Curvature-dependent tension and tangential flows at the interface of motility-induced phases

Purely repulsive active particles spontaneously undergo motility-induced phase separation (MIPS) into condensed and dilute phases. Remarkably, the mechanical tension measured along the interface between these phases is negative. In equilibrium, this would imply an unstable, expanding interface. However, these out-of-equilibrium systems display long-time stability and intrinsically stiff phase boundaries. In this work, we use active Brownian particle simulations and a novel frame of reference at the phase boundary to carefully study the emergent tangential currents at the interface, finding correlations between local interface curvature and measured values of interfacial tension. The combined observation of tangential currents in the gas and local “self-shearing” of the surface of the dense phase suggest a stiffening interface that redirects particles along itself to heal local fluctuations. In this way, the wildly fluctuating MIPS interface restores itself via an out-of-equilibrium Marangoni effect.