Effects of Dielectric Relaxation on the Director Dynamics of Uniaxial Nematic Liquid Crystals
Oleg D Lavrentovich
,Sergij V. Shiyanovskii
,Oleg D. Lavrentovich
Liquid Crystal Institute, KSU
Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent State University, Kent, OH 44242
Dielectric anisotropy of liquid crystals (LC) causes director reorientation in the applied electric field. The components of the dielectric tensor are frequency-dependent. Till recently, this frequency dependence was not accounted for in the description of director dynamics in the electric field. In this work, we theoretically derive the reorienting dielectric torque acting on the director, taking into account the entire frequency spectrum of the dielectric tensor. The model allows one to include the effects of multiple relaxations in both parallel and perpendicular components of the dielectric tensor, thus generalizing the recent model [Yin, Golovin, Shiyanovskii, and Lavrentovich, Phys. Rev. Lett. 95, 087801 (2005)] limited by the single relaxation approach. The model predicts the “dielectric memory effect” (DME), i.e., dependence of the dielectric torque on both the “present” and “past” values of the electric field and the director.
The new model allows one to describe the experimentally observed director reorientation in the case when the rise time of the applied voltage is smaller than the dielectric relaxation time. In the typical materials such as 5CB, in which the dielectric anisotropy is positive at low frequencies, the DME slows down director reorientation in a sharply rising electric field, as the sharp front is perceived as a high-frequency excitation for which the dielectric anisotropy is small or even of a negative sign. In the materials that are dielectrically negative, the DME speeds up the response when a sharp pulse is applied.