Influence of ions on stability of planar and homeotropic structures in hybrid aligned flexoelectric nematic layers subjected to electric field

Abstract

Weakly anchored hybrid aligned flexoelectric nematic layers subjected to a dc external electric field can adopt not only a hybrid distorted structure but also a planar or homeotropic one depending on thickness of the layer, d, applied voltage, U, anchoring strengths on both boundary surfaces, W ₁ and W ₂, as well as on dielectric anisotropy, Δε, and flexoelectric properties determined by the sum of the flexoelectric coefficients, e = e ₁₁+e ₃₃, of the nematic material. Recently, this problem was treated theoretically for the case of a perfectly insulating flexoelectric nematic. The formulae describing the boundaries of regions of existence of planar and homeotropic structures in the control parameters plane U,d were proposed to be used for determination of the sum of flexoelectric coefficients. In this paper, the problem is studied numerically for the more realistic case of a nematic containing ions. The regions of existence of the planar and homeotropic structures are determined for a well‐purified as well as for typically contaminated nematics. The results can be described by the theoretical formulae derived for insulating nematic with sufficient accuracy only when the ion concentration does not exceed 5×10¹⁸ m⁻³. Methods proposed for the measurement of flexoelectric coefficients are critically analysed.

Keywords:

• nematic liquid crystals
• flexoelectric
• hybrid alignment
• ionic conductivity

Notes

Note that the factor 1/2 was erroneously omitted in equation (51) of Derfel 13.