Hollow droplet impact on a solid surface

Abstract

Droplet impact on a solid surface occurs in many natural phenomena and industrial applications such as raindrop impact on the ground, drip irrigation, spray cooling, spray coating, inkjet printing, etc. Importantly, in several cases such as aerosol transfer from the sea, oxygen dissolution, and thermally sprayed coatings the impinging droplets contain air bubble dealing with liquid-bubble-solid interaction. These hollow droplets upon impacting on the target surface, due to their controllable weight and embedded medium, control the splat properties by triggering the cavitation and releasing the inner substance. To better understand the flattening process of a hollow droplet, in this work a comprehensive experimental and numerical study is performed on water and glycerol droplets impacting on a rigid surface. The experiments are repeated on different types of surfaces including aluminum, sand-blasted steel, and superhydrophobic. The results show that the mechanisms of the post-impact process of hollow droplets are different from those of dense droplets in several aspects. We study the role of surface wettability, liquid properties, and impact velocity on the droplet flattening process. In the numerical work, we compare pressure contours and velocity vectors for dense and hollow droplets that provide an in-depth insight through the formation of splat and counter jets.