Generalized Thermodynamics of Phase Equilibria in Scalar Active Matter

TL;DR

This paper develops a generalized thermodynamic framework for understanding phase separation in scalar active matter, providing a unified description of motility-induced phase separation (MIPS) through an effective free energy approach.

Contribution

It introduces a continuum model based on a generalized Cahn-Hilliard equation to predict phase coexistence in scalar active matter systems.

Findings

Quantitative agreement with phase diagrams of models with density-dependent propulsion.

Unified thermodynamic description of MIPS in different active matter models.

Provides a general prescription for phase coexistence in flux-free steady states.

Abstract

Motility-induced phase separation (MIPS) arises generically in fluids of self-propelled particles when interactions lead to a kinetic slowdown at high densities. Starting from a continuum description of scalar active matter, akin to a generalized Cahn-Hilliard equation, we give a general prescription for the mean densities of coexisting phases in flux-free steady states that amounts, at a hydrodynamics scale, to extremizing an effective free energy. We illustrate our approach on two well-known models: self-propelled particles interacting either through a density-dependent propulsion speed or via direct pairwise forces. Our theory accounts quantitatively for their phase diagrams, providing a unified description of MIPS.