A comparative study of the immiscibility effect on liquid drop impacting onto very thin films

The liquid drop impacting onto a very thin film is investigated experimentally through time-resolved shadowgraph images for both miscible and immiscible liquids. Regime maps for five impact morphologies, including deposition, crown formation, crown breakdown, crown-type splashing, and crown-breakdown splashing, are reported and compared comprehensively. Besides the thresholds of splashing/non-splashing regimes, the divisions between crown-type and crown-breakdown splashing are also figured out. It is found that the immiscibility can promote splashing, especially the crown-breakdown type, mainly due to the interfacial instability. The crown-breakdown splashing can be suppressed by increasing film viscosity in immiscible systems, while the viscosity effect is not significant in other test cases. The statistics of crown and secondary droplets are measured and analyzed. It is found that the immiscibility can enlarge the crown diameter but decrease the height, while the effect of viscosity on the crown structure is minor. When the film viscosity is similar, more but smaller secondary droplets are generated in immiscible systems. With the increase in film viscosity, larger but less secondary droplets are observed in both miscible and immiscible systems.