Flexoelectricity in cholesterics

For weak anchoring and 0 by analogy with a nematic (see Eq. 10.77), the free energy of the distortion includes the elastic term due to the bend-distortion (we assume K = Kn = K s) and the flexoelectric term with an average coefficient e. The second elastic term is due to the cholesteric helical structure (modulus K22).  

The rise and decay of the flexoelectric distortion is controlled by periodicity of the helix. 

Therefore, for short pitch cholesterics with pitch about 0.3 pm, the characteristic time of the director switching is short, x 100 ps (yi 1 P or 0.1 Pa s in SI, qa = InlPo 2 x 10 cm or 2 x 10 m /sT22 3 x 10 dyn or 3 x 10 N). Indeed, experiments show that the effect of the realignment of the helical axis is less than 100 ps, and the speed of the response is independent of the field strength. 

The theory of liquid crystals. Trans. Faraday Soc. 29, 883-899 (1933) 

de Gennes, P.G., Prost, J. The Physics of Liquid Crystals, 2nd edn. Oxford Science Publications, Oxford (1995) 

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