Non-negative Interfacial Tension in Phase-Separated Active Brownian Particles

TL;DR

This paper develops a microscopic theory for the interfacial tension in active Brownian particles, showing it is non-negative and explaining the stability of gas-liquid interfaces in nonequilibrium conditions.

Contribution

It introduces a general square gradient theory for nonequilibrium interfacial tension in active particles, providing a theoretical basis for observed interface stability.

Findings

Interfacial tension $ ext{γ}_{ ext{gl}}$ is non-negative.

The theory explains the stability of gas-liquid interfaces.

Supports previous simulation observations.

Abstract

We present a microscopic theory for the nonequilibrium interfacial tension $\gamma_{\rm gl}$ of the free interface between gas and liquid phases of active Brownian particles. The underlying square gradient treatment and the splitting of the force balance in flow and structural contributions is general and applies to inhomogeneous nonequilibrium steady states. We find $\gamma_{\rm gl}\geq 0$, which opposes claims by Bialk\'e et al. [Phys. Rev. Lett. $\bf{115}$, 098301 (2015)] and delivers the theoretical justification for the widely observed interfacial stability in active Brownian dynamics many-body simulations.