Theory of Nonequilibrium Coexistence with Coupled Conserved and Nonconserved Order Parameters

TL;DR

This paper develops a dynamical framework for understanding phase coexistence in nonequilibrium systems with coupled conserved and nonconserved order parameters, extending thermodynamic concepts to active and driven systems.

Contribution

It introduces a generalized theory of coexistence criteria for nonequilibrium phases, including a numerical verification using the Active Model C+.

Findings

Generalizes thermodynamic notions like chemical potential to nonequilibrium systems.

Numerically verifies coexistence criteria in active matter models.

Provides a foundation for high-dimensional nonequilibrium phase diagrams.

Abstract

Phase separation routinely occurs in both living and synthetic systems. These phases are often complex and distinguished by features including crystallinity, nematic order, and a host of other nonconserved order parameters. For systems at equilibrium, the phase boundaries that characterize these transitions can be straightforwardly determined through the framework of thermodynamics. The prevalence of phase separation in active and driven systems motivates the need for a genuinely nonequilibrium theory for the coexistence of complex phases. Here, we develop a dynamical theory of coexistence when both conserved and nonconserved order parameters are present, casting coexistence criteria into the familiar form of equality of state functions. Our theory generalizes thermodynamic notions such as the chemical potential and Gibbs-Duhem relation to systems out of equilibrium. While these notions may not exist for all nonequilibrium systems, we numerically verify their existence for a variety of systems by introducing the phenomenological Active Model C+. We hope our work aids in the development of a comprehensive theory of high-dimensional nonequilibrium phase diagrams.