Interface dynamics of active/passive mixtures

Understanding the interfacial properties of phase separating mixtures of active and passive fluids is relevant to both biological processes and to the design of new functional materials. Recent experiments on mixtures of active microtubule-based liquid crystals and passive fluids have shown that activity can arrest and suppress phase separation, drive propagating surface waves, and strongly enhance interfacial fluctuations, resulting in new nonequilibrium scaling forms of the height fluctuation spectrum. We present analytical work on the behavior of the interface between an active nematic and a passive fluid. Starting from a continuum model of the mixture formulated in terms of the nematic concentration, the active liquid crystalline order parameter and flow, we derive analytically a reduced description of the interface dynamics in terms of coupled equations for the interface height and the nematic director field at the interface. We show that the linearized form of the equations captures activity-driven instabilities and propagating waves, leading to microphase separated states with droplet size distribution and growth laws observed experimentally and numerically. The nonlinear terms in the equations might shed light on the nonequilibrium properties of the height correlation spectrum.