On the nature of the orientational effect of ultrasound on nematic liquid crystal
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On the Nature of the Orientational Effect of Ultrasound on Nematic Liquid Crystal O. A. Kapustina and E. K. Negazina Andreev Institute of Acoustics, Russian Academy of Sciences, ul. Shvernika 4, 117036 Russia e-mail: [email protected] Received June 10, 2015
Abstract—Experimental substantiation of the validity of the model of orientational distortion in a homeotropic layer of nematic liquid crystal under an ultrasonic beam with a sharp boundary is presented for the first time. The model is constructed within the concepts of nonequilibrium thermodynamics and statistical hydrodynamics, taking into account the processes of structural relaxation of the mesophase. It establishes the relationship between the characteristics specifying the homeotropic structure deformation (layer thickness, ultrasound frequency, parameters of the molecular micromodel of liquid crystal, and its material constants) and the layer transparency for a linearly polarized light beam. The calculation results are compared with the experimental data in the frequency range of 0.1–3 MHz. DOI: 10.1134/S106377451604009X
INTRODUCTION In contemporary acoustics of liquid crystals (LCs), the problem of stability and variability of the orientational ordering of a thin mesophase layer holds a particular position and has been studied quite extensively in recent decades [1–4]. Utmost attention was paid to the structural distortions of a nonthrershold nature1 in a homeotropic layer of nematic liquid crystal (NLC) under ultrasound. One specific feature of the homeotropic-layer structural distortion is that it reproduces the wave field pattern and correlates with the elasticwave wavelength. The models [3] of the mechanism of this phenomenon, developed within the Leslie‒Ericksen hydrodynamics [5] at various research centers, postulated a unified concept of its nature (the convective mechanism of structure deformation) but different hypotheses of the origin of steady-state flows (radiation pressure, interaction of waves, anisotropy of static elastic modulus, sound absorption, and its anisotropy). The fact that none of these hypotheses was confirmed experimentally is related to not only their incorrectness, but also to the inability of classical hydrodynamics to describe the NLC properties at ultrasonic frequencies. The point is that within the conventional approach the viscous stresses and torques that make molecules rotate were represented as a linear combination of the velocity gradients and the rotational velocity of the NLC director n with respect to the surrounding liquid, while the processes 1 In
actual experiments the effects become “threshold,” but the value of the “conditional threshold” in this case is determined by the sensitivity limits of the measuring equipment.
of structural relaxation of LCs were taken into account only when analyzing the anisotropy of LC acoustic properties [6]. An important step in developing adequate theoretical models of the mechanism of the phenomenon, which could explain experimental data obtained at ultrasoni
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