Monte Carlo Simulations of Biaxial Molecules Near Hard Wall

A. Kapanowski, S. Dawidowicz Faculty of Physics, Astronomy and Applied Computer Science, Jagiellonian University, Łojasiewicza 11, 30-348 Kraków, Poland

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A system of optimal biaxial molecules placed at the sites of a cubic lattice is studied in an extended Lebwohl-Lasher model. Molecules interact only with their nearest neighbors through the pair potential that depends on the molecule orientations. It is known that in a homogeneous system there is a direct second-order transition from the isotropic to the biaxial nematic phase, however the properties of confined systems are less known. In the present paper, the lattice has periodic boundary conditions in the X and Y directions and has two walls with planar anchoring, perpendicular to the Z direction. We have investigated the model using Monte Carlo simulations on Nx×Ny×Nz lattices, for Nx=Ny=10,16, Nz from 3 to 19, with and without mirror symmetry. This study is complementary to the statistical description of hard spheroplatelets near a hard wall (Phys. Rev. E 89, 062503 (2014) 10.1103/PhysRevE.89.062503). The temperature dependence of the order-parameter profiles between walls is calculated for many cases of wall separations. For large wall separations, there are surface layers with biaxial ordering on both walls (~4-5 lattice constants wide) and, beyond the surface layers, the order parameters have values as in a homogeneous system. For small wall separations, the isotropic-biaxial transition is shifted and the surface layers are thinner. Above the isotropic-biaxial transition the preferable orientations in both surface layers can be different. It is interesting that planar anchoring for biaxial molecules leads to uniaxial interactions on the wall. As a result, we get the planar Lebwohl-Lasher model with additional (biaxial) interactions with neighbors from the second layer, where the Kosterlitz-Thouless transition is present on the wall.

DOI:10.12693/APhysPolA.140.365 topics: liquid crystals, biaxial nematics, Monte Carlo simulations