Control of alignment for liquid crystal on inhomogeneous substrate

In this thesis work, we first investigated the origin of an orientation transition found in nematic LC deposited on microtextured substrates comprising domains of parallel grooves alternating between the x and y directions, in which the LC alignment underwent an abrupt transition from being inhomogeneous planar to more uniform possessing a large pretilt angle of 40° when the pattern period was decreased to ~1 μm. The LC alignment near transition homogenizes towards either the +45° or the -45° azimuthal directions in order to relax the elastic strain from copying the alternating groove orientations, and in turn the ±45° azimuthal alignment states, by being misaligned from the local grooves, possess a large surface energy and may cause the LC director to tilt out of plane. Based on this mechanism, we successfully devised controls on the pattern parameters that allowed us to easily manipulate the LC pre-tilt angle. Then we developed the photoalignment method to make the inhomogeneous alignment patterns with sizes in the centimeter scale. We measured their azimuthal anchoring energies to be at least 80% smaller than those of similarly prepared uniform surfaces. The anchoring of LC on a uniformly photoaligned surface is due to chemo-physio interactions between the LC molecules and the alignment surface. But that on an inhomogeneous patterned surface probably has an elastic origin, which may explain why a different anchoring energy is demonstrated. Finally, we explored applications of the inhomogeneous alignment patterns in display devices. Based on the orientation degeneracy of the ±45° azimuthal alignment states on these patterns, we devised the protocol of a bistable twist nematic LC cell and successfully constructed bistable twist nematic cells in which the two different twist states can be switched back and forth by backflow dynamics induced by out-of-plane electric pulses of 20 - 30 V