Anomalous relaxation of a twisted cell under an external magnetic field: The nearly homogeneous relaxation

Abstract

We report experimental and theoretical investigations of the dynamic behaviour of a π/2 twisted NLC layer in a magnetic field. When a magnetic field H is applied in the layer plane at a suitable angle β with respect to the easy axis on the first surface, the relaxation towards the equilibrium texture occurs through a slow decay of unstable textures. Depending on the values of H and β, the relaxation of the system can be nearly homogeneous or strongly inhomogeneous. In this paper we restrict our attention to the case where the relaxation occurs in a nearly homogeneous way. The theoretical relaxation time τw of the unstable textures is found to depend strongly on the angle β and on the amplitude of the magnetic field. The experimental dependence of τw on H and β is found to be in good agreement with the theoretical predictions. The relaxation process is extremely sensitive to small dishomogeneities of the director easy alignment on the surfaces. From the measured relaxation we are able to estimate a spread of 0.3° on the surface easy axes at a planar anchored SiO surface.