Mechanical Activity: The Elastic Counterpart of Optical Activity

Abstract

Recent advances in 3D-printing technologies enable more and more sophisticated designs of artificial materials, so called metamaterials. Thereby, properties well beyond the limits of ordinary Cauchy mechanics can be achieved [1]. In our work, we design, fabricate and characterize chiral micropolar mechanical metamaterials, that show mechanical activity – the elastic analogue of optical activity [2]. Finite-element band structure calculations indicate, that the lowest two transversal modes are circular polarized. In addition, the degeneracy of these modes is lifted and the relative phase velocities differ by up to 5%. Hence, the polarization vector of linear polarized waves would rotate when propagating through such media. To unambiguously identify effects originating from chirality, static experiments have been performed showing a twist upon pushing onto a bar. This rotational degree of freedom is absent in ordinary Cauchy mechanics. The necessary sliding boundary conditions have been realized experimentally by putting a right handed structure on top of left handed structure. Thereby, the face between these two structures is free to rotate and rotation angles exceeding 2 per percent strain have been measured. Additionally, the micropolar theory predicts a breakdown of scalability due to the presence of a local length scale [3]. Intuitively, this lenght scale can be associated with the substructure of the material, like the distance between atoms or the lattice constant a of an artificial material. Therefore, the crucial experiments are probing the twist per strain and the effective Young’s modulus as a function of L∕a, with L being the sidelength of the sample.