Dynamics of droplet-film collision

The head-on collision of a droplet onto a liquid layer of the same material, backed by a solid surface, was experimentally and computationally investigated, with emphasis on the transition from bouncing of the droplet to its absorption by the film for given droplet Weber number, We, and the film thickness scaled by the droplet radius, H-f. Experimental results show that while absorption is favoured with increasing We, there exists a range around H-f. approximate to 1 over which this tendency is moderated. This local moderation in turn corresponds to a regime, 11 less than or similar to We less than or similar to 14, over which increasing H-f from a small value leads to a triple reversal behaviour of absorption, bouncing, absorption again, and bouncing again. The collision dynamics including evolution of the surface contours of the droplet and film, as well as the energy budgets, were then simulated by using a front-tracking technique. For collisions leading to absorption and partial absorption, for which part of the absorbed droplet is subsequently ejected from the film, rupture and hence merging of the interfaces were manually imposed at an instant that leads to agreement between the subsequent calculated and experimental images. The simulation satisfactorily identified the different factors influencing the observed non-monotonic response of the collision event.