MULTIFUNCTIONAL MATERIALS

BIOMATERIALS

Millions of years of evolution have resulted in natural materials exhibiting unique photonic, mechanical, and thermal properties. Given the limited pool of constituents and synthesis temperatures available in Nature, natural materials exhibit a combination of hierarchical structures,  heterogeneity, and exquisite organization, to achieve superior mechanical performance.

The Micro and Nanomechanics Group has a long tradition of not only unravelling the mechanics of natural materials, but also in extracting design rules and translating them into synthetic materials produced by 3D printing. Ultimately, the group aims at the design of advanced composites able to withstand a combination of extreme pressures, strain rates, and temperatures.

Among research directions in biomaterials our lab has pursued are:

• • Identification of the inelastic mechanisms responsible for the remarkable strength and toughness of Nacre, a unique biocomposite made of mineral and polymeric phases arranged in a brick-and-mortar structure. We showed that a composite, 95% ceramic and 5% polymer, achieves the toughness of aluminum. This was achieved via a unique combination of multiscale experimental techniques, using TEM, SEM, AFM, nanoindentation, and microtesters, which revealed that tablet sliding and transverse expansion (negative Poisson’s ratio) in the direction perpendicular to the tablet planes was responsible for the material toughness. Leveraging on these findings, the group devised design rules and demonstrated similar mechanisms in synthetic materials made by 3D printing.
  • Characterization of the Bouligand microstructure found in the elytra of beetles. Using rigorous contact mechanics calculations and AFM nanoscale experiments, precise determination of elastic constants and geometric parameters of the helicoidal fiber arrangement was achieved for the first time.
  • Direct observation of the hypothesized self-sharpening phenomenon in the teeth of the pink sea-urchin. The development of a novel in-situ SEM scratch test enabled the first observation and quantification for teeth sharpness preservation, a phenomenon relevant to the design of machine tools, tunnel boring, and other material removal processes.

*Movie: In-situ SEM Scratch test

Origami Metamaterial with Reversible Poisson’s Ratio

Similarly, we have investigated planar Kirigami metamaterials by focusing on a particular Kirigami cut layout (motif), which despite its simplicity has revealed a plethora of out-of-plane deformation modes because of tensile-induced instabilities. We studied these structures at the nanoscale, using silicon nitride freestanding membranes fabricated in a silicon wafer. By producing cuts with a focused ion beam (FIB), symmetric and asymmetric out-of-plane deformations resulted from instabilities caused by membrane tensile residual stresses. In a subsequent investigation, we studied a larger number of symmetric and anti-symmetric cut variations, within the same motif, and used the shadow Moire technique to experimentally verify finite element predictions. Interestingly, we observe concurrent and sequential bifurcations giving rise to complex out-of-plane shapes eliciting tilt and twist.

*Movies: FEM Analysis of Kirigami

Currently, we are exploring the vast space of Kirigami motifs and 3D deformations via Machine and Deep Learning techniques. Using these methods, we are advancing surrogate models that enable the inverse design of Kirigami structures for a range of functionalities. Moreover, we are using symbolic regression, via genetic programming schemes, to predict the buckling and post-buckling behavior of these structures.

Both Kirigami and Origami metamaterials have tremendous potential in the fields of soft-robotics, programmable surfaces (e.g., tunable friction, hydro- and aerodynamic flow and thermal control, tunable thin optics), MEMS and NEMS. In the future, we are planning to investigate functional origami/kirigami structures (arrays) as well as their behavior in dynamic regimes (wave propagation, dynamic instabilities, etc.)

2D NANOCOMPOSITES

Materials Genome Initiative

The demand for lightweight, strong, and tough materials, in transportation, space exploration, and energy applications, continues to drive the search for novel composites that can exploit advances in material synthesis and characterization. Among them, the emergence and rise of atomically thin 2D materials epitomizes nanoscale refinement, offering reinforcing constituents with unprecedented optical, electrical, and mechanical properties. When combined with intercalated polymers and metals, a plethora of new materials is envisioned. Exploration of such combinatorial problem calls for acceleration strategies, the so-called Materials Genome Initiative (MGI), that could enable identification, synthesis, and characterization of conceptualized nanocomposites system. In this respect, establishing research frameworks for assessing the predictive capabilities of computational models to screen and select promising candidates is highly needed. In the spirit of MGI, we advanced a combined experimental-computational framework with graphene oxide (GO)-polymer nacre-like nanocomposites as a case study. By employing in situ TEM and AFM experiments, we built a fundamental understanding of deformation and fracture of GO flakes, GO intralayer, and GO-polymer interfacial properties. We also assessed the predictive capabilities of atomistic simulations based on commonly used force fields. By selecting those that agree with experiments, we used atomistic simulation outputs to formulate continuum models for predicting mechanical properties of nanocomposites at larger scales. Current work involves expanding this approach to other 2D Materials-synthetic polymer systems.