Auxetic granular metamaterials

Granular materials are materials that consist of many compartments. The collective behaviour of these smaller grains is typically very different from the behaviour of the individual grains. A clear sample is sand. The individual sand grains are solid. On the beach the grains support weight like a solid. However, when pushing the sand aside, the grains resemble a liquid. This liquid-solid transition always occurs in granular materials, and it is largely dependent on the density of the grains. Confined grains are typically very hard to move. For moving a single grain, the surrounding grains need to facilitate this movement. Even when grains are elastic, this is challenging. Most materials expand once they are compressed. Pushing on a granular material thus causes the grains to expand laterally. This will create a strong force network that can only lead to a granular flow with sufficiently high stresses.
In this thesis, we made auxetic grains –- the grains shrink in all directions under confinement. The design is predominantly geometrical, meaning that the grains are metamaterials. Once a force acts on an auxetic granular metamaterial, the grains shrink in all directions. Therefore, the density does not increase as quickly as in a regular granular material. This strongly affects the liquid-solid transition. We show in experiments and numerical models that auxetic grains flow better than regular grains. As the grains allow for large confinements, the total energy absorption is very high. Therefore, the auxetic grains may have great perspective in shock absorption applications.