The hydrodynamics of drop impact onto chemically structured surfaces

The normal and inclined impact of liquid drops onto chemically structured surfaces has been studied experimentally and theoretically. The surface functionalization comprised a self-assembly process of a covalently bound monochlorosilane on a silicon substrate, followed by a photochemical attachment of a polymer of defined hydrophilicity and a subsequent deep UV ablation step to create a local spot of high wettability in a region of low wettability so that a self-centring effect of the impacting liquid could be achieved. Experimentally the impact is observed using a high-speed camera, changing the impact velocity, the impact displacement from the wettable spot and the inclination of the surface. The temporal spread of the drop was recorded, yielding also the macroscopic dynamic contact angle as a function of time. A theoretical model of the drop impact is developed, based on a mass balance and on a momentum balance which includes capillary forces and viscous drag, and which accounts for the inertial and wettability effects. The theoretical predictions for the time evolution of the drop edges agree well with the experimental data.