Optical measurement of the director relaxation time in a periodically reoriented nematic liquid crystal

Abstract

The director field of an electrohydrodynamically excited nematic liquid crystal layer exhibits a spatially periodic reorientation, provided the ac voltage applied to the layer is sufficiently close to its threshold value. This fact, combined with the strong optical anisotropy which, as a rule, characterizes any nematic phase, causes the nematic layer to act as a diffraction grating when illuminated by a monochromatic light beam. The time dependence of the intensities of diffraction fringes so formed are a manifestation of the motion of the nematic director. In the present article we propose an experimental method for the determination of the decay time of the nematic director under conditions where the electric field across the nematic layer is temporarily zero. The method is based on a linearized theory of the electrohydrodynamic instability of nematic liquid crystals, as well as, on an optical theory of the diffraction grating action of a periodically reoriented nematic layer. We applied the proposed method in a case where one can compute the respective theoretical value of the relaxation time of the nematic director. The agreement of the experimental result with what one obtains on the grounds of the electrohydrodynamic threshold theory, is found to be fairly satisfactory. Thus, taking advantage of the relatively simple dependence of the measured relaxation time on (a) the experimentally measurable distortion wave number of the electrohydrodynamically excited nematic layer, and (b) the elastic constants and the viscosity coefficients of the nematic material, we get the possibility to obtain further experimental information about the mentioned material parameters, by experimental investigation of the dependence of the director relaxation time on the distortion wave number.