The Mechanical Theory of Nonequilibrium Coexistence

Nonequilibrium phase transitions are routinely observed in both natural and synthetic systems. The ubiquity of these transitions highlights the conspicuous absence of a general theory of phase coexistence that is broadly applicable to both nonequilibrium and equilibrium systems. In this talk, we present a general mechanical theory for phase separation rooted in ideas explored nearly a half-century ago in the study of inhomogeneous fluids. The core idea is that the mechanical forces within the interface separating two coexisting phases uniquely determine coexistence criteria, regardless of whether a system is in equilibrium or not. We demonstrate the power and utility of this theory by applying it to active Brownian particles, predicting, from first principles, a quantitative phase diagram for motility-induced phase separation in both two and three dimensions. Finally, we extend and apply this mechanical perspective of phase coexistence to nonequilibrium order-disorder transitions and systems comprised of particles with nonreciprocal interactions.