| In many cholesteric applications, such as cholesteric polarizers and light shutters, a defect-free planar state is highly desired. In practice, there are usually disclinations in cholesteric liquid crystals. Although the disclinations have a higher free energy, they are metastable because the energy barrier against their annihilation. We studied the evolution of defect lines (oily streaks) in cholesteric liquid crystals. We observed experimentally that the total length of the defect lines decreased with time logarithmically, though the individual defect lines shrank linearly with time. The decay constant depended on the helical pitch of the cholesteric liquid crystal and the temperature. The logarithmic temporal behavior of the total defect length was produced by statistical effect. We studied the defects and learned that the defects could be eliminated by applying external perturbations. We found that the efficient techniques were heating or applying external fields.; In some thermochromatic cholesteric liquid crystal devices, such as thermometer and temperature field detector, the temperature dependence of helical pitch is a must as it determines the color of the reflected light. The cholesteric liquid crystal used usually exhibit a Smectic A phase at a lower temperature. For practical applications, it is of great importance to study the cholesteric (ChLC) to smectic A (SA) phase transition as the variation in pitch with temperature is profoundly affected by the fluctuations of Smectic A order in the cholesteric phase. On the other hand, for fundamental science, the ChLC to SA phase transition is analogous to the superconducting to normal transition in metals. We investigated the change of the helical pitch P and the twist elastic constant K 22 with temperature in the cholesteric phase. We observed that as the temperature was lowered toward the ChLC-SA transition, both the pitch and the elastic constant increased dramatically due to the formation of short range SA order in the cholesteric phase. Our results showed that K22/P 2 was approximately a constant independent of temperature.; Two cholesteric dichroic dye displays were developed using polymer stabilization. In the first display, with dispersed polymer networks (cured in the homeotropic texture) perpendicular to cell surface, the hysteresis was enhanced in the focal conic-homeotropic transition; the material was bistable under a bias voltage in the hysteresis loop. In the second display, with random polymer network (cured in the isotropic phase), the hysteresis in the focal conic-homeotropic transition was reduced; and the material could be operated in gray scales. We systematically studied the electro-optical performance of the displays which were suitable for direct view display applications. |
