Experimental investigation of a single droplet impact onto a sessile drop

This paper presents an experimental study of the impact of a single water droplet onto a stationary liquid bulk built by the previous impact of one or two droplets. The experiments were performed with two different film thicknesses, three different Weber (We) numbers, and two surface contact angles. In both cases we have hydrophilic conditions, which do not allow for a good investigation of this parameter. The morphology of the drop impact was studied using a charge-coupled device (CCD) camera, and the corresponding qualitative and quantitative characteristics regarding the time evolution of the phenomena, such as the diameter and height of the evolving crown, were obtained by image analysis. Analysis of the experimental data evidences that the phenomenon has a strong similarity to the impact of a single drop on shallow films, although the effect of the surface wetting characteristics plays, in this case, a negligible role. The regimes of deposition and splashing are identified as a function of the We number and the maximum thickness of the steady film, which is affected by the surface wettability properties.