Walls in Nematics

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Abstract
At the Freedericks transition of a nematic slab, the molecules can rotate by two different ways, giving rise to adjacent domains separated by a wall. We present observations on the structure and dynamics of these walls at magnetic fields above the transition. The local distortion through the wall agrees with theoretical prediction. The ellipticity of a wall surrounding a closed domain allows to determine some elastic constant ratios. If the magnetic field is decreased to the Freedericks critical field, the thickness and the velocity of the walls are observed to diverge respectively as (H - Hc )−1/2 and (H - Hc )−1. From the velocity of the walls we can deduce the twist viscosity γ1 in planar geometry, and a renormalized viscosity γ including backflow corrections in homeotropic geometry.