Unwinding of The Helix in Thin Planar Cholesteric Layers

Abstract

Unwinding of a helix in thin planar cholesteric layers induced by the varying external parameters (temperature, electric or magnetic field) and its dependence on the surface anchoring are theoretically investigated for the defectless mechanism of the cholesteric pitch changing connected with a slipping of the director on the surface through the anchoring potential barrier. It is shown that the critical field of the helix unwinding Ec in a thin layer may be as essentially lower than the critical field in a bulk cholesteric so larger than this field and the unwinding process in a layer is a jump-like one contrary to the case of bulk cholesteric. The changes of Ec in a layer relative to the bulk critical field (growth or lowering) are dependent, in particular, on the initial (i.e. at zero field) director distribution in the layer. The expressions connecting Ec (the field of the pitch jump) with the cholesteric layer parameters, anchoring potential and the initial director distribution in the layer are found. The hysteresis of Ec and the corresponding bistability of the cholesteric layer for the opposite directions of the field changes are investigated. In particular, it is shown that for some range of the relevant to the problem parameter values the helix unwound by the field remains to be unwound after removing of the field. The analytically revealed qualitative features of the unwinding are illustrated by computations for the specific values of the cholesteric layer parameters.