Towards a 'Thermodynamics' of Active Matter

TL;DR

This paper develops a thermodynamics-inspired theoretical framework for active matter, enabling prediction of phase separation and interpretation of nonequilibrium behaviors using generalized thermodynamic concepts.

Contribution

It introduces a simple theory based on swim pressure to predict phase behavior in active matter, extending thermodynamic ideas to nonequilibrium systems.

Findings

The phase diagram includes a spinodal and critical point.

The theory quantitatively matches simulation data.

A nonequilibrium chemical potential is defined for active systems.

Abstract

Self-propulsion allows living systems to display unusual collective behavior. Unlike passive systems in thermal equilibrium, active matter systems are not constrained by conventional thermodynamic laws. A question arises however as to what extent, if any, can concepts from classical thermodynamics be applied to nonequilibrium systems like active matter. Here we use the new swim pressure perspective to develop a simple theory for predicting phase separation in active matter. Using purely mechanical arguments we generate a phase diagram with a spinodal and critical point, and define a nonequilibrium chemical potential to interpret the "binodal." We provide a generalization of thermodynamic concepts like the free energy and temperature for nonequilibrium active systems. Our theory agrees with existing simulation data both qualitatively and quantitatively and may provide a framework for understanding and predicting the behavior of nonequilibrium active systems.