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

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.