Smart Honeycomb Mechanical Metamaterials with Tunable Poisson's Ratios

Mechanical metamaterials represent a class of deformable systems, which exhibit macroscopic deformations, mechanical, and/or thermal properties. These emerge due to the structure of their subunits rather than their materials composition and typically exhibit anomalous (normally negative) macroscopic structural, mechanical, or thermal property/properties caused by a change in shape/size of the system. Herein, a class of honeycombs is discussed, which push to the extreme the classical definition of mechanical metamaterials, exhibiting temperature-tunable Poisson's ratio properties. More specifically, centrosymmetric honeycombs with T-shaped joints constructed from different materials are shown to exhibit temperature-dependent Poisson's ratio values, which can be either positive or negative (auxetic) depending on the external stimulus the system is subjected to. The sign and magnitudes of the Poisson's ratio values are explained in terms of particular geometries that these composite honeycomb systems attain at different temperature conditions. In particular, auxeticity is attributed to the transformation of the T-shaped units to re-entrant units. Practical aspects on how these properties may be achieved are discussed, including the possibility that the changes in shape, and hence Poisson's ratios, are inducedviadifferent extents of dryness on opposite surfaces of ligaments made from absorbent materials.