  We study mixtures of self-propelled and passive rod-like particles in two
dimensions using Brownian dynamics simulations. The simulations demonstrate
that the two species spontaneously segregate to generate a rich array of
dynamical domain structures whose properties depend on the propulsion velocity,
density, and composition. In addition to presenting phase diagrams as a
function of the system parameters, we investigate the mechanisms driving
segregation. We show that the difference in collision frequencies between
self-propelled and passive rods provides a driving force for segregation, which
is amplified by the tendency of the self-propelled rods to swarm or cluster.
Finally, both self-propelled and passive rods exhibit giant number fluctuations
for sufficient propulsion velocities.
