Nonequilibrium pathways between cluster morphologies in active phase separation: Necking, rupture, and cavitation

We investigate the dynamical pathways of a morphological transition in a two-dimensional active lattice gas undergoing motility-induced phase separation. The transition is between two locally stable morphologies of the liquid cluster: a system-spanning "slab" and a compact "droplet." We generate trajectories of this transition in both directions using forward flux sampling. We find that the droplet-to-slab transition always follows a similar mechanism to its equilibrium counterpart, but the reverse (slab-to-droplet) transition depends on rare nonequilibrium fluctuations. At low Péclet numbers the equilibrium and nonequilibrium pathways compete, while at high Péclet numbers the equilibrium pathway is entirely suppressed, and the only allowed mechanism involves a large vapor bubble. We discuss the implications of these findings for active matter systems more generally.