Preparation and Characterization of Chiral Mechanical Metamaterials

Chirality of nanostructures and nanocomposites have been instinctively associated with their chiroptical activities, <i>e.g.</i>, circular dichroism and optical rotation. Chiral effects have also been predicted to exist in the mechanics of materials by theories more than 50 years ago.^[1]Two key phenomena have been identified: 1) A chiral material can rotate the linear polarization plane of a transverse mechanical wave, analogous to the optical rotation of electromagnetic waves. 2) A moment is induced when a chiral solid is subjected to longitudinal stress. Notwithstanding the theoretical achievements, it has only been recently that a few experimental progresses have emerged, because challenges remain in both the preparation and characterization of such materials. The goal of this talk is to present our most recent work in addressing these two challenges.<br/>Currently reported chiral mechanical structures were mostly created via 3D printing. We propose a material engineering strategy to fabricate chiral mechanical composites with helical inclusions. A variety of chiral nano- and micro-particles and assemblies can be obtained self-assembly methods.^[2] The morphology of these particles, including their size and twisting pattern, can be tuned by the self-assembly process. The chiral particles are then included in an aerogel matrix of aramid nanofibers.^[3] The micro-rotation of the particles, induced by external linear mechanical waves, transmits through the nanofiber network and couples into a collective macroscopic rotation, which can be experimentally measured.<br/>In contrast to electromagnetics, there are few commercially available tools for the detection of the polarization of mechanical waves. We built up a digital image correlation (DIC) system that enables the measurements of the displacement vector of a surface with nanometer resolution.^[4] A piezoelectric transducer, on which a chiral solid is mounted, produces a linear transverse mechanical wave up to 250 kHz. The DIC measurements are used to identify the polarization directions of the transvers waves on the piezoelectric transducer and the sample surface. The twisting angle between the two directions is an indication of the mechanical chirality of the solids. This presentation outlines our ongoing efforts on the preparation and characterization of chiral mechanical effects in self-assembled composite metamaterials.<br/><br/>[1] A. C. Eringen, Microcontinuum Field Theories, Springer New York, New York, NY, <b>1999</b>.<br/>[2] W. Jiang, Z. Qu, P. Kumar, D. Vecchio, Y. Wang, Y. Ma, J. H. Bahng, K. Bernardino, W. R. Gomes, F. M. Colombari, A. Lozada-Blanco, M. Veksler, E. Marino, A. Simon, C. Murray, S. R. Muniz, A. F. de Moura, N. A. Kotov, SCIENCE <b>2020</b>, 368, 642+.<br/>[3] M. Yang, K. Cao, L. Sui, Y. Qi, J. Zhu, A. Waas, E. M. Arruda, J. Kieffer, M. D. Thouless, N. A. Kotov, ACS Nano <b>2011</b>, 5, 6945.<br/>[4] T. Frenzel, J. Köpfler, A. Naber, M. Wegener, Sci Rep <b>2021</b>, 11, 2304.