Cage Breaking Far from Equilibrium

TL;DR

This study investigates how activity in self-propelling particles alters the caging environment in dense matter, revealing a link between microscopic scales and enhanced dynamics, and demonstrating broken detailed balance in active systems.

Contribution

The paper introduces a minimal active-matter model to analyze cage breaking, showing how activity reshapes the entropic landscape and breaks detailed balance, advancing understanding of nonequilibrium dense matter.

Findings

Cage-breaking landscape becomes metastable at high activity.

Maximum cage-breaking speed occurs when persistence length matches particle radius.

Detailed balance is broken in both landscape dynamics and basin transitions.

Abstract

Active matter can flow and yield under conditions where passive matter jams and slows down, as self-propulsion significantly modulates particle escape from local cages. How activity microscopically reshapes the caging environment to produce this effect, however, remains poorly understood. Here we study a minimal active-matter model of cage breaking: three distinguishable self-propelling disks under circular confinement. This simple setting allows us to construct an entropic landscape for rearrangements and to compare it exactly with its equilibrium counterpart. At low activity the landscape is effectively bistable, whereas at high activity it develops additional metastable basins associated with frustrated clusters at the boundary. We quantify the system's departure from equilibrium and show that cage breaking is fastest when the persistence length matches the particle radius, linking a geometric microscopic scale to the enhanced dynamics of active glasses. Extending the landscape to two dimensions reveals circulating probability currents, and a Markov-state description shows that detailed balance is broken both in the continuous landscape dynamics and in the coarse-grained transitions between entropic basins. Our results provide a minimal microscopic framework for understanding how activity reshapes caging, relaxation, and irreversibility in dense nonequilibrium matter.