(316i) Coarsening Dynamics in Binary Liquids Under Active Rotation

Active materials comprised of many self-propelled units (e.g., colloidal swimmers) exhibit emergent collective behaviors such as clustering, swarming, and segregating that depend on the nature of the local energy input and the interactions between the individual units.   In a recent simulation study [Phys. Rev. Lett. 112, 075701, (2014)], it was shown that binary mixtures of actively rotating particles phase separate in 2-dimensions to form domains of like rotating particles.  The size ℓ of the actively rotating domains were found to grow in time as ℓ ~ t^1/3 in agreement with classic models of liquid-liquid unmixing.  Other more exotic types of coarsening dynamics are anticipated in systems driven further from equilibrium.  Here, we develop a continuum model of phase separation in actively rotating liquids and systematically explore the relative importance of active rotation, frictional damping, and viscous coupling on the system’s macroscopic dynamics.  The continuum model combines the convective Cahn-Hilliard equation governing the local composition and the Stokes equation with active rotation governing the velocity field.  In addition to reproducing previous results (“weak” rotation regime), this model predicts several new types of coarsening dynamics in the “strong” rotation regime as well as the formation of dissipative structures that move about autonomously within an otherwise homogeneous system.  Numerical results are reproduced and explained by scaling arguments that outline the different dynamical regimes and elucidate the diverse behaviors therein.