(757c) Effect of Geometric Roughness On the Wetting Behavior of Water

Knowledge of the contact angle made by a fluid on a solid surface is crucial in designing various engineering devices. It is well known that a combination of geometrical roughness and surface chemistry can impart an entire spectrum of wetting behavior. We have used molecular simulation to investigate the manner in which these factors influence wetting behavior.  More specifically, we have constructed wetting diagrams for several model systems with regular nanoscale features. Such diagrams provide the cosine of the contact angle a fluid adopts on a geometrically rough surface cos(θr) in terms of the cosine of the contact angle the same fluid adopts on a chemically-identical flat surface cos(θ_f). We work with two model fluids: Lennard-Jonesium and SPCE water, which interact with atomistically-detailed surfaces with features characterized by amplitudes and periodicities of 1-25 nm. It is observed that the macroscopically-based Cassie and Wenzel models become unreliable as the length scale of substrate features drops below 10 fluid diameters. Interestingly, our results also indicate similar qualitative behavior for Lennard-Jonesium and water.