Ising-Bloch transition in a nematic liquid crystal

Abstract

We consider a nematic slab with homeotropic anchoring and investigate the effect of a magnetic field, H, parallel to the plates and an electric field, E, perpendicular to them $(\epsilon_{\rm a} < 0,~\chi_{\rm a} > 0)$. The magnetic field is either fixed, or rotating at a frequency ω around an axis perpendicular to the plate. The main effect consists in an Ising-Bloch transition of the walls observed either in the ($H^2,~E^2$) or ($H^2,~\omega$) parameter space. The latter case corresponds to the experimental observation recently made at Brandeis University [1]. Some of these results are understood within the framework of a Ginzburg-Landau model, directly derived from the Oseen-Zocher-Frank expression for nematic elasticity, which accounts for the main static and dynamic experimental phenomena observed. The spiral shaped textures obtained in the dynamic case probably represent one of the first simple physical analogs of the chemical and biological phenomena observed in excitable media