Polar-horizontal versus polar-vertical reverse-tilt-domain walls: Influence of a pretilt angle below the nematic-isotropic phase transition

On cooling through the isotropic-to-nematic phase transition in a cell whose substrates induce a large pretilt angle θ0 from the vertical direction, but with no preferential azimuthal orientation, tilt domains appear. The boundary walls between reverse tilt domains are found to be bendlike and twistlike when θ0 (T= TNI) is sufficiently large just below the isotropic-nematic phase transition temperature TNI -i.e., for a nearly planar orientation. Here the director becomes planar approximately midway through the wall, and we refer to this type of wall as "polar horizontal," which is topologically stable. However, if θ0 (T= TNI) is sufficiently small just below TNI -i.e., closer to vertical orientation-a splay like and twistlike domain wall obtains, where the director is vertically oriented approximately midway through the wall; we refer to this type of wall as "polar vertical," whose stability depends on the anchoring. On cooling through the nematic phase, the pretilt angle θ0 decreases, with the director aligning closer to the vertical orientation. Nevertheless, the structures of both types of domain walls remain unchanged on variation of θ0 with temperature owing to topological constraints and also are unchanged after the application and removal of a large electric field. We examine the structure of domain walls for the liquid crystal ZLI-4330 (Merck) as a function of pretilt angle θ0 (T= TNI) and calculate a critical value θ0c (T= TNI) of the pretilt angle just below TNI for which the predominance of domain walls crosses over from polar horizontal to polar vertical.