a source of inspiration for novel designs of composites. Despite many years of research on nacre, an accurate characterization of its constituents is lacking. In this work nacre was tested as a layered composite material using low depth indentation and uniaxial compression. The first test was modeled using finite element analysis and the second test was modeled as a Reuss composite in compression. A micromechanical model of the interface was also pursued to gain insight on the relevance of the interface features such as tablet roughness and biopolymer hydrated response. 

Fracture Size Effect in Ultrananocrystalline Diamond: Weibull Theory Applicability

B. Peng, H.D. Espinosa†

Proceedings of the 2004 ASME International Mechanical Engineering Congress, 2004, pp. 13-19

Strength characterization and analysis of fracture size effect in ultrananocrystalline diamond (UNCD) thin films are presented. In this work, we report the changes in mechanical properties of UNCD by the addition of nitrogen gas to the Ar/CH4 microwave plasma. Both undoped and doped UNCD films show a decrease in fracture strength with an increase in specimen size. The strength data, obtained by using the membrane deflection experiment (MDE) developed at Nothwestern University, is interpreted using Weibull statistics. The capability of the theory is examined in conjunction with detailed fractographic analysis. The Weibull parameters are estimated by maximum likelihood estimation (MLE) based on 480 tests when the specimen volume varies from 500 to 1600 cubic microns. The results show that one can predict the fracture strength of a component possessing any arbitrary volume to within ±3% from the fracture strength identified from the tested specimens. The failure mode of UNCD is suggested to be volume controlled.

Strength of Ultrananocrystalline Diamond Thin films–Identification of Weibull Parameters

B. Peng†, H.D. Espinosa†, N. Moldovan, X. Xiao, O. Auciello, J.A. Carlisle, D.M Gruen, R.S. Divan, D.C. Mancini, J.E. Gerbi, J. Birrell

MRS Online Proceedings Library 778(1), 2004, pp. 2101-2106

The fracture strength of ultrananocrystalline diamond (UNCD) thin films, grown by microwaveplasma- enhanced chemical-vapor deposition (PECVD), was measured using the membrane deflection experiment (MDE) developed by Espinosa and coworkers. The data show that UNCD fracture strength appears to follow a Weibull distribution. Furthermore, we show that the Weibull parameters are highly dependent on the seeding process used in the growth of the films. When seeding was performed with micron-size diamond particles, using mechanical polishing of the substrate, the stress, resulting in a probability of failure of 67%, was found to be 1.74 GPa, and the Weibull modulus was 5.74. By contrast, when seeding was performed with nano-size diamond particles, using ultrasonic agitation, the stress, resulting in a probability of failure of 67%, increased to 4.13 GPa and the Weibull modulus was 10.76. 

A membrane deflection fracture experiment to investigate fracture toughness of freestanding MEMS materials

H.D. Espinosa†, B. Peng

MRS Online Proceedings Library 795(1), 2003, pp. 294-300

This paper presents a novel Membrane Deflection Fracture Experiment (MDFE) to investigate the fracture toughness of MEMS and other advanced materials in thin film form. It involves the stretching of freestanding thin-film membranes, in a fixed-fixed configuration, containing pre-existing cracks. The fracture behavior of ultrananocrystalline diamond (UNCD), a material developed at Argonne National Laboratory, is investigated to illustrate the methodology. When the fracture initiates from sharp cracks, produced by indentation, the fracture toughness was found to be 4.7 MPa m^1/2. When the fracture initiates from blunt notches with radii about 100 nm, machined by focused ion beam (FIB), the mean value of the apparent fracture toughness was found to be 7.2 MPa m^1/2. Comparison of these two values, using the model proposed by Drory et al. [9], provides a correction factor of 2/3, which corresponds to a mean value of ρ/2x=1/2.

Fracture strength of ultrananocrystalline diamond thin films—identification of Weibull parameters

H.D. Espinosa†, B. Peng, B.C. Prorok, N. Moldovan, O. Auciello, J.A. Carlisle, D.M. Gruen, D.C. Mancini

Journal of Applied Physics 94(9), 2003, pp. 6076-6084

Size effects on the mechanical behavior of gold thin films

H.D. Espinosa†, B.C. Prorok

Journal of Materials Science 38(20), 2003, pp. 4125-4128

Microelectromechanical systems and nanomaterials: Experimental and computational mechanics aspects

H.D. Espinosa†

Experimental Mechanics 43(3), 2003, pp. 225-227

Dynamic torsion testing of nanocrystalline coatings using high-speed photography and digital image correlation

F. Barthelat, Z. Wu, B.C. Prorok, H.D. Espinosa†

Experimental Mechanics 43(3), 2003, pp. 331-340

An experimental/computational approach to identify moduli and residual stress in MEMS radio-frequency switches

H.D. Espinosa†, Y. Zhu, M. Fischer, J. Hutchinson

Experimental Mechanics 43(3), 2003, pp. 309-316

Mechanical properties of ultrananocrystalline diamond thin films relevant to MEMS/NEMS devices

H.D. Espinosa†, B.C. Prorok, B. Peng, K.H. Kim, N. Moldovan, O. Auciello, J.A. Carlisle, D.M. Gruen, D.C. Mancini

Experimental Mechanics 43(3), 2003, pp. 256-268

Nanoscale displacement and strain measurement

Y. Zhu, F. Barthelat, P.E. Labossiere, N. Moldovan, H.D. Espinosa†

Proceedings of the 2003 SEM Annual Conference and Exposition on Experimental and Applied Mechanics, 2003.

Electromechanical Modeling and Simulation of RF MEMS Switches

Y. Zhu, H.D. Espinosa†

Proceedings of the 2003 SEM Annual Conference and Exposition on Experimental and Applied Mechanics, 2003.

Elastic properties of nacre aragonite tablets

F Barthelat, H.D. Espinosa†

Proceedings of the 2003 SEM Annual Conference and Exposition on Experimental and Applied Mechanics, 2003.

Massively parallel multi-tip nanoscale writer with fluidic capabilities-fountain pen nanolithography (FPN)

K.H. Kim, C. Ke, N. Moldovan, H.D. Espinosa†

Proceedings of the 2003 SEM Annual Conference and Exposition on Experimental and Applied Mechanics, 2003.

An examination of the competition between bulk behavior and interfacial behavior of ceramics subjected to dynamic pressure–shear loading

P.D. Zavattieri, H.D. Espinosa†

Journal of the Mechanics and Physics of Solids 51(4), 2003, pp. 607-635

Modeling dynamic crack propagation in fiber reinforced composites including frictional effects

S.K. Dwivedi, H.D. Espinosa†

Mechanics of Materials 35(3-6), 2003, pp. 481-509

A grain level model for the study of failure initiation and evolution in polycrystalline brittle materials. Part II: numerical examples

H.D. Espinosa†, P.D. Zavattieri

Mechanics of Materials 35(3-6), 2003, pp. 365-394

A grain level model for the study of failure initiation and evolution in polycrystalline brittle materials. Part I: Theory and numerical implementation

H.D. Espinosa†, P.D. Zavattieri

Mechanics of Materials 35(3-6), 2003, pp. 333-364

Strain rate effects in metallic cellular materials

S. Lee, F. Barthelat, H.D. Espinosa†

Proceedings of the 2003 SEM Annual Conference and Exposition on Experimental and Applied Mechanics, 2003.

Design of radio frequency (RF) MEMS switches: Modeling

Y. Zhu, H.D. Espinosa†

ASME 2003 International Mechanical Engineering Congress and Exposition, 2003.

A novel AFM chip for fountain pen nanolithography-Design and microfabrication

K-H. Kim, N. Moldovan, C. Ke, H.D. Espinosa†

MRS Online Proceedings 782, 2003.

A methodology for determining mechanical properties of freestanding thin films and MEMS materials

H.D. Espinosa†, B.C. Prorok, M. Fischer

Journal of the Mechanics and Physics of Solids 51(1), 2003, pp. 47-67

Mechanical properties of ultrananocrystalline diamond thin films for MEMS applications

H.D. Espinosa†, B. Peng, K-H. Kim, B.C. Prorok, N. Moldovan, X.C. Xiao, J.E. Gerbi, J. Birrell, O. Auciello, J.A. Carlisle, D.M. Gruen, D.C. Mancini

MRS Online Proceedings Library 741, 2002, pp. 921-926

Modelling of failure mode transition in ballistic penetration with a continuum model describing microcracking and flow of pulverized media

B.A. Gailly, H.D. Espinosa†

International Journal for Numerical Methods in Engineering 54(3), 2002, pp. 365-398

Effects of nanometer-thick passivation layers on the mechanical response of thin gold films

B.C. Prorok, H.D. Espinosa†

Journal of nanoscience and nanotechnology 2(3-4), 2002, pp. 427-433

Effects of Film Thickness on the Yielding Behavior of Polycrystalline Gold Films

H.D. Espinosa†, B.C. Prorok

Thin Films: Stresses And Mechanical Properties IX, Vol. 695, 2002, pp. 349-354

Advances in micro scale modeling of failure mechanisms in ceramics and fiber composites

H.D. Espinosa†, S. Lee

Proceedings of the First Sino-US Joint Symposium on Multi-Scale Analysis in Material Sciences and Engineering 2002

Report on ONR workshop on fracture scaling

Z.P. Bažant, Y.D.S. Rajapakse, D.H. Allen, R. Ballarini, H.D. Espinosa, H. Gao, R. Gettu, M. Jirasek, G. Pijaudier-Cabot, J. Planas, F.J. Ulm

International journal of fracture 113(4), 2002, pp. 345-366

Quasi-Static and Dynamic Torsion testing of ceramic coatings using High-Speed photography

F. Barthelat, K. Malukhin, H. Espinosa†

Recent Advances in Experimental Mechanics In Honor of Isaac M. Daniel, edited by E.E. Gdoutos

Size Effects and Passivation Effects on the Plasticity of Freestanding Submicron Gold Films

H.D. Espinosa†, B.C. Prorok

2002 Annual meeting of the Society of Experimental Mechanics, 2002

A MEMS Device for In Situ TEM/AFM/SEM/STM Testing of Carbon Nanotubes and Nanowires

H.D. Espinosa†, Y. Zhu, B. Peng

2002 Annual meeting of the Society of Experimental Mechanics, 2002

Wafer Level Mechanical testing of RF MEMS Switches Testing at Low and High Temperatures

H.D. Espinosa†, B.C. Prorok

2002 Annual meeting of the Society of Experimental Mechanics, 2002

Tensile Testing of Abalone Nacre Miniature Specimens Using Microscopy and Speckle Correlation

F. Barthelat, H.D. Espinosa†

2002 Annual meeting of the Society of Experimental Mechanics, 2002

Grain level analysis of crack initiation and propagation in brittle materials

P.D. Zavattieri, H.D. Espinosa†

Acta Materialia 49(20), 2001, pp. 4291-4311

Modeling of shear instabilities observed in cylinder collapse experiments performed on ceramic powders

H.D. Espinosa†, B.A. Gailly

Acta Materialia 49(19), 2001, pp. 4135-4147

The role of thermal activation on dynamic stress-induced inelasticity and damage in Ti–6Al–4V

H.V. Arrieta, H.D Espinosa†

Mechanics of Materials 33(10), 2001, pp. 573-591

A 3-D finite deformation anisotropic visco-plasticity model for fiber composites

H.D. Espinosa†, H.C. Lu, P.D. Zavattieri, S. Dwivedi

Journal of Composite Materials 35(5), 2001, pp. 369-410

A computational model of ceramic microstructures subjected to multi-axial dynamic loading

P.D. Zavattieri, P.V. Raghuram, H.D Espinosa†

Journal of the Mechanics and Physics of Solids 49(1), 2001, pp. 27-68

A model is presented for the dynamic finite element analysis of ceramic microstructures subjected to multi-axial dynamic loading. This model solves an initial-boundary value problem using a multi-body contact model integrated with interface elements to simulate microcracking at grain boundaries and subsequent large sliding, opening and closing of microcracks. An explicit time integration scheme is adopted to integrate the system of spatially discretized ordinary differential equations. A systematic and parametric study of the effect of interface element parameters, grain anisotropy, stochastic distribution of interface properties, grain size and grain morphology is carried out. Numerical results are shown in terms of microcrack patterns and evolution of crack density, i.e., damage kinetics. The brittle behavior of the microstructure as the interfacial strength decreases is investigated. Crack patterns on the representative volume element vary from grains totally detached from each other to a few short cracks, nucleated at voids, except, for the case of microstructures with initial flaws. Grain elastic anisotropy seems to play an important role in microfracture presenting higher values of crack density than the isotropic case. 

Modeling intersonic crack propagation in fiber reinforced composites with contact/cohesive laws

S.K. Dwivedi, H.D. Espinosa†

2001 ASME International Mechanical Engineering Congress and Exposition, Vol. 66, 2001, pp. 121-153

Dynamic crack propagation in an unidirectional Carbon/Epoxy composite was studied. It was shown that the friction coefficient along the crack surface plays an important role by smearing the discontinuous field that develops behind the crack tip and by reducing crack speed in the intersonic regime. The results showed that the contour integral computed at near field contours are path independent and can serve as a parameter for characterizing intersonic crack propagation.

Identification of residual stress state in an RF-MEMS device

H.D. Espinosa†, M. Fischer, E. Herbert, W. Oliver

White paper, MTS Systems Corporation

Microelectromechanical Systems (MEMS) are among the most significant technological advances of this decade. The objective of this technology is to manufacture “systems” whose dimensions are only a few hundred microns. Devices with applications ranging from drug delivery systems to telecommunications are currently under development. Their reduced size and weight give them unique advantages.

An investigation of plasticity in MEMS materials

H.D. Espinosa†, B.C. Prorok, Y. Zhu, M. Fischer

Proceedings of InterPACK’01, the Pacific RIM/International, Intersociety, Electronic Packaging Technical/Business Conference & Exhibition, 2001.

We have developed a membrane deflection experiment particularly suitable for the investigation of sub-micron thin films that directly measures actual load and film stretch. The experiment consists of loading a fixed-fixed membrane with a line load that is applied to the middle of the span with a nanoindenter column. A Mirau microscope-interferometer is conveniently aligned with the nano-indenter to directly measure strains. This is accomplished through a specially manufactured wafer containing a window to expose the bottom surface of the membrane. The sample stage incorporates the interferometer to allow continuous monitoring of the membrane deflection during both loading and unloading. As the nanoindenter engages and deflects the sample downward, fringes are formed due to the motion of the bottom surface of the membrane and are acquired through the use of a CCD camera. Digital monochromatic images are obtained and stored at periodic intervals of time to map the strain field.

A novel experimental technique for testing thin films and MEMS materials

H.D. Espinosa†, B.C. Prorok, M. Fischer

Proceedings of the SEM Annual Conference on Experimental and Applied Mechanics, 2001, pp. 446-446.

We have developed a novel µ–scale membrane deflection experiment particularly suited for the investigation of submicron thin films and MEMS materials. The experiment consists of loading a fixed-fixed membrane with a line load that is applied to the middle of the span with a nanoindenter column. A Mirau microscope-interferometer is positioned below the membrane to observe its response to loading. This is accomplished through a specially micromachined wafer containing a window to expose the bottom surface of the membrane. The sample stage incorporates the interferometer to allow continuous monitoring of the membrane deflection during both loading and unloading. As the nanoindenter engages and deflects the sample downward, fringes are formed due to the motion of the bottom surface of the membrane and are acquired through a CCD camera. Digital monochromatic images are obtained and stored at periodic intervals of time to map the strain field.

3-D computational modeling of RF MEMS switches

H.D. Espinosa†, M. Fischer, Y. Zhu, S. Lee

Technical Proceedings of the 2001 International Conference on Modeling and Simulation of Microsystems, 2001, pp. 402-405.

Young’s modulus and residual stress state of freestanding thin membranes are characterized in this work by means of wafer level experimental techniques. RF MEMS Switches manufactured by Raytheon Systems Co. are investigated using a new method that combines a Membrane Deflection Experiment (MDE) and numerical simulations. It is found that the thin aluminum alloy membranes used in the RF MEMS devices have a Young’s modulus of 70±10 GPa in the plane of the membrane, and a residual tensile stress of 4±1 MPa. The accuracy of the identified parameters is confirmed by sensitivity studies to geometric aspects of the specimens and loads. It is found that changes in initial residual stress affect the loaddeflection curves at small values of deflection. By contrast, variations in Young’s modulus result in changes in loaddeflection curvature at large displacements. These features are very important to decouple both effects in the process of identification of the parameters.

Dynamic friction measurements at sliding velocities representative of high-speed machining processes

H.D. Espinosa†, A.J. Patanella, M. Fischer

Journal of Tribology 122(4), 2000, pp. 834-848.

Understanding high speed machining processes requires knowledge of the dynamic friction response at the tool-workpiece interface, the high strain rate response of the workpiece material and its fracture mechanisms. In this paper, a novel experimental technique, consisting in the independent application of an axial static load and a dynamic torque, is used to investigate time resolved dynamic friction. Shear stress wave propagation along an input bar, pressing statically against an output bar, is analyzed. The quasi-static and kinetic friction coefficients of Ti-6Al-4V sliding against 1080 Steel, Al 6061-T6 sliding against 1080 Steel, and Al 6061-T6 sliding against Al 7075-T6, with various surface characteristics, are investigated. Sliding velocities up to 6.9 m/s are achieved. Surface roughness is varied to understand its role on the frictional response of the sliding interfaces. The dependence of friction coefficient on …

Dynamic compression-shear response of brittle materials with specimen recovery

H.D. Espinosa†, A.J. Patanella, Y. Xu

Experimental Mechanics 4(3), 2000, pp. 321-330.

A new configuration for compression-shear soft-recovery experiments is presented. This technique is used to investigate various failure mechanisms during dynamic multiaxial loading of an Al₂O₃/SiC nanocomposite and TiB₂. Velocity profiles of the target surface are measured with a variable sensitivity displacement interferometer, yielding normal and transverse velocity-time histories. A dynamic shear stress of approximately 280 MPa is obtained, in the Al₂O₃/SiC nanocomposite, for an imposed axial stress of about 3.45 GPa on a 540 μm thick sample. This dynamic shear stress is well below the value predicted by elastic wave propagation theory. This could be the result of stress-induced damage and inelasticity in the bulk of the sample or inelasticity on the sample surface due to frictional sliding. To gain further insight into the possible failure mechanisms, an investigation of compression-shear recovery …

Enhanced ballistic performance of confined multi-layered ceramic targets against long rod penetrators through interface defeat

H.D. Espinosa†, N.S. Brar, G. Yuan, Y. Xu, V. Arrieta

International Journal of Solids and Structures 37(36), 2000, pp. 4893-4913.

Impact recovery experiments on confined multi-layered ceramic targets are performed to identify materials and structural design issues in interface defeat of long rod tungsten heavy alloy (WHA) penetrators. In-situ stress measurements are made, with embedded manganin/constantan gauges, and velocity histories of the target rear surface are measured using an interferometric technique. Material response to penetration is examined by considering different hardness of the cover steel plate and two types of ceramics, viz., Alumina and TiB2. The combined material-structural response is examined by changing the thickness of the graphite plate, used to accommodate the deforming WHA penetrator, and by welding top and bottom plates with the middle plate to increase the stiffness of the assembled multi-layered target. In total, eight shots are performed in the velocity range of 1.5–1.7 km/s. Ceramic damage is studied …

A novel dynamic friction experiment using a modified Kolsky bar apparatus

H.D Espinosa†, A. Patanella, M. Fischer

Experimental Mechanics 40(2), 2000, pp. 138-153.

A novel dynamic friction experiment using the Kolsky bar concept was developed. The technique is complementary to the plate impact and other macroscopic friction experiments in the sense that sliding velocities and pressures not attainable otherwise can be investigated. The experimental results reported in this article show that the technique provides accurate and repeatable measurement of time-resolved friction. The apparatus is simpler and easier to operate than the plate impact facility. However, it cannot achieve the same level of contact pressure. Several material pairs have been investigated. In particular, the kinetic friction coefficient of Ti-6Al-4V sliding against WC/Co (cermet) and 4340 steel sliding against WC/Co were measured and compared with the values reported by Prakash and Clifton in 1993. Atomic force microscopy is used to characterize the surface topography before and after the…

Modeling impact induced delamination of woven fiber reinforced composites with contact/cohesive laws

H.D. Espinosa†, S. Dwivedi, H-C. Lu

Computer Methods in Applied Mechanics and Engineering 183(3-4), 2000, pp. 259-290.

The dynamic delamination in woven glass fiber reinforced plastic (GRP) composite is studied with a 3D finite deformation anisotropic viscoplastic model in conjunction with contact/cohesive laws. The large deformation of the material during impact loading is described through an anisotropic plasticity model in total Lagrangian co-ordinates whose coefficients are determined experimentally. The interaction between lamina is analyzed through a contact/interface model. The tensile and shear tractions in zero thickness interface elements, embedded between lamina, are calculated from interface cohesive law. The interface cohesive law describes the evolution of these tractions in terms of normal and tangential displacement jumps and other interface parameters. The compressive traction at the interface is calculated through the impenetrability condition employed in the contact module. Once the effective displacement…

Low-velocity impact testing

H.D. Espinosa, S. Nemat-Nasser

ASM Handbook 8(6), 2000, pp. 539-559.

Impact tests are used to study dynamic deformation and failure modes of materials. Low-velocity impact techniques can be classified as plate-on-plate, rod-on-plate, plate-onrod, or rod-on-rod experiments. Two types of plate-on-plate impact tests have been developed: wave propagation experiments and thin-layer high-strain-rate experiments. The plate-on-plate experiments are further classified as nonrecovery or recovery experiments. The focus of this article is on plate-on-plate experimental techniques. At the end of this article, rod-on-plate and plate-on-rod experiments are briefly examined.

Modeling of ceramic microstructures: dynamic damage initiation and evolution

H.D. Espinosa†, P.D. Zavattieri

Shock Compression of Condensed Matter – 1999 505(1), 2000, pp. 333-338.

A model is presented for the dynamic finite element analysis of ceramic microstructures subjected to multi-axial dynamic loading. This model solves an initial-boundary value problem using a multi-body contact scheme integrated with interface elements to simulate microcracking at grain boundaries and subsequent large sliding, opening and closing of interfaces. A systematic and parametric study of the effect of interface element parameters, grain anisotropy, stochastic distribution of interface properties, grain size and grain morphology is carried out. Numerical results are shown in terms of microcrack patterns and evolution of crack density. The qualitative and quantitative results presented in this article are useful in developing more refined continuum theories of fracture properties of ceramics.

Dynamic friction of nano-materials

H. Zhang, A. Patanella, H.D. Espinosa†, K.D. Pae

Shock Compression of Condensed Matter – 1999 505(1), 2000, pp. 1225-1228.

A modified Kolsky bar method consisting in the dynamic loading in shear of a pre-compressed thin-wall sample has been developed. The technique allows the identification of the transient response from static to kinetic friction under sliding velocities of about 2–6 m/s. The normal and tangential tractions are measured independently and hence a dynamic friction coefficient identified. The sliding velocities obtained with the Kolsky bar are smaller than those obtained in pressure-shear friction experiments. Hence, the techniques are complementary and provide valuable information for the formulation of friction laws at sliding velocities, pressures and temperatures typical of manufacturing processes, dynamic shear fracture, metal forming, etc. The friction properties of two nano-ceramics sliding against metals, cermets and other ceramics are here reported.

High and low temperature dynamic testing of advanced materials

H.V. Arrieta, H.D. Espinosa†

Shock Compression of Condensed Matter – 1999 505(1), 2000, pp. 1075-1078.

A novel experimental research approach that differs substantially from the majority of previous studies is pursued. It consists of performing novel plate impact experiments at low and elevated temperatures. Through variations in temperature, it is possible to study the role of thermal activation on damage and inelastic mechanisms. The technique was used to study variation in the Hugoniot elastic limit (HEL), plastic flow and spallation of Ti-6Al-4V, and to explore thermal activation mechanisms in the formation of the so-called failure waves in soda-lime glass.

Ballistic penetration of multi-layered ceramic/steel targets

P.D. Zavattieri, H.D. Espinosa†

Shock Compression of Condensed Matter – 1999 505(1), 2000, pp. 1117-2220.

The response of multi-layered ceramic/steel targets to high velocity impact and penetration has been investigated through experiments and finite element simulations. Damage quantification and the material stresses and velocity histories provided by experiments are used as constraints to be satisfied by numerical simulations of the ballistic penetration event. Experimental and numerical observations demonstrate that the penetration process does not strongly depend on the ceramic material as usually assumed by most investigators. Instead, local and global effects which are related to material performance and structural features have been found to be very important factors that affect the overall target performance. These findings show that meaningful light weight armor design can only be accomplished through a combined experimental/numerical study in which relevant ballistic materials and structures are…

A review of micromechanics of failure waves in silicate glasses

N.S. Brar, H.D. Espinosa†

Journal of Chemical Physics (Russian), Chemical Physics Reports (English) 17(1-2), 1998, pp. 317-342.

Three mechanisms have been proposed for the recently observed failure waves (fronts) in plate impact experiments on silicate glasses. The first based on the phase transformation in glass does not explain the observed features in measured wave profiles. The second involves the comparison of the transfer of elastic shear strain energy in glass specimen due to 1-D compression to dilatant strain energy as a result of microcracking. No simulations of wave profiles were performed using this mechanism. The third is based on the microcracking multiple-plane model and is very rigorously derived. Numerical simulations of the measured wave profiles were carried out following the model. The simulations show that the failure wave phenomenon can be modeled by propagating surfaces of discontinuity from the specimen surface to its interior. Lateral stress increase and reduction of spall strength behind the failure front are successfully predicted by the multiple-plane model.

A finite deformation continuum\\discrete model for the description of fragmentation and damage in brittle materials

H.D. Espinosa†, P.D. Zavattieri, S.K. Dwivedi

Journal of the Mechanics and Physics of Solids 46(10), 1998, pp. 1909-1942.

A dynamic finite element analysis of large displacements, high strain rate deformation behavior of brittle materials is presented in total Lagrangian coordinates. A continuum\\discrete damage model capable of capturing fragmentation at two size scales is derived by combining a continuum damage model and a discrete damage model for brittle failure. It is assumed that size and distribution of potential fragments are known a priori, through either experimental findings or materials properties, and that macrocracks can nucleate and propagate along the boundaries of these potential fragments. The finite deformation continuum multiple-plane microcracking damage model accounts for microcracks within fragments. Interface elements, with cohesive strength and reversible unloading before debonding, between potential fragments describe the initiation of macrocracks, their propagation, and coalescence leading to the…

Special issue on advances in failure mechanisms in brittle materials-Preface

H.D. Espinosa, R. Clifton

Mechanics of Materials 29(3-4), 1998, pp. 141-142.

In October of 1996, a symposium on “Advances in Failure Mechanisms in Brittle Materials,” was held at the ASME International Conference and Exposition, in Atlanta, GA. The symposium, cosponsored by the Materials and Applied Mechanics Divisions, addressed the need for identication and modeling of failure initiation and growth in structures and machine components made of high strength polycrystalline brittle materials and fiber composites. Several methodologies were presented ranging from the derivation of thermodynamically consistent constitutive equations, calculation of interface crack stress fields, the use of impact experiments to examine the loss of shear resistance in glass ceramics, molecular dynamics calculations of brittle fracture, statistical aspects of brittle fragmentation, the measurement of penetrator tail velocities during penetration, the use of computer vision to obtain full field displacement maps during damage and failure, and finite element modeling of impact damage in brittle and quasi-brittle materials.

Recent developments in velocity and stress measurements applied to the dynamic characterization of brittle materials

H.D. Espinosa†

Mechanics of Materials 29(3-4), 1998, pp. 219-232.

The synthesis of novel brittle materials with tailored microstructures requires the understanding of new physical phenomena related to the failure of these materials. Observation capabilities with spatial resolution of atomic dimensions, e.g., scanning tunneling microscopy (STM) and high resolution electron microscopy (HREM), have opened new frontiers in the mechanical characterization of these advanced materials. The challenge is to design experiments capable of loading the material in a controlled fashion such that defects, resulting in well defined macroscopic stress and velocity features, are produced. In this article, techniques for the measurement of surface and in-material particle velocities and in-material axial and transverse stress measurements are reviewed. Examples on the usefulness of these techniques in the study of brittle failure are provided. A variable sensitivity displacement interferometer is used…

Adaptive FEM computation of geometric and material nonlinearities with application to brittle failure

H.D. Espinosa†, P.D. Zavattieri, G.L. Emore

Mechanics of Materials 29(3-4), 1998, pp. 275-305.

A model is presented for the dynamic finite element analysis of large-strain, high strain rate deformation behavior of materials. A total Lagrangian formulation is used in the derivation of discrete equations of motion. Both an isochoric finite deformation plasticity model, including rate and temperature effects, for metals, and a multiple-plane microcracking model for ceramics are introduced. In addition, algorithms are presented for correcting finite element mesh distortion through mesh rezoning, optimization, and refinement. A surface-defined multibody contact algorithm designed to handle large relative displacements between bodies, with addition for friction, is included. Extensions of the mechanical contact to account for heat fluxes between sliding bodies and the treatment of body interfaces with cohesive strength are presented within a unified framework. Two test examples are examined, simulating a modified Taylor…

A numerical investigation of penetration in multilayered material/structure systems

H.D. Espinosa†, S. Dwivedi, P.D. Zavattieri, G. Yuan

International journal of solids and structures 35(22), 1998, pp. 2975-3001.

The response of multilayered ceramic/steel targets to high velocity impact and penetration has been investigated through finite element simulations. A multiple-plane microcracking model has been used to describe the inelastic constitutive behavior of ceramics in the presence of damage. The model has been integrated into the finite element code EPIC95, which possesses contact and erosion capabilities particularly suitable for ballistic simulations. The integrated code has been used to analyze the depth of penetration (DOP) and interface defeat (ID) ceramic target configurations. Parametric analyses have been carried out to establish the effect of ceramic materials, target configuration design for ceramic confinement, diameter/length (d/L) ratio of the penetrator, material erosion threshold levels and the use of a shock attenuator on the response of multilayered targets subjected to high velocity impact. The response…

A novel technique for penetrator velocity measurement and damage identification in ballistic penetration experiments

H.D. Espinosa†, H-C. Lu, Y. Xu

Journal of Composite Materials 32(8), 1998, pp. 722-743.

Enhanced ballistic performance of confined ceramic targets

N.S. Brar, H.D. Espinosa†

Proceedings of the 3rd International Symposium on Impact Engineering 1998, pp. 357-362.

Dynamic compression-shear loading of brittle materials with specimen recovery

H.D. Espinosa†, A. Patanella, Y. Xu

Proceedings of the 11th International Conference on Experimental Mechanics 1998, pp. 223-229.

Experimental study of interface defeat in confined ceramic targets

N.S. Brar, H.D. Espinosa, Y. Guang

The Review of High Pressure Science and Technology 7, 1998, pp. 855-857.

Experimental study of interface defeat in confined ceramic targets

N.S. Brar, H.D. Espinosa, G. Yuan, P.D. Zavattieri

AIP Conference Proceedings 429, 1998, pp. 497-500.

Numerical Study Of Penetration in Ceramic Targets with a Multiple-Plane Model

H.D. Espinosa, G. Yuan, S. Dwivedi, P.D. Zavattieri

AIP Conference Proceedings 429, 1998, pp. 901-904.

Inelastic behavior of fiber composites subjected to out-of-plane high strain rate shearing

H.D. Espinosa, Y. Xu, H-C. Lu

Acta Materialia 45(11), 1997, pp. 4855-4865.

Micromechanics of failure waves in glass: II, modeling

H.D. Espinosa, Y. Xu, N.S. Brar

Journal of the American Ceramic Society 80(8), 1997, pp. 2074-2085.

Micromechanics of failure waves in glass: I, experiments

H.D. Espinosa, Y. Xu, N.S. Brar

Journal of the American Ceramic Society 80(8), 1997, pp. 2061-2073.

A variable sensitivity displacement interferometer with application to wave propagation experiments

H.D. Espinosa, M. Mello, Y. Xu

Journal of Applied Mechanics 64(1), 1997, pp. 123-131.

A finite deformation anisotropic plasticity model for fiber reinforced composites

H.D. Espinosa, H.C. Lu, S.K. Dwivedi, P.D. Zavattieri

Proceedings of 12th Annual Technical Conference of the American Society for Composites, 1997, pp. 429-441.

Damage quantification in confined ceramics

Y. Xu, H.D. Espinosa

AIP Conference Proceedings 429(431), 1997, pp. 431-434.

A desensitized displacement interferometer applied to impact recovery experiments

H.D. Espinosa, M. Mello, Y. Xu

Applied Physics Letters 69(21), 1996, pp. 3161-3163.

Dynamic compression‐shear loading with in‐material interferometric measurements

H.D. Espinosa

Review of Scientific Instruments 67(11), 1996, pp. 3931-3939.

Computational modeling of geometric and material nonlinearities with an application to impact damage in brittle materials

H.D. Espinosa, G. Emore, P. Zavattieri

Advances in Failure Mechanisms in Brittle Materials, edited by R.J. Clifton and H.D. Espinosa, ASME Winter Annual Meeting, 1996, pp. 119-161.

Modeling Failure Waves in Brittle Materials

H.D. Espinosa

Fourth International Conference on Structures Under Shock and Impact IV, 1996, pp. 449-458.

Novel technique for penetrator velocity measurement in ballistic penetration studies

H.D. Espinosa, H.C. Lu, Y. Xu

Advances in Failure Mechanisms in Brittle Materials, edited by R.J. Clifton and H.D. Espinosa, ASME Winter Annual Meeting, 1996, pp. 23-47.

Dynamic failure mechanisms of ceramic bars: Experiments and numerical simulations

H.D. Espinosa, N.S. Brar

Journal of the Mechanics and Physics of Solids 43(10), 1995, pp. 1615-1638.

On the dynamic shear resistance of ceramic composites and its dependence on applied multiaxial deformation

H.D. Espinosa

International Journal of Solids and Structures 32(21), 1995, pp. 3105-3128.

High strain rate behavior of composites with continuous fibers

H.D. Espinosa, G. Emore, Y. Xu

Proceedings of the 1995 ASME International Mechanical Engineering Congress and Exposition, 1995, pp. 7-18.

Dynamic Inelasticity of Polymer-Matrix Composites with Continuous Fibers

H.D. Espinosa, G. Emore

Fourth International Conference on Computational Mechanics, 1995.

A Desensitized Normal Displacement Interferometer Applied to Fast Moving Particles in a Continuum

H.D. Espinosa, Y. Xu, M. Mello

AIP Conference Proceedings 370, 1995, pp. 1011-1014.

Dynamic Failure of Brittle Materials

H.D. Espinosa, N.S. Brar

Panamerican Congress on Applied Mechanics (PACAM IV), 1995.

High strain rate modeling of ceramics and ceramic composites

H.D. Espinosa

AIP Conference Proceedings 309(1), 1994, pp. 721-724.

Experimental observations and numerical modeling of inelasticity in dynamically loaded ceramics

H.D. Espinosa, G. Raiser, R.J. Clifton, M. Ortiz

Journal of Hard Materials 3(3-4), 1992, pp. 285-313.

Performance of the star‐shaped flyer in the study of brittle materials: Three dimensional computer simulations and experimental observations

H.D. Espinosa, G. Raiser, R.J. Clifton, M. Ortiz

Journal of Applied Physics 72(8), 1992, pp. 3451-3457.

Micromechanics of the dynamic response of ceramics and ceramic composites (Ph.D. Thesis)

H.D. Espinosa

PhD Thesis, Brown University, 1992.

Plate impact experiments for investigating inelastic deformation and damage of advanced materials

H.D. Espinosa, R.J. Clifton

Winter Annual Meeting of the American Society of Mechanical Engineers, 1991, pp. 37-56.

Inelastic mechanisms in dynamically loaded ceramics

H.D. Espinosa, G.F. Raiser, R.J. Clifton, M. Ortiz

ASCE Engineering Mechanics Specialty Conference, Mechanics Computing in 1990’s and Beyond, 1991, pp. 293-297.

A soft recovery experiment for ceramics

R.J. Clifton, G. Raiser, M. Ortiz, H.D. Espinosa

Shock Compression of Condensed Matter, 1990, pp. 437-440.

Finite element analysis of stress induced damage in ceramics (M.Sc. Thesis)

H.D. Espinosa

MSc Thesis, Brown University, 1989.