Structural Transformations in Nematic Liquid Crystals with a Hybrid Orientation
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TICAL, NONLINEAR, AND SOFT MATTER PHYSICS
Structural Transformations in Nematic Liquid Crystals with a Hybrid Orientation V. A. Delev* and A. P. Krekhov Institute of Molecular and Crystal Physics, Ufa Scientific Center, Russian Academy of Sciences, Ufa, Bashkortostan, 450075 Russia *e-mail: [email protected] Received June 23, 2017
Abstract—The structural transformations in a nematic liquid crystal (NLC) layer with a hybrid orientation (planar director orientation is created on one substrate and homeotropic director orientation is created on the other) are studied. In the case of a dc voltage applied to the NLC layer, the primary instability is flexoelectric. It causes the appearance of flexoelectric domains oriented along the director on the substrate with a planar orientation. When the voltage increases further, an electroconvective instability in the form of rolls moving almost normal to flexoelectric domains develops along with these domains. Thus, the following spatially periodic structures of different natures coexist in one system: equilibrium static flexoelectric deformation of a director and dissipative moving oblique electroconvection rolls. The primary instability in the case of an ac voltage is represented by electroconvection, which leads to moving oblique or normal rolls depending on the electric field frequency. Above the electroconvection threshold, a transition to moving “abnormal” rolls is detected. The wavevector of the rolls coincides with the initial director orientation on the substrate with a planar orientation, and the projection of the director at the midplane of the NLC layer on the layer plane makes a certain angle with the wavevector. The results of numerical calculations of the threshold characteristics of the primary instabilities agree well with the obtained experimental data. DOI: 10.1134/S1063776117120032
1. INTRODUCTION The systems exhibiting the appearance of a supramolecular order are being extensively studied in condensed-matter physics. This attention is related to the fundamental problems of structural transformation mechanisms, the appearance of chaos and turbulence in such systems, and the possibility of using investigation results in various fields of engineering. Liquid crystals (LCs) are anisotropic liquids and occupy an intermediate position between isotropic liquids and crystalline solids due to their physical properties [1, 2]. One of the characteristic features of LCs is their ability to change an orientation structure under a very weak external action. Due to the anisotropy of physical properties, LCs exhibit a wide spectrum of structural transformations under the action of an applied electric or magnetic field, a hydrodynamic flow, and a temperature gradient [2, 3]. The mechanisms of formation of a supramolecular order are most actively studied in nematic liquid crystals (NLCs) in an electric field. The unique feature of this system is the possibility of formation of a large number of equilibrium (spatially periodic Fréedericksz transition, flexoelectric domains) and di
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