Experimental comparisons between droplet-droplet collision and single-droplet impacts on a solid surface

The behavior of droplet-droplet collision on dry and wet solid surfaces was studied experimentally. We show that the well-documented similarity between droplet-droplet collision on a solid surface and a single-droplet impact on a dry surface is limited to low impact velocities, while the similarity between droplet-droplet collision on a solid surface and a single-droplet impact on a wet surface is limited to high impact velocities. The major phenomenological differences between the above two cases, in terms of morphology, spreading behavior, secondary droplet formation, different crown behavior, and the formation of a central (Worthington) jet, are discussed here in detail.