Field-induced phases of an orientable charged particle in a dilute background of point charges

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.