Colloidal crystal motion in electric fields

We report on the short and long time flow behaviour of colloidal crystals subjected to a homogeneous electrical square wave a.c.-field in closed cells of rectangular cross section. Local velocities are determined from Laser Doppler velocimetry, sample structure and morphology were observed by time resolved polarisation microscopy. We observe a complex time dependence of the flow behaviour. After each field reversal we find a transition from an initial parabola-like flow to a final plug-like flow. On the time scale of minutes to hours channel narrowing occurs, i.e. flow initially restricted by the cell walls becomes restricted to the cell centre by wall based stationary crystals. Within the narrowed channel, however, a qualitatively similar time dependence of flow is observed.