Recently it has become clear, however, that bulk phase separation in active systems generically displays strongly non-equilibrium features. In experiments, phase separation is often arrested to a finite length-scale (micro-phase separation). Bubbly phase separation, the coexistence between a seemingly boiling liquid and vapor, or active foam states, are also often observed.
I will describe our recent efforts to describe phase separation in active matter and, more generally, in systems where detailed balance is broken locally. Most of these results will be rationalized via field-theoretical analysis, extending Model B to include the leading-order terms that break detailed balance (Active Model B+). Particle models simulations further strengthen the figure. If time allows, I will conclude the talk by discussing phase separation in systems undergoing collective motion and hydrodynamic interactions.