Microscopic structure of molecularly thin confined liquid-crystal films

Abstract

The microscopic structure of a molecularly thin liquid-crystal film confined between two plane parallel surfaces (i.e., walls) composed of rigidly fixed atoms is investigated in grand canonical ensemble Monte Carlo simulations in which the temperature T, the chemical potential \ensuremath{\mu}, and the wall separation ${\mathrm{s}}_{\mathrm{z}}$ are the relevant thermodynamic state variables. These conditions correspond to those encountered in related experiments employing the surface forces apparatus (SFA). Wall atoms are distributed according to the (100) configuration of a face-centered cubic (fcc) lattice. Film molecules interact with each other via the Gay-Berne potential which may be viewed as a Lennard-Jones (12,6) potential modified to account for the anisotropy of the interaction between two ellipsoidal film molecules. Parameters governing the film-wall interaction are chosen such that molecules tend to arrange their symmetry axes parallel with the plane of a wall (i.e., the x-y plane). The thermodynamic state of a bulk phase in equilibrium with the confined film pertains to the isotropic phase of the Gay-Berne fluid, so that preferred orientations in the film are unambiguously ascribed to confinement (i.e., to the presence of the walls). In general, film structure is characterized by stratification, that is, the tendency of film molecules to arrange their centers of mass in individual strata parallel with the walls. The strata are more diffuse than in films composed of ``simple'' molecules without rotational degrees of freedom due to a larger geometric incompatibility between film and wall structure and to orientability of film molecules in the present model. As ${\mathrm{s}}_{\mathrm{z}}$ is increased at fixed T and \ensuremath{\mu}, molecularly thin liquid-crystal films undergo complex structural changes resulting from a competition between wall-induced orientation and lack of space. These effects are analyzed in depth by density-alignment histograms and correlated with variations of the normal stress ${\mathrm{T}}_{\mathrm{zz}}$ exerted by the film on the walls. The normal stress, which is in principle accessible in SFA experiments, depends strongly on ${\mathrm{s}}_{\mathrm{z}}$ even in rather thick films, indicating the importance of cooperative wall-induced phenomena for materials properties of confined liquid-crystal films.