Fluid phase transitions at chemically heterogeneous, nonplanar solid substrates: Surface versus confinement effects

The phase behavior of a "simple" Lennard-Jones (12,6) fluid confined between planar substrates has been investigated by means of Monte Carlo simulations in the grand canonical ensemble (GCEMC). The interaction of fluid molecules with the upper wall is purely repulsive and attractive with the lower one. A spherical cap of radius xi is embedded in the surface of the upper wall. The cap also attracts fluid molecules. For xi=0, that is, in the case of two planar homogeneous substrates, one observes classical prewetting, namely a first-order phase transition from thin to thick films adsorbed on the lower substrate. When xinot equal0, that is, when the upper substrate is decorated with a three-dimensional pattern of finite size, system properties are no longer translationally invariant in any spatial dimension. Thus, the grand potential Omega is not a homogeneous function of degree one in any of its extensive variables which precludes a "mechanical" expression for Omega in terms of stresses and conjugate strains. Therefore, in order to determine the phase behavior through plots of Omega versus chemical potential mu we employ a thermodynamic integration scheme under isothermal conditions (T=const). We observe a partially condensed phase, where the molecules are preferentially adsorbed in the gap between the spherical cap and the lower substrate, associated with either a thin or a thick film adsorbed on the lower substrate. (C) 2003 American Institute of Physics.