Electrohydrodynamic Instability in Smectic-C* Liquid Crystal Films

We have studied theoretically and experimentally the effect of applying an AC field E(omega) to a free-standing chiral smectic-c* liquid crystal film. Besides the usual dielectric coupling, there is a linear coupling of the field to the spontaneous polarization which leads to unique effects. For low amplitude fields, the molecular director oscillates uniformly in space at frequency omega. Above a threshold field proportional to omega the director field breaks up into domains which grow and disappear in phase with the field. We explain these results in terms of a dynamical model describing the director field and the mobile charged impurities.