The Jammed Phase of Infinitely Persistent Active Matter

TL;DR

This paper investigates the jammed phase of dense active matter with infinite persistence, revealing how activity influences force distributions, stability, and yielding behavior through simulations and force network analysis.

Contribution

It introduces a scaling law for the critical active force in jammed active matter and analyzes force redistribution, plasticity, and the limitations of Hessian-based predictions.

Findings

Critical active force scales with pressure as f_c ~ p^1.17.

Force distribution deviates from passive power-law at small forces.

Active systems exhibit abrupt plasticity not captured by Hessian spectrum.

Abstract

We study an extreme active matter system, which is essentially a dense assembly of athermal, soft and infinitely persistent active particles. Using extensive numerical simulations we obtain jammed configurations of this system in two dimensions and probe the stability of such structures under increasing active forcing magnitude. We show that the critical active forcing magnitude for the jammed phase to yield scales with virial pressure as $f_c\sim p^\alpha$, with $\alpha=1.17$, describing the yielding line. Using a Laplacian framework, we redistribute the active forces into a modified contact force network. By analysing the statistics of these redistributed forces, we obtain a very robust scaling law consistent with the passive limit, not just near the unjamming line, but in the entire jammed active phase. The probability distribution of the magnitude of the contact force deviates from the power-law form found in passive systems for values smaller than the active force. Moreover, within the jammed phase, the system displays elastic, plastic, and yielding events with increasing active forcing. This active plasticity appears abruptly and can not be captured by the continuous softening of the Hessian spectrum. However, we demonstrate that the Hessian still retains the ability to predict relaxation times. These results clarify how activity modifies force distributions and leads to deformation, plasticity and yielding in dense, jammed, infinitely persistent active matter.