Numerical Simulation Of A Voltage-Dependent Surface-Induced Molecular Reorientation In A Confined Nematic Liquid Crystal
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Numerical Simulation Of A Voltage-Dependent Surface-Induced Molecular Reorientation In A Confined Nematic Liquid Crystal J.J. Castro¹, A. Calles² , and R. M. Valladares² ¹Departamento
de Física, CINVESTAV del IPN Apdo. Postal 14-740, 07300 México, D.F. ²Departamento de Física, Facultad de Ciencias, UNAM Apdo. Postal 70-646, 04510 México, D.F. ABSTRACT We present a computer simulation of the voltage-dependent orientational ordering of a nematic liquid crystal within the lattice approximation, confined between two substrates with different anchoring for each surface. The system is studied by analyzing the response of the molecular reorientation near the surfaces as a function of the applied voltage. The model considers a molecular interaction responsible for the nematic phase that includes a superposition of the anisotropic induced-dipole-induced-dipole and isotropic Maier-Saupe interactions, whereas for the nematic-surface intermolecular interaction we assume the Rapini-Papoular form. The response to the external electric field is studied by calculating the dielectric constant tensor, based on a model through the molecular polarizability. We particularly discussed the case for negative dielectric anisotropy that could have relevance for the voltage controlled twist effect. The simulation is carried through a numerical relaxation method for the total energy of the system. INTRODUCTION One basic problem in the study of liquid crystals, both for a theoretical description as well as for practical application, is the understanding of the behavior of the director near a surface. Most liquid crystal displays consist of a liquid crystal layer contained between two substrate surfaces. The surfaces impose a specific orientation of the liquid crystal director near its boundary that might considerable influence the electro-optical properties of the material. In particular we can mention the case when a nematic liquid crystal is placed between a flat surface, which induced a parallel anchoring of the director, and a grating surface treated to induced a perpendicular anchoring. It has been shown [1] that for this case, the effective anchoring becomes azimuthal at the grating surface and depends on the applied voltage when the nematic phase has negative dielectric anisotropy (that is, the director tends to align perpendicular to the applied field), allowing for a voltage-controlled twist effect. This has suggested the possibility of constructing new liquid crystal materials for high-quality displays that could electrically manipulate liquid crystal orientation near the surfaces, lowering the traditional operating voltages. This can be achieved making liquid crystals with weak surface anchoring [2]. Simulations of such materials with different kind of anchoring on each surface and its response to applied voltages might help in gaining physical insight on the understanding of the response of the molecular orientation near the surface. Here we present the study of the director orientation of a nematic liquid crystal CC5.30.1
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