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Overstability and inverted bifurcation in homeotropic nematics heated from below

In nematic homeotropic films (director n perpendicular to the horizontal limiting plates) heated from below, the distortion of the director which is coupled to the ordinary heat convection mechanism responsible for the Rayleigh–Bénard instability exerts a strongly stabilizing influence. Owing to the difference in time scales, an oscillatory instability results whose characteristics are investigated experimentally here. An inverted bifurcation with an associated hysteresis is also obtained and this was studied in some detail. A vertical magnetic field H is also used to align the sample. The decrease of the orientational time constant when H increases leads to marked changes in the overstable regime, which are well described by a simple analysis.