Dynamic Dewetting Regimes Explored

To resolve issues concerning the manner in which substrate wettability and heterogeneity influence contact line motion, we investigate the dewetting process that occurs when a bubble contacts a hydrophobic surface immersed in water. Titania surfaces, partly coated with a self-assembled monolayer of octadecyltrihydrosilane, were used to achieve a wide range of surface composition and wettability. For all surfaces, two distinct dynamic regimes were detected: the initial stage of the dewetting process is captured by hydrodynamics (partly affected by inertia), whereas the regime of low contact line speeds is well described by the molecular kinetic approach, indicating a thermally activated process. In the hydrodynamic regime, physically reasonable model parameter values were obtained where inertia is negligible. In the low speed regime, however, the results contradicted the common expectation that the activation free energy of displacements correlates with the work of adhesion. Rather quantitative agreement with a microscopic pinning energy was obtained. Our data and analysis strongly indicate that pinning controls contact line motion in the low velocity regime of contact line motion on chemically heterogeneous surfaces.