We report on experiments in nematics with homeotropic alignment where the electrohydrodynamic instability occurs after a bend Freedericksz transition. The threshold of convection, the wave number at onset, and the threshold for the transition between order and disorder in the presence of a stabilizing magnetic field agree with theory. Whereas in the oblique-roll range the behavior of the correlation time of the disordered state exhibits near onset the expected scaling behavior, the disordered state in the normal-roll range at zero magnetic field shows no dynamics below a well-defined value s of the reduced control parameter, in contrast to theoretical predictions. At s one has reproducibly a continuous and reversible transition to a dynamic state. We attribute these features to the appearance of chevronlike structures.

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