Electrohydrodynamic Effects in Cholesteric Liquid Crystals under ac Electric Fields

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Abstract
When an electric field is applied perpendicularly to a cholesteric planar texture, periodic perturbations arise. The frequency dependence of the threshold of this instability is reported for nematic-cholesteric mixtures with both positive and negative dielectric anisotropy. In the latter case a "conduction" and a "dielectric" regime are observed, as in nematics. Our results are in agreement with a recent extension to ac excitations of Helfrich's theories for cholesterics.