Changing granular flows and impact dynamics with negative Poisson's ratio grains

Granular materials are known to have the unique property of being able to act both as a solid when jammed and as a liquid when unjammed. Due to dilation, it is generally difficult to unjam a jammed packing by applying simple shear. Here, we introduce a negative Poisson's ratio to all the individual grains. A packing of auxetic grains is observed to locally generate some freedom to move and reduce the occurrence of force chains upon compaction due to their radial size reduction. This size reduction leads to a sharp decrease in yield stress when sheared and distinctive unclogging behaviour when obstructed in a hopper configuration. When impacted by a circular object, part of the kinetic energy is shown to be absorbed and returned elastically, causing the packing to heal inflicted damage.