A grand canonical ensemble Monte Carlo study of confined planar and homeotropically anchored Gay-Berne films

Abstract

A molecularly thin film composed of Gay-Berne molecules is investigated in grand canonical ensemble Monte Carlo simulations in which the thermodynamic state of the film is determined by the temperature T and the chemical potential μ. These parameters are chosen such that a corresponding bulk Gay-Berne fluid is isotropic. The film is confined between two plane parallel solid substrates (i.e., walls) composed of individual atoms (wall atoms). Wall atoms are distributed according to the (100) configuration of the face centred cubic lattice. The focus is on the impact of the film-wall interaction potential on the microscopic structure and its consequences for the normal-stress curve T_zz (s_z ) (s_z is the distance between the walls) which is accessible in principle in corresponding experiments employing the surface forces apparatus. Depending on the values of film-wall interaction potential parameters planar or homeotropic orientations of the symmetry axes of the film molecules can be realized. In general, the confined film is stratified, i.e., film molecules arrange their centres of mass in molecular strata parallel with the plane of the walls. Stratification depends sensitively on the wall-induced orientation of film molecules. If the walls support a homeotropic orientation they induce a highly ordered, solid-like film structure which does not extend beyond the stratum closest to a wall; the inner portion of the film remains isotropic and nearly homogeneous because of competition between orientation favoured by the walls and lack of space. Results for the excess coverage, which can be determined experimentally, are correlated with the MayerSaupe order parameter for the entire film.