MICROSCOPIC STRUCTURE OF MOLECULARLY THIN CONFINED LIQUID-CRYSTAL FILMS

The microscopic structure of a molecularly thin liquid-crystal film co nfined between two plane parallel surfaces (i.e., walls) composed of r igidly fixed atoms is investigated in grand canonical ensemble Monte C arlo simulations in which the temperature T, the chemical potential mu , and the wall separation s(z) are the relevant thermodynamic state va riables. These conditions correspond to those encountered in related e xperiments employing the surface forces apparatus (SFA). Wall atoms ar e distributed according to the (100) configuration of a face-centered cubic (fee) lattice. Film molecules interact with each other via the G ay Berne potential which may be viewed as a Lennard-Jones (12,6) poten tial modified to account for the anisotropy of the interaction between two ellipsoidal film molecules. Parameters governing the film-wall in teraction are chosen such that molecules tend to arrange their symmetr y axes parallel with the plane of a wall (i.e., the x-y plane). The th ermodynamic state of a bulk phase in equilibrium with the confined fil m pertains to the isotropic phase of the Gay-Berne fluid, so that pref erred orientations in the film are unambiguously ascribed to confineme nt (i.e., to the presence of the walls). In general, film structure is characterized by stratification, that is, the tendency of film molecu les to arrange their centers of mass in individual strata parallel wit h the walls. The strata are more diffuse than in films composed of ''s imple'' molecules without rotational degrees of freedom due to a large r geometric incompatibility between him and wall structure and to orie ntability of film molecules in the present model. As s(z) is increased at fixed T and mu, molecularly thin liquid-crystal films undergo comp lex structural changes resulting from a competition between wall-induc ed orientation and lack of space. These effects are analyzed in depth by density-alignment histograms and correlated with variations of the normal stress T-zz exerted by the film on the walls. The normal stress , which is in principle accessible in SFA experiments, depends strongl y on s(z) even in rather thick films, indicating the importance of coo perative wall-induced phenomena for materials properties of confined l iquid-crystal films.