Flexoelectric deformations of homeotropic nematic layers in the presence of ionic conductivity

Abstract

Elastic deformations of homeotropic nematic liquid crystal layers subjected to a d.c. electric field were studied numerically in order to find the dependence of threshold voltage on the properties of such a system. A nematic material characterized by a negative sum of flexoelectric coefficients and by a small negative dielectric anisotropy was considered. The flow of ionic current was taken into account. The electric properties are described in terms of a weak electrolyte model. Finite surface anchoring strength was assumed. The director orientation, the electric potential and the ion concentrations were calculated as functions of the coordinate normal to the layer. It was found that the threshold for the deformation depends on the distributions of the ions, governed by the generation constant and by the properties of the electrodes. The effects observed may be interpreted as a consequence of the separation of the ions. When the electrodes have pronounced blocking character, a high and non‐uniform electric field, created by the subelectrode ion space charges, causes drastic decrease of the threshold voltage, much below the value U f valid for the insulating nematic. On the other hand, the electric field gradient arising in the bulk at moderate concentrations has a stabilizing effect and remarkably enhances the threshold above U f. When the electrodes are conducting there are no significant space charges and the threshold voltage remains close to U f. These results indicate that phenomena related to the charge transport should be taken into account in the analysis of the elastic deformations of ion‐containing flexoelectric nematics.