Abstract

Active materials, like passive ones, can undergo phase separation into regions of higher and lower particle density divided by a sharp interface. In the passive case, there are long established field theories for such phase separations which must respect the underlying time-reversal symmetry of dynamics in systems close to thermal equilibrium. In active systems, this symmetry is broken, and new terms need to be added to such models which can entirely change their behaviour. One striking example is that the Ostwald process, in which phase separation proceeds by growth of large droplets at the expense of smaller ones, can go into reverse. This happens when one of several interfacial tensions (all of which coincide in the passive case) becomes negative, and leads to incomplete phase separation exhibiting clusters or bubbles. At higher activity, a second tension, governing capillary fluctuations, can also become negative; the resulting instability leads instead to an ‘active foam’ phase. In systems where all relevant interfacial tensions remain positive, activity can still lead to major effects. Specifically, (i) the rate of droplet nucleation, which (remarkably) can be calculated via an active counterpart of Classical Nucleation Theory, depends exponentially on active parameters as well as passive ones; and (ii) activity changes the universality class and scaling exponents for dynamical roughening of an interface between phases via buildup of capillary waves.