On the mechanics of active interfaces

Active Brownian particles (ABPs), purely repulsive particles that can undergo liquid-gas phase separation, are among the most widely studied systems exhibiting nonequilibrium phase coexistence. The interface between coexisting active phases has been the subject of recent controversy due to measurements of a negative mechanical surface tension despite being manifestly stable. In this talk we outline a generic route to derive an interfacial equation of motion (EOM) beginning from the microscopic particle dynamics. Our explicit coarse-graining allows for the description of interfaces arbitrarily far from equilibrium. In general, the nonequiblrium interfacial EOM is KPZ-like with a non-linearity that vanishes when the fluctuation-dissipation theorem is satisfied (i.e., for systems in equilibrium). We apply this perspective to ABPs, demonstrating that the effective surface tension governing the interfacial EOM is indeed distinct from the mechanical surface tension and strictly positive. In addition, we discuss how the effective surface tension and driving coefficient scale with activity and its implications for the stability of active interfaces.