Electrohydrodynamic instabilities in nematic liquids of positive dielectric ansiotropy

True hydrodynamic instabilities in nematic and weakly cholesteric liquids of positive dielectric anisotropy were observed under ac excitation. Static and dynamic periodic structures occur under two regimes. The low-conductance mode is characterized by an instability threshold voltage which is independent of the driving frequency and almost independent of temperature until a transition frequency is reached. Above that transition frequency, the threshold is a field, proportional to the square root of the driving frequency and varying with temperature as the square root of the viscosity. The high-conductance mode is characterized by a field threshold at all driving frequencies. The conductance mode type is determined by both the charge relaxation frequency and the electrode spacing. The existence of a cutoff frequency for the instability lends additional support to the identification of the phenomenon as a conductance mode.