Isotropic Auxetic Metamaterials from Disordered Networks*

Recent work indicates that selective pruning of disordered networks consisting of nodes connected by bonds can generate materials with non-trivial mechanical properties, including auxetic networks having a negative Poisson's ratio. Until now, auxetic networks created based on this strategy have not been successfully realized in experiment. Here a new model that includes angle-bending forces and the experimental boundary conditions is introduced for pruning-based design of auxetic materials. By pruning the appropriate bonds, the Poisson's ratio can be tuned to values approaching the lower mechanical limit of -1, and the corresponding laboratory networks exhibit good agreement with model predictions. While pruning algorithms allow for the creation of highly auxetic materials with an anisotropic Poisson's ratio, the auxetic effect is limited in the isotropic case. To address this, we employ optimization algorithms which selectively modify the moduli of individual bonds. Using such procedures, we are able to create isotropic highly auxetic networks in both simulation and experiment.