Computational investigation of the effects of surface roughness on wetting
Grzelak, Eric Michael
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This dissertation focuses on the study of wetting phenomena by use of molecular simulation. Within this work we generate a better understanding of how surface structure affects the interfacial properties of a system. We begin by developing a series of molecular simulation tools that enable us to compute the macroscopic contact angle and solid-fluid interfacial tension of model systems. These techniques are subsequently used to explore two issues: (1) the anisotropic wetting of crystalline solids and (2) the influence of nanoscopic heterogeneities in the 2 to 25 nm range on wetting behavior. We anticipate that our findings will help facilitate the design of technologies wherein a fluid contacts a solid. We focus on a computational approach wherein one obtains interfacial properties via analysis of the surface excess free energy as a function of surface density. We show that previous sampling limitations can be overcome by employing transition matrix Monte Carlo techniques to acquire the surface free energy. After outlining the key features of this approach we turn our attention to probing the extent to which extracted interfacial properties are sensitive to the finite size of the simulation cell. Our findings show that finite-size effects are essentially negligible for relatively short-ranged substrate potentials but become significant for longer-ranged interactions. Next, we outline a means to account for these finite-size effects and show its utility. We then extend the project to the study of anisotropic wetting and find that the microscopic topography of a substrate can significantly influence the wetting behavior a system exhibits. Several quantitative metrics are examined in an effort to identify a means to predict the variation in the contact angle via a structural and/or energetic characteristic of the substrate. The planar density of the first layer of the surface appeared to be the most practical. We next increase the length scale of substrate heterogeneities into the nanometer range and evaluate the efficacy of the Wenzel equation. Nanoscopically rough contours are created on atomistic surfaces and contact angles are determined as functions of roughness factors. We find that the Wenzel equation provides an adequate description of the contact angle when the periodicity of the heterogeneity exceeds approximately 10 nm.