Monte Carlo simulation strategies for computing the wetting properties of fluids at geometrically rough surfaces

We introduce Monte Carlo simulation methods for determining the wetting properties of model systems at geometrically rough interfaces. The techniques described here enable one to calculate the macroscopic contact angle of a droplet that organizes in one of the three wetting states commonly observed for fluids at geometrically rough surfaces: the Cassie, Wenzel, and impregnation states. We adopt an interface potential approach in which the wetting properties of a system are related to the surface density dependence of the surface excess free energy of a thin liquid film in contact with the substrate. We first describe challenges and inefficiencies encountered when implementing a direct version of this approach to compute the properties of fluids at rough surfaces. Next, we detail a series of convenient thermodynamic paths that enable one to obtain free energy information at relevant surface densities over a wide range of temperatures and substrate strengths in an efficient manner. We then show how this information is assembled to construct complete wetting diagrams at a temperature of interest. The strategy pursued within this work is general and is expected to be applicable to a wide range of molecular systems. To demonstrate the utility of the approach, we present results for a Lennard-Jones fluid in contact with a substrate containing rectangular-shaped grooves characterized by feature sizes of order ten fluid diameters. For this particular fluid-substrate combination, we find that the macroscopic theories of Cassie and Wenzel provide a reasonable description of simulation data.