The role of air entrainment on the outcome of drop impact on a solid surface

The characteristic conditions causing spreading or splashing after drop impact on solid surfaces are considered together with the underlying mechanisms. To this end, the results of the various studies published over the past few years that have addressed the issue of splashing after droplet impact, specifically in terms of the definition of a splashing threshold, are critically compared and synthesized. The discussion aims at clarifying some of the conflicting findings. Information drawn from these considerations is used to distinguish between various splashing thresholds and it is shown that there exists a distinct difference between splashing on smooth and on rough surfaces, both in terms of the splashing thresholds and in terms of the mechanisms. Finally, a physical mechanism akin to air entrainment in dynamic wetting is proposed that may be of primary importance for the inception of splashing as well as fingering on smooth surfaces.