Hydrodynamic instabilities in nematic liquid crystals under oscillatory shear

We present the results of an experimental study of hydrodynamic instabiliti es in two classes of nematic liquid crystal material subjected to linear os cillatory shear. The materials are distinguished by their viscosity coeffic ient as, which is negative in one case and positive in the other. The insta bilities appear above a critical amplitude of the shear whose value also de pends on the applied frequency. In the material with negative as. the insta bility has the form of microscopic Williams domains, which align with the s hear and are uniformly spread throughout the sample. The principal set of r esults relates to instabilities in a material with positive alpha(3). They concern two qualitatively distinct macroscopic features that are on the len gth-scale of hundreds of la-er thicknesses. One of them consists of a unifo rm distortion of the director, and the other contains microscopic structure s in the form of Williams domains. It is proposed that an observed large-sc ale mean flow is responsible for the modulation of the latter instability.