Electrohydrodynamic Instabilities in Cholesteric Liquid Crystals with Negative Dielectric Anisotropy

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Abstract
Electrohydrodynamic instabilities have been investigated in cholesteric liquid crystals with negative dielectric anisotropy. We have used a room-temperature mixture of nematic p-methoxy benzilidene p-n butyl aniline (MBBA) and cholesteryl nonanoate(CN). The ac electric field is applied parallel to the helical axis of the planar texture. The onset of a two-dimensional periodic pattern is optically observed at threshold. According to the excitation frequency, two regimes must be distinguished, namely a low frequency (conduction) regime and a high frequency (dielectric) regime. For both regimes, we give a full experimental analysis of the threshold voltage and of the period of the instability as a function of the various parameters (frequency, sample conductivity, cholesteric pitch, sample thickness). The dependence of the threshold conditions vs. frequency can be quantitatively accounted for by Hurault's theory in the low frequency regime. As far as the high frequency regime is concerned, no theory is available. It appears nevertheless that part of our results can be interpreted along the lines of the model developed by Dubois-Violette et al. for nematics. Finally, the behavior above threshold is briefly described.