Drops make a splash

Abstract

Splashing, as a result of the drop impact onto a liquid film, occurs in various natural phenomena and technical processes. Examples include rain drops impacting on the ground, fuel injection impacting chamber walls, spray cleaning, spray cooling, or spray coating. Inertia, viscous and capillary forces determine the impact outcome if both liquids are the same. In the case of two different liquids, also the miscibility and the interfacial forces influence the drop impact phenomenon. This latter case is much less understood. The main objective of the present experimental work is to elucidate the impact phenomena of a single Newtonian drop onto a liquid wall film of different viscosity. The experimental setup consist of a drop-on-demand drop generator, a wetted, horizontal, glass substrate, and an observation system, consisting of the illumination source and a high-speed video camera. A high frame rate of up to 40,000 fps is used to investigate the impact dynamics of the fluids and to observe the fluid distribution after the drop impact. The liquid of the drop is marked by a dye to distinguish liquid interfaces. The viscosity and surface tension of the two liquids are varied. The drop and wall liquids are not miscible. The experiments revealed several interesting phenomena, typical only to the case of collision of different liquids.