Water drop impact on oil layer floating on water pool

The occurrence of oil slicks on water surfaces is common in various aquatic environments. Raindrops impact can potentially contribute to the transport of oil slicks to the atmosphere and deep water, leading to more widespread and persistent contamination. This experimental study investigates the impact of a water drop on an oil layer floating on a water pool, aiming to reveal the mesoscale mechanisms and characteristics of oil transport. The experiment identifies secondary droplets and oil-encapsulated water (O-E-W) particles, which elucidate how oil slicks are transported into atmosphere and deep water. A regime map with thresholds is established to predict if and how oil slicks would be transported. The formation mechanisms and morphological variations of O-E-W particles are clarified, showing that their number is proportional to the maximum crater depth. The amount of oil volume carried by O-E-W particles generally increases with the impact Weber number, reaching up to two times the impact drop volume. Secondary droplets are generated via central jet or crown splash. A crown splash can generate nearly 600 secondary droplets, with diameters smaller than 0.24 times the impact drop and velocities exceeding two times the impact velocity. Large-sized drops impacting thin and low-viscosity oil layers at high speeds promote the generation of greater numbers of high-velocity and small-sized secondary droplets and more numerous and diverse O-E-W particles, indicating increasing oil transport capacity and environmental risks. These results enhance the understanding of the physical dynamics of drop impact, contributing to the assessment of oil slicks pollution caused by rainfall.