On bouncing and absorption of drop impacting liquid film

Drop impact on liquid surfaces is ubiquitous in many natural and industrial processes, such as soil erosion through rain drop impact, ink-jet printing, and fuel spray impacting the cylinder walls in direct-injection internal combustion engines. In this study the impact outcome of either bouncing or absorption was first experimentally mapped in the H∗-W e space, where H∗ is the liquid thickness normalized by the drop radius, and W e the drop Weber number. It is observed that there exists a critical W e_cr beyond which the drop always merges with the liquid film. However, for smaller values than W e_cr, the impact exhibits a nonmonotonic transition with increasing film thickness, from absorption to bouncing to absorption and finally to bouncing again. Using side-view shadowgraphy to observe the penetration process below the liquid surface, and through analysis of surface deformation for both the drop and film, the controlling mechanisms of these nonmonotonic transitions are identified, leading to the derivation of the scaling laws for the transition boundaries. These results provide useful insight into the role of the bottom surface and the film thickness on the impact dynamics and outcome.