CRITICAL THICKNESS OF A HYBRID ALIGNED NEMATIC LIQUID-CRYSTAL CELL

We discuss the director deformation inside a hybrid aligned nematic (HAN) cell, assuming that the surface interaction is of the Berreman kind, and we suppose that the anchoring energy for homeotropic alignment is greater than the planar one. In the absence of an external electric field, we show that a critical thickness dc, below which the nematic liquid crystal (NLC) is undistorted, exists. Furthermore we investigate theoretically dc vs. the reduced electric field (E/Ec, where Ec is a threshold field depending upon the anchoring energies and the intrinsic constants of the NLC). The last analysis shows that there are two critical thicknesses dc(H) and dc(P): for d<dc(H) the sample is homeotropic (HOM),while for d>dc(P) it is planar (PLAN). Finally we point out that if the anchoring energies are the same on the upper and lower plates of the sample, the HAN-configuration is realizable only if E/Ec 1.