The role of the substrate roughness in contact angle hysteresis and dynamic deviation

Static contact angle hysteresis and dynamic angle variation are two fundamental phenomena about the contact angle deviating from the equilibrium state. Various factors including solid roughness and disjoining pressure have been considered as the origins of the phenomena. This work made a reduction to absurdity by employing absolutely smooth solid surfaces in large-scale molecular dynamics simulations. The results showed that the equilibrium angles were well established on the absolutely smooth surface just as regular solid surfaces, while the hysteresis and the dynamic deviation vanished. In contrast, the solids made of atoms, even with atomic roughness, could bring significant static hysteresis and dynamic deviation. The convex nanobending, which was recently experimentally confirmed as a feature structure at advancing contact lines, was reproduced on atomic solids while vanished on the absolutely smooth one. The results indicated that the angle deviations could be unified to originate from the friction on solids, either static or dynamic, and that even the atomic roughness could be effective. As comparison, 3D observations were made using state-of-the-art helium ion microscopy for the first time revealing the ubiquitous nanoscopic distortion along the contact line on atomically smooth surfaces. (C) 2019 Elsevier Ltd. All rights reserved.