Can Continuum Thermodynamics Characterize Wenzel Wetting States of Water at the Nanometer Scale?

We address the question of how solid-liquid surface free energy is affected by nanometer scale roughness. We proceed through molecular dynamics calculations of the solid-liquid surface free energy of water in a collapsed wetting state on rough nonpolar surfaces characterized by an array of parallel linear grooves. We establish a continuum equation based on Wenzel and Cassie-Baxter approaches to predict the solid-liquid surface free energy variations of systems, where the vertical and horizontal surfaces of the groove have different interactions with water and where the groove geometry can be varied. We show that the molecular simulations results agree with the prediction of the continuum equation under the condition that the groove is wider than three water molecular diameters and deeper than one molecular diameter.