Additivity, density fluctuations, and nonequilibrium thermodynamics for active Brownian particles

TL;DR

This paper develops a thermodynamic framework based on additivity to analyze particle-number fluctuations in active Brownian particles, successfully describing phase transitions and distribution properties in nonequilibrium conditions.

Contribution

It introduces a fluctuation-response based thermodynamic theory for ABPs, linking additivity to nonequilibrium phase behavior and particle-number fluctuations.

Findings

Analytical particle-number distributions match simulations.

Thermodynamic theory captures phase transition features.

Additivity underpins nonequilibrium fluctuation analysis.

Abstract

Using an additivity property, we study particle-number fluctuations in a system of interacting self-propelled particles, called active Brownian particles (ABPs), which consists of repulsive disks with random self-propulsion velocities. From a fluctuation-response relation - a direct consequence of additivity, we formulate a thermodynamic theory which captures the previously observed features of nonequilibrium phase transition in the ABPs from a homogeneous fluid phase to an inhomogeneous phase of coexisting gas and liquid. We substantiate the predictions of additivity by analytically calculating the subsystem particle-number distributions in the homogeneous fluid phase away from criticality where analytically obtained distributions are compatible with simulations in the ABPs.