Characterization of splashing and regime thresholds for oblique droplet impact on thin wall films

This study experimentally investigates oblique droplet impacts on thin wall films, providing new insights into crown formation, splashing types, and threshold modelling. Different splashing types were identified by varying the impact angle $\alpha$ and Weber number $We$, keeping the film thickness $\delta =0.22$ and fluid (isopropanol) constant. The splashing types include secondary droplet ejection only from the front or only from the sides of the crown, splashing caused by crown-film interaction, and 90°-like splashing. Combinations of these mechanisms, such as simultaneous front and side splashing, were also observed, as their triggering mechanisms are independent and allow for a superposition of regimes. A regime map and threshold formulations were developed to describe the distinct splashing types. The front splashing limit lies at $cos\left(\alpha \right)We=128$, which reveals that the $cos\left(\alpha \right)$ is the relevant physical quantity beside the Weber number. Side splashing depends solely on the wall-normal Weber number ($W{e}_n=425$). Crown-film interactions occur below $\alpha <60°$ if the Weber number is high enough, while splashing similar to that seen when $\alpha =90°$ is observed if the impact angle exceeds $\alpha >81°$.