Interfacial tension of motility-induced phases*

A striking feature of motility-induced phase separation (MIPS) is the apparent negative interfacial tension that arises during spinodal decomposition. In equilibrium, a negative interfacial tension indicates an ever-expanding surface and dividing volumes, but active Brownian particles (ABPs) with purely repulsive interactions are different. Using ABP simulations, we study interface height fluctuations for different system sizes and persistence lengths. We find that the statistics of this interface are surprisingly equilibrium-like, and that interface rigidity increases with motility persistence. Using local frames of reference, we study particle orientations relative to the normal and find that this negative value of interfacial tension is due to strong tangential currents, which seem to help stabilize the interface.