SUBSTRATE-INDUCED ORDER IN CONFINED NEMATIC LIQUID-CRYSTAL FILMS

In orientationally biased grand canonical ensemble Monte Carlo (GCEMC) simulations we investigated the microscopic structure of liquid-cryst alline films confined between two plane parallel solid surfaces (i.e., walls) consisting of N-s discrete, rigdly fixed atoms. These wall ato ms are distributed across the plane of a wall according to the (100) s tructure of the face-centered cubic lattice. Parameters of the film-wa ll interaction potential are chosen such chat a homeotropic alignment of film molecules is favored. In the simulations the thermodynamic sta te of the film is determined by the temperature T, the chemical potent ial mu, the distance between the walls s(z), and the film-wall interfa cial area A. Thermodynamic states of the him are chosen such that a co rresponding bulk liquid crystal is nematic. To simulate nematic phases in the GCEMC we modified the classic Gay-Berne potential for the inte raction between a pair of film molecules so that the isotropic-nematic phase transition in the bulk occurs at sufficiently low densities. Re liability of the GCEMC method under these conditions is illustrated by a self-consistent comparison between Monte Carlo simulations in the c anonical and grand canonical ensembles. In the bulk the nematic nature of the modified Gay-Berne fluid is established by computing the Mayer -Saupe order parameter S and suitably defined pair correlation functio ns which show that the bulk phase is not smectic even though S is fair ly large. For a single temperature we investigate the isotropic-nemati c phase transition in the modified Gay-Berne fluid which turns our to be a first-order phase transition. In the corresponding confined film variations of the microscopic structure with increasing s(z) are corre lated with the normal component of the stress tensor T-zz(s(z)). Our r esults show that molecules in inner portions of the film undergo a reo rganization from an originally planar orientation of their symmetry ax es to a perpendicular one with respect to the plane of a wall. This or ientational change is manifested as a periodic sequence of shoulders i n T-zz(s(z)) where the periodicity length Delta s(z) is close to the l arger diameter of the ellipsoidal film molecules. (C) 1998 American In stitute of Physics. [S0021-9606(98)51220-2].