A continuum theory of phase separation kinetics for active Brownian particles

TL;DR

This paper develops a continuum theory for phase separation in active Brownian particles, accurately capturing dynamics and structures observed in simulations, and revealing similarities to equilibrium phase separation despite non-equilibrium conditions.

Contribution

The authors derive a coarse-grained continuum model for ABP phase separation that includes density gradient effects and matches simulation results.

Findings

The continuum theory reproduces domain growth kinetics.

It accurately predicts domain topologies and coexistence densities.

Coarsening dynamics resemble equilibrium phase separation.

Abstract

Active Brownian particles (ABPs), when subject to purely repulsive interactions, are known to undergo activity-induced phase separation broadly resembling an equilibrium (attraction-induced) gas-liquid coexistence. Here we present an accurate continuum theory for the dynamics of phase-separating ABPs, derived by direct coarse-graining, capturing leading-order density gradient terms alongside an effective bulk free energy. Such gradient terms do not obey detailed balance; yet we find coarsening dynamics closely resembling that of equilibrium phase separation. Our continuum theory is numerically compared to large-scale direct simulations of ABPs and accurately accounts for domain growth kinetics, domain topologies and coexistence densities.