ANOMALOUS RELAXATION OF A TWISTED CELL UNDER AN EXTERNAL MAGNETIC-FIELD - THE NEARLY HOMOGENEOUS RELAXATION

We report experimental and theoretical investigations of the dynamic b ehaviour of a pi/2 twisted NLC layer in a magnetic field. When a magne tic field H is applied in the layer plane at a suitable angle beta wit h respect to the easy axis on the first surface, the relaxation toward s the equilibrium texture occurs through a slow decay of unstable text ures. Depending on the values of H and beta, the relaxation of the sys tem 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 tau(W) of th e unstable textures is found to depend strongly on the angle beta and on the amplitude of the magnetic field. The experimental dependence of tau(W) on H and beta is found to be in good agreement with the theore tical predictions. The relaxation process is extremely sensitive to sm all dishomogeneities of the director easy alignment on the surfaces. F rom the measured relaxation we are able to estimate a spread of 0.3 de grees on the surface easy axes at a planar anchored SiO surface.