Searching for structural predictors of plasticity in dense active packings

TL;DR

This study investigates whether structural indicators used in passive amorphous solids can predict plasticity in dense active materials, revealing limitations due to pressure gradients and boundary effects.

Contribution

It maps an active matter system onto a granular packing with external potential and extends linear response indicators to active systems with noisy dynamics.

Findings

Linear response predicts plasticity in low-pressure regions

Pressure gradients and boundaries affect indicator effectiveness

Active packings differ from passive systems in response behavior

Abstract

In amorphous solids subject to shear or thermal excitation, so-called structural indicators have been developed that predict locations of future plasticity or particle rearrangements. An open question is whether similar tools can be used in dense active materials, but a challenge is that under most circumstances, active systems do not possess well-defined solid reference configurations. We develop a computational model for a dense active crowd attracted to a point of interest, which does permit a mechanically stable reference state in the limit of infinitely persistent motion. Previous work on a similar system suggested that the collective motion of crowds could be predicted by inverting a matrix of time-averaged two-particle correlation functions. Seeking a first-principles understanding of this result, we demonstrate that this active matter system maps directly onto a granular packing in the presence of an external potential, and extend an existing structural indicator based on linear response to predict plasticity in the presence of noisy dynamics. We find that the strong pressure gradient necessitated by the directed activity, as well as a self-generated free boundary, strongly impact the linear response of the system. In low-pressure regions the linear-response-based indicator is predictive, but it does not work well in the high-pressure interior of our active packings. Our findings motivate and inform future work that could better formulate structure-dynamics predictions in systems with strong pressure gradients.