Auxetic Granular Metamaterials

TL;DR

This paper demonstrates that granular metamaterials with negative Poisson's ratio exhibit easier yielding and smoother flow, especially in complex confined geometries, due to their high compressibility and altered elastic properties.

Contribution

It introduces auxetic granular particles with negative Poisson's ratio and shows their impact on flow and yield behavior in granular materials, bridging complex fluids and metamaterials.

Findings

Auxetic grains exhibit negative Poisson's ratio regardless of compression direction.

Granular metamaterials have lower shear modulus and yield stress than ordinary grains.

Flow is enhanced in complex geometries with auxetic grains, even in small fractions.

Abstract

The flowing, jamming and avalanche behavior of granular materials is satisfyingly universal and vexingly hard to tune: a granular flow is typically intermittent and will irremediably jam if too confined. Here, we show that granular metamaterials made from particles with a negative Poisson's ratio yield more easily and flow more smoothly than ordinary granular materials. We first create a collection of auxetic grains based on a re-entrant mechanism and show that each grain exhibits a negative Poisson's ratio regardless of the direction of compression. Interestingly, we find that the elastic and yielding properties are governed by the high compressibility of granular metamaterials: at a given confinement they exhibit lower shear modulus, lower yield stress and more frequent, smaller avalanches than materials made from ordinary grains. We further demonstrate that granular metamaterials promote flow in more complex confined geometries, such as intruder and hopper geometries, even when the packing contains only a fraction of auxetic grains. Our findings blur the boundary between complex fluids and metamaterials and could help in scenarios that involve process, transport and reconfiguration of granular materials.