  We present results of molecular dynamics simulations for two-dimensional
repulsively interacting colloids driven by a one dimensional asymmetric and
commensurate ratchet potential, switching on and off stochastically. This
drives a time-averaged directed current of colloids, exhibiting resonance with
change in ratcheting frequency, where the resonance frequency itself depends
non-monotonically on density. Using scaling arguments, we obtain analytic
results that show good agreement with numerical simulations. With increasing
ratcheting frequency, we find non-equilibrium re-entrant transitions between
solid and modulated liquid phases.
