  A Quincke roller is a unique active particle that can run and tumble freely
on a flat plate due to the torque generated by a uniform DC electric field
applied perpendicular to the plate. A system involving many such particles
exhibits a variety of collective dynamics, such as the disordered gas, polar
liquid, and active crystal states. We performed direct numerical simulations of
a three-dimensional system containing many self-rotating particles to
explicitly resolve the hydrodynamic interactions among rotating particles. The
collective motion depends on the magnitude of the dipole moments induced on the
dielectric particles, the area fraction of particles, and the strength of
interparticle attraction. We find that the highly ordered polar liquid state is
destabilized by the hydrodynamic interaction between rotating particles at high
densities: the near-field lubrication interaction becomes dominant over
far-field effects as the interparticle separation becomes shorter.
