  We study a dynamical model of a rod-like particle surrounded by a cloud of
smaller particles of the same charge and we show that, in the presence of a
low-frequency alternating electric field, the rod displays the same type of
anomalous orientation (perpendicular to the field) that was recently observed
in laboratory colloids. This indicates that the anomalous orientation is due to
the collective dynamics of the colloidal particles, and does not require
electro-osmotic effects. We also confirm the experimental observation that for
higher field frequencies the standard orientation (parallel to the field)
prevails. In the simulations, these changes are abrupt enough to resemble a
phase transition.
