  Active matter comprised of many self-driven units can exhibit emergent
collective behaviors such as pattern formation and phase separation in both
biologica and synthetic systems. While these behaviors are increasingly well
understood for ensembles of linearly self-propelled particles, less is known
about the collective behaviors of active rotating particles where energy input
at the particle level gives rise to rotational particle motion. A recent
simulation study revealed that active rotation can induce phase separation in
mixtures of counter-rotating particles in 2D. In contrast to that of linearly
self-propelled particles, the phase separation of counter-rotating fluids is
accompanied by steady convective flows that originate at the fluid-fluid
interface. Here, we investigate the influence of these flows on the coarsening
dynamics of actively rotating binary liquids using a phenomenological,
hydrodynamic model that combines a Cahn-Hilliard equation for the fluid
composition with a Navier-Stokes equation for the fluid velocity. The effect of
active rotation is introduced though an additional force within the
Navier-Stokes equations that arises due to gradients in the concentrations of
clockwise and counter-clockwise rotating particles. Depending on the strength
of active rotation and that of frictional interactions with the stationary
surroundings, we observe and explain new dynamical behaviors such as "active
coarsening" via self-generated flows as well as the emergence of self-propelled
vortex doublets. We confirm that many of the qualitative behaviors identified
by the continuum model can also be found in discrete, particle-based
simulations of actively rotating liquids. Our results highlight further
opportunities for achieving complex dissipative structures in active materials
subject to distributed actuation.
