Role of kinematic constraints in the time reversal symmetry breaking of a model active matter

TL;DR

This paper introduces a new, experimentally accessible measure of time-reversal symmetry breaking in active matter systems, using collision event statistics in a novel robotic setup, revealing insights into non-equilibrium energetics and emergent collective behavior.

Contribution

The study presents a robust measure of BTRS based on Kullback-Leibler divergence, linking dissipation, energetics, and kinematic constraints in active matter.

Findings

Quantitative measure of BTRS using collision event divergence

Demonstration of a universal fluctuation symmetry in active energetics

Observation of activity-induced herding phenomena

Abstract

Active-matter systems are inherently out-of-equilibrium and perform mechanical work by utilizing their internal energy sources. Breakdown of time-reversal symmetry (BTRS) is a hallmark of such dissipative non-equilibrium dynamics. We introduce a robust, experimentally accessible, noninvasive, quantitative measure of BTRS in terms of the Kullback-Leibler divergence in collision events, demonstrated in our novel artificial active matter, comprised of battery-powered spherical rolling robots whose energetics in different modes of motion can be measured with high precision. Our dimensionless measure characterizes how dissipation and internal energetics are influenced by kinematic constraints from interactions with the environment. We propose this measure of BTRS as an empirical estimate of the distance from equilibrium. An energetic insight into this departure of active matter from equilibrium comes from our demonstration of a non-trivial fluctuation symmetry, which reveals a potentially universal thermodynamic characteristic of active energetics. As a many-body consequence of BTRS in our experimental active system, we demonstrate the emergence of activity-induced herding, which has no equilibrium analogue.