Orientational transition in nematic liquid crystals under oscillatory Poiseuille flow

We investigate the orientational behaviour of a homeotropically aligned nem atic liquid crystal subjected to an oscillatory plane Poiseuille flow produ ced by an alternating pressure gradient. For small pressure amplitudes the director oscillates within the flow plane around the initial homeotropic po sition, whereas for higher amplitudes a spatially homogeneous transition to out-of-plane director motion was observed for the first time. The orientat ional transition was found to be supercritical and the measured frequency d ependence of the critical pressure amplitude in the range between 2 and 20 Hz was in quantitative agreement with a recent theory.