(219e) Monte Carlo Simulation Methods for Calculating the Interfacial Properties of Fluids at Heterogeneous Surfaces

We describe our recent efforts aimed at development of Monte Carlo simulation methods for determining the wetting properties of fluids on heterogeneous surfaces. Our strategy involves calculation of the surface excess free energy (also referred to as the interface potential) as a function of the surface density of a fluid in contact with the substrate at a specified temperature and chemical potential. The shape of this curve provides qualitative insight regarding the system's wetting behavior (e.g. partial wetting, complete wetting) and quantitative analysis provides macroscopic interfacial properties, such as the spreading coefficient and interfacial tension. Our standard approach involves the use of grand canonical transition matrix Monte Carlo simulation to obtain the interface potential. When studying heterogeneous substrates, this direct method fails under some conditions. Within this presentation, we describe strategies that we employ to overcome this obstacle. Specifically, we address the use of various expanded ensemble techniques that provide alternative paths to compute the necessary free energy information. Results are presented for the evolution of the contact angle with substrate strength on a substrate with nanoscale geometric roughness.