A GRAND-CANONICAL ENSEMBLE MONTE-CARLO STUDY OF CONFINED PLANAR AND HOMEOTROPICALLY ANCHORED GAY-BERNE FILMS

A molecularly thin film composed of Gay-Berne molecules is investigate d in grand canonical ensemble Monte Carlo simulations in which the the rmodynamic state of the him is determined by the temperature T and the chemical potential mu. These parameters are chosen such that a corres ponding bulk Gay-Berne fluid is isotropic. The him is confined between two plane parallel solid substrates (i.e., walls) composed of individ ual atoms (wall atoms). Wall atoms are distributed according to the (1 00) configuration of the face centred cubic lattice. The focus is on t he impact of the him-wall interaction potential on the microscopic str ucture 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 mo lecules 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 sensitiv ely on the wall-induced orientation of film molecules. If the walls su pport a homeotropic orientation they induce a highly ordered, solid-li ke film structure which does not extend beyond the stratum closest to a wall; the inner portion of the film remains isotropic and nearly hom ogeneous because of competition between orientation favoured by the wa lls and lack of space. Results for the excess coverage, which can be d etermined experimentally, are correlated with the Mayer-Saupe order pa rameter for the entire film.