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 ξ is embedded in the surface of the upper wall. The cap also attracts fluid molecules. For ξ=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 ξ≠0, 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 Ω is not a homogeneous function of degree one in any of its extensive variables which precludes a “mechanical” expression for Ω in terms of stresses and conjugate strains. Therefore, in order to determine the phase behavior through plots of Ω versus chemical potential μ 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.