Thursday, 6 July 2006 - 11:40 PM

Effects of dielectric relaxation on the dynamics and dielectric heating of nematic liquid crystals

Sergij V. Shiyanovskii, Ye Yin, Andrii B. Golovin, and Oleg D. Lavrentovich. Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent State University, Summit Rd, Kent, OH 44242

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. Reorientation of the director is caused by a dielectric torque that is a vector product of the electric displacement and electric field. 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 previously ignored effect is the subject of our study. We show that it causes profound effects when the characteristic times of the field and director changes are 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. Nematic devices are capable of switching in microsecond [1] and nanosecond [2] ranges, thus the issue of finite rate of dielectric relaxation is also of practical importance. We present recent experimental and theoretical results that link the phenomenon of dielectric relaxation in nematic liquid crystals to their fast electro-optical switching and 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 [3]. Figure shows transmitted light intensity modulated by the changes of optical retardation for the nematic cell driven by an abruptly applied voltage pulse at 1 kHz; at the time scale shown, the pulse looks like a DC pulse. In the top part, the solid line is the oscilloscope's trace for the experimentally determined light transmittance, the dashed line represents the transmitted intensity as calculated from the theoretical model, and the dotted line represents the standard approach. The director initially reorients in the "wrong" direction up to the point "Y" and this initial reorientation limits the switching rate. We also analyze the effect of dielectric heating that is especially profound at the frequencies corresponding to dielectric relaxation. We found that the temperature dependence of dielectric heating on the state of director orientation provide new mechanisms of multistability.

The work was partially supported by NSF grants DMS 0456286 and DMR 0504516.

[1] A. B. Golovin, S. V. Shiyanovskii, and O. D. Lavrentovich, Appl. Phys. Lett. 83, 3864 (2003). [2] H. Takanashi, J. E. Maclennan, and N. A. Clark, Jpn. J. Appl. Phys 37, 2587 (1998). [3] Y. Yin, S. V. Shiyanovskii, A. B. Golovin, and O. D. Lavrentovich, Phys. Rev. Lett. 95, 087801 (2005).

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