Non-equilibrium pathways between cluster morphologies in active phase separation: necking, rupture and cavitation

We investigate the dynamical pathways of a geometric phase transition in a two-dimensional active lattice gas undergoing motility-induced phase separation. The transition is between metastable 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 non-equilibrium fluctuations. At low Peclet numbers the equilibrium and non-equilibrium pathways compete, while at high Peclet numbers the equilibrium pathway is entirely suppressed, and the only allowed mechanism involves a large vapour bubble. We discuss the implications of these findings for active matter systems more generally.