Field-induced transient periodic structures in nematic liquid crystals : the splay Frederiks transition

2014 A beautiful striped texture is often observed as the initial response of a uniformly aligned nematic liquid crystal to a suddenly applied reorienting field The structure of the instability depends on the elastic and viscous anisotropy of the liquid crystal, the field strength, and in an important way on the boundary conditions imposed by the sample cell. We have studied the geometry in which a magnetic field is applied normal to a parallel-plate cell containing a planar-aligned liquid crystal composed of suspended tobacco mosaic virus particles. In this geometry, parallel striped domains develop in two directions with a field-dependent wavelength and angle relative to the initial director. The initial distortion appears as a cross-hatched pattern of intersecting sets of parallel lines. We have analysed the pattern by a linear hydrodynamic stability analysis in two ways : First, a general analysis of the equations of motion is presented which describes the angle and wavelength with reasonable values for the material parameters and assuming free boundary conditions. Second, a simple theory based on energetics for the limit of very long molecules is given, which reproduces the essential features of the tobacco mosaic virus samples. Finally the effects of rigid boundaries are investigated for a simplified case and are found not to alter the conclusions drawn for free boundaries. J. Physique 46 (1985) 905-917 JUIN 1985,