Field-Induced Dielectric Instabilities of Cholesterics

The untwisting of a helix in a cell with homeotropically orienting walls coated with lecithin was indentified experimentally [15]. Complete untwisting of the helix actually occurs at thicknesses below a critical value of the order of the pitch. 

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The EHD behavior of cholesteric liquid crystals is very similar to that of nematics. When the anisotropy of the electrical conductivity is positive (cTa>0), the planar texture of a cholesteric liquid crystal in a field parallel to the helical axis is unstable for any sign of [263, 264]. The instability is caused by the torque induced by the electrical conductivity acting against the elastic torque of the cholesteric and, although the cause is different from the purely dielectric case (see Sec. 9.3.2.2 of this Chapter), the result obtained is the same that is, the appearance of a two-dimensional periodic pattern for the distribution of the director. 

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