PATTERN-FORMATION IN A TWISTED NEMATIC LIQUID-CRYSTAL CELL UNDER AN EXTERNAL MAGNETIC-FIELD

We analyze the spatially inhomogeneous dynamic behaviour of a +pi/2 tw isted NLC layer in a magnetic field. When a magnetic field H is applie d in the layer plane at a suitable angle beta with respect to the easy axis on the first surface, the relaxation toward the equilibrium text ure occurs through a slow decay of unstable textures. For some values of beta and H, the relaxation of the unstable texture is strongly inho mogeneous and spatial domains occur everywhere. In this special case, a very rich dynamical behaviour is observed together with the appearan ce of domains corresponding to unstable, metastable, and stable textur es coupled with line defects and walls. We have studied and modelled t he form and the properties of the different domains and interfaces, th e velocities of the defect lines and the inhomogeneous-homogeneous cha racter of the unstable texture relaxation. The static and dynamic feat ures of the observed domains are in satisfactory agreement with the th eoretical predictions.