Splay and bend disclinations in multidomain vertical-alignment liquid-crystal cells.

We investigated the domain boundary formations in multidomain vertical-alignment nematic liquid-crystal (VA LC) cells. There are two types of domain boundaries in multidomain VA cells, splay and bend type domains, the boundaries of which are dominated by the splay and bend deformations, respectively. Each type could be achieved by either the single-4-domain pair or the 2-2-domain pair of two substrates and have four different domain arrangements. We demonstrated that the disclination width of the splay type domain is about 50% wider than that of the bend type domain even when the splay and bend elastic constants are the same. The difference is so large that it cannot be compensated for by adjusting the elastic constants of the LCs within realistic material parameter ranges. The mechanism of the phenomenon was investigated by disassembling the elastic deformation strains into each component and analyzing them individually. The large difference was revealed to arise from the different twist deformation strains near the surface, which is significantly larger in the bend type than in the splay type. We also suggest an asymmetric pretilt structure, which dramatically reduces the domain boundary width. However, simple adoption of a large pretilt angle is not effective for reducing the width. The experimental results obtained in real LC display panels qualitatively agree well with the simulation results.