Regimes of drop morphology in oblique impact on deep fluids

Abstract

Oblique impact of single drops on deep fluids at various impact angles $\alpha $ is studied for a wide Weber number range (15$\,{\le}\,$We$\,{\le}\,$249), where $\alpha $ is varied between 5.4$^\circ$ and 64.4$^\circ$ with respect to the target surface, and drop and target consist of the same fluid (water/glycerol). In the range of studied parameters, coalescence always takes place. Below a critical level of the Weber number We$_{N}$ formed with the normal velocity component, We$_{N}\,{<}\, 10$, capillary waves are produced on the drop and target surface in the first stage after impact. For We$_{N}\,{>}\, 10$, a lamella is ejected from the target surface and drop volume for which both experimental and numerical evidence is given. For We$\,{<}\, 140$ and $\alpha \,{<}\, 23^\circ$, the drop spreads on the target surface with no visible immersion of drop fluid into the target fluid. Outside this range, a partial immersion of drop fluid can be seen: for We$_{N}\,{>}\,$10, it occurs in the front part of the spreading drop; for $\alpha \,{>}\,$23$^\circ$, in the rear part of the impact crater. The spreading patterns of the drop fluid are visualized with dyed drop fluid once it has come to rest after impact. In many cases, patterns of high complexity and strong sensitivity to the impact conditions can be seen which are very reproducible for similar impact conditions. The mechanisms underlying capillary wave or lamella formation and the partial immersions of drop fluid are discussed and their relevance for pattern formation is taken into account.