Role of compressible air film entrapped during droplet impact on a solid surface

The impact of liquid droplets on solid surfaces occurs widely in the natural world and in various industrial applications such as falling raindrops, inkjet printing, spray coating and spray cooling. Before impact, as the droplet approaches the solid surface, a thin air layer is formed between them. The air in this thin region gets compressed as it cannot escape completely when the droplet comes closer to the surface. The high pressure of the compressed air layer deforms the bottom portion of the drop into a dimple shape, and a thin peripheral air disk surrounds this central dimple. The first liquid-solid contact occurs due to the collapse of this peripheral air disk. In this work, we explore the mechanism and the exact location of collapse using high-fidelity numerical simulations. We also monitor the evolution of the complete entrapment region using direct numerical simulation and quantitatively validate the air layer dynamics with existing experimental data.