Active waves in liquid-liquid phase separation

Phase separation of liquids, as illustrated by everyday oil and water mixtures, provides the basis for creating diverse soft materials, from lipid vesicles to emulsions. These states of matter are typically formed upon the addition of surface adsorbing agents that stabilize liquid interfaces. I will show that active matter provides a new way for controlling phase separation. Active fluids contain microscopic energy consuming objects that drive far-from-equilibrium large-scale flows. In phase separating mixtures of an active isotropic fluid and a passive fluid, bulk active flows produce interfacial waves whose amplitude is controlled by activity. Motility of the phase separated domains leads to finite-size steady states, where coalescence of smaller droplets is balanced by the break-up of larger ones. When interfaces meet a solid boundary, active waves also drive non-equilibrium wetting transitions. These results demonstrate the promise of mechanically-driven interfaces for creating a new class of soft active matter.