Elastic Deformations and Electrohydrodynamic Instabilities in Large Pitch Cholesteric Liquid Crystals under an Electric Field

Abstract

We report experimental observations on large pitch cholesteric liquid crystals in the planar geometry submitted to DC or low frequency AC fields applied along the helical axis. An electric field can cause orientation of the molecules in the field direction (tilting of the helical axis leading to a fingerprint texture and unwinding of the cholesteric spiral) or disruption of the orientation due to the hydrodynamic effect of current carriers (periodic two-dimensional deformations). The behavior of the samples depends upon the sign and the absolute value of the dielectric anisotropy ε_a. We have investigated a wide range of systems by using nematic matrices with ε_a between -4 and +33 doped with small amounts of cholesteryl chloride (ε_a > 0) and cholesteryl benzoate (ε_a < 0).

Instabilities are observed in negative ε_a mixtures; depending upon the frequency, two regimes can be found, as in nematics. The behavior above threshold depends largely on the magnitude of the negative anisotropy. In the case of a small positive ε_a, domain instabilities and elastic deformations occur. The nature and the amount of the cholesteric dopant affect the threshold for the square grid deformation. The response of mixtures with strong ε_a involves processes in which the orientation of the molecules by the field is the principal effect. Upon increasing the voltage, the instabilities of the nematic phase in the homeotropic geometry appear in the form of a conduction and a dielectric regime of splay. The Frank elastic constants are derived from the threshold field of the different deformations.