On the role of flexoeffect in synchronization of electroconvective roll oscillations in nematics
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TICAL, NONLINEAR, AND SOFT MATTER PHYSICS
On the Role of Flexoeffect in Synchronization of Electroconvective Roll Oscillations in Nematics E. S. Batyrshin, A. P. Krekhov, O. A. Scaldin, and V. A. Delev Institute of Molecular and Crystal Physics, Ufa Research Center, Russian Academy of Sciences, Ufa, 450075 Russia email: [email protected] Received September 29, 2011
Abstract—We describe the dynamics of zigzag oscillations in a system of convective rolls in a nematic liquid crystal above the electroconvection threshold under the action of an ac voltage with a biased position of the mean value. It is found that an increase in the contribution from the constant component leads to a substantial increase in the spatiotemporal ordering of zigzag rolls and their synchronization with the homogeneous twist mode. The results confirm the flexoelectric mechanism of locking. DOI: 10.1134/S1063776112040061
1. INTRODUCTION Nematic liquid crystals (NLCs) constitute one of the most attractive model systems for studying univer sal aspects of formation and evolution of spatiotem poral order in various nonequilibrium physical, chemical, and biological systems [1, 2]. NLCs are anisotropic liquids characterized by orientational ordering of elongated molecules. The preferred direc tion of orientation of NLC molecules is described by the field of director n [3]. Electroconvective instabil ity occurs when a voltage exceeding a certain critical value is applied to the NLC layer enclosed between conducting substrates. In this case, a periodic system of stripes is formed representing spatially periodic modulation of the director field and of the NLC flow velocity (electroconvective rolls). Incessant activity in the study of electroconvective structures is due to the importance of analyzing the possible mechanisms of selforganization in complex anisotropic systems. The relative simplicity of measuring the control parameters (amplitude and frequency of the applied voltage) and the high optical contrast of the formed structures due to optical anisotropy of NLCs ensure considerable advantages in experimental studies of electroconvection. The ideas put forth by Carr [4] and Helfrich [5] concerning the mechanism of electroconvection in anisotropic liquids have led to the construction of the standard electroconvection model [3, 6–9], which has been used to compute the threshold characteris tics of instabilities. The instability scenario is deter mined by the signs of anisotropy of permittivity ⑀a and conductivity σa and by the initial distribution of the director field [10, 11]. In accordance with the standard model, the necessary condition for the occurrence of electroconvective instability is the pos itive sign of conductivity anisotropy (σa > 0). The dis
covery of the socalled nonstandard electroconvec tion regime in NLCs with σa < 0 [11–13], for which the Carr–Helfrich convective mechanism does not operate, initiated further development of theoretical models. Nonstandard electroconvection has been explained in the model taking into
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