Mechanical Theory of Nonequilibrium Coexistence and Motility-Induced Phase Separation

TL;DR

This paper introduces a mechanical theory for nonequilibrium phase coexistence based on interface forces, successfully predicting phase diagrams and novel interfacial phenomena in active matter systems.

Contribution

It develops a general mechanical framework for nonequilibrium phase separation, extending classical ideas to active particles and predicting new interfacial effects.

Findings

Predicted phase diagram for motility-induced phase separation.

Confirmed increasing interface width with deeper two-phase region.

Provided a unified approach for equilibrium and nonequilibrium phase coexistence.

Abstract

Nonequilibrium phase transitions are routinely observed in both natural and synthetic systems. The ubiquity of these transitions highlights the conspicuous absence of a general theory of phase coexistence that is broadly applicable to both nonequilibrium and equilibrium systems. Here, we present a general mechanical theory for phase separation rooted in ideas explored nearly a half-century ago in the study of inhomogeneous fluids. The core idea is that the mechanical forces within the interface separating two coexisting phases uniquely determine coexistence criteria, regardless of whether a system is in equilibrium or not. We demonstrate the power and utility of this theory by applying it to active Brownian particles, predicting a quantitative phase diagram for motility-induced phase separation in both two and three dimensions. This formulation additionally allows for the prediction of novel interfacial phenomena, such as an increasing interface width while moving deeper into the two-phase region, a uniquely nonequilibrium effect confirmed by computer simulations. The self-consistent determination of bulk phase behavior and interfacial phenomena offered by this mechanical perspective provide a concrete path forward towards a general theory for nonequilibrium phase transitions.