Michael Cates– University of Cambridge
Wednesday, March 5th , 10:45AM EST
Multiple interfacial tensions in active phase separation
In thermal equilibrium, the interfacial tension between coexisting phases is a free energy derivative with respect to the interfacial area. Consistent with thermodynamics, many different measurements or definitions of the tension (via, say, the capillary wave spectrum or the Laplace pressure at a curved interface) all give the same answer. There is no such consistency for interfaces in active systems. Indeed a minimal scalar field theory of active phase separation (Active Model B+) gives at least three distinct tensions, some of which can be negative without the interface losing stability. The various tensions have interesting consequences for the phase diagram and also for nucleation, where (luckily) the quasipotential is calculable at the level of classical nucleation theory. Using that quasipotential, I will attempt to argue that AMB+'s mismatch in tension between liquid-in-vapor and vapor-in-liquid droplets shows its stationary measure to be nonlocal -- not just in certain phases or parameter regimes, but in general. This augurs against recent proposals to construct effective field theories for active systems via a Landau Ginzburg expansion within the stationary measure, rather than within the equations of motion.