Disordered packings like sand piles and metallic glasses have arrangements of their constituent particles that appear very similar to those of a liquid. It is a very hard and long-standing problem to be able “look” at the particle arrangement and tell if the the system is rigid, and where flow will initiate if the system is deformed.
One key concept has been that of “free volume” — the idea that regions that are underpacked are more vulnerable to rearrangement than overpacked ones. However, it has been difficult to identify underpacked regions from experimental images without introducing thresholds and in a way that is valid for systems with interactions that are not either infinitely-repulsive or zero.
We made major progress on this issue in Ref.[1] by devising a new tool based on the pink arrows shown in the accompanying figure. These identify the under-packed weak regions, without the usual need to calibrate a threshold. We also showed how the tool can be used to structurally identify the transition from liquid-like to solid-like behavior that occurs with increasing particle concentration.
