Dielectric relaxation and electrooptical effects in nematic liquid crystals
Chemical Physics Interdisciplinary Program
Orientational dynamics of the nematic director in the applied electric field is a fundamental physical phenomenon that is at the heart of numerous modern technologies. Dielectric reorientation of the director is caused by a torque that depends on both the electric field and electric displacement. The finite rate of dielectric relaxation causes a time lag of the electric displacement as compared to the instantaneous value of the electric field. This effect which has been previously ignored is the subject of this review. We show that it causes profound effects when the characteristic time of the field changes is close to the dielectric relaxation time. Depending on the type of the nematic material, the characteristic relaxation times range from milliseconds (dual-frequency materials) to nanoseconds ("regular" materials such as pentylcyanobiphenyl). From the fundamental viewpoint, the problem is to determine the dielectric torque as a function of not only the present values of the electric field and director (as in the current theory) but of the previous values of these variables. As the modern industry demands nematic devices capable of switching within 1 ms or less, the issue of finite rate of dielectric relaxation becomes also of practical importance. We review recent experimental and theoretical works that link the phenomenon of dielectric relaxation in nematic liquid crystals to their fast electro-optical switching and also to the problem of dielectric heating. We discuss the theoretical model that allows one to relate director reorientation to the "history" of the electric field and the experimental verification of its validity. We analyze the effect of dielectric heating that is especially profound at the frequencies corresponding to dielectric relaxation. One of the important new results here is that the temperature raise depends on the state of director orientation. We also reported on a thermodielectric bistability in DFNs caused by the anisotropic nature of dielectric heating and director reorientation in an electric field. The bistability is a result of the positive feedback loop: director reorientation -> anisotropic dielectric heating -> dielectric anisotropy -> director reorientation. We demonstrate both experimentally and theoretically that two states with different temperature and director orientation, namely, a cold planar state and a hot homeotropic state coexist in a LC cell for a certain frequency and amplitude range of the applied voltage.