Capillary surface wave formation and mixing of miscible liquids during droplet impact onto a liquid film

In order to advance the understanding of the process of droplet impact on wet surfaces, realized in various applications such as droplet-based coating methods (inkjet printing, aerosol-jet, and spray coating), we studied the impact of a dyed water droplet onto a clear water film. The color contrast in images allowed investigation of mixing process of the like liquids during the rapid dynamic stage and beyond. Four Weber numbers (We), in the range of 121–304, and four dimensionless film thickness to droplet diameter ratios (h*), in the range of 0.092–0.367, were considered. The aforementioned numbers correspond to the film thickness of 0.4–1.6 mm, droplet size of 4.36 mm, and impact velocity of 1.4–2.2 m/s. While the experimental database is rather comprehensive and can be used for further detailed analysis, here we focused on less-explored topics of capillary surface waves formed outside the crater and found the wave characteristics and their role in mixing. Within the range of parameters studied here, we found that the outer capillary surface waves were created as a result of perturbing the liquid film by droplet impact, but the wave characteristics such as frequency (400-500 Hz) were not a strong function of the impact We number. We also observed six mixing mechanisms of miscible liquids, including the expansion/compression waves and turbulence created upon impact, stable crown wall formation with an acute wall angle, which causes a tsunami-type of flow, unstable crown leading to fingering and splashing, capillary waves, and molecular diffusion.