How Rainfall Impacts Oil Slicks

Water drop impacts cause macroscopic effects that are quite common in the natural world, from seed transportation to soil erosion. Unfortunately, rainfall can also exacerbate oil slick pollution — in the ocean and in the atmosphere. Although drop impact has been studied extensively, little is known about the physical dynamics of such impacts on oil.

Zeng et al. conducted an investigation into the understudied mechanisms at play between water and oil. The researchers used a liquid pool with silicone oil on its surface, a simple waterdrop generation setup, and a high-speed visualization system, then experimented with drop volumes and velocities.

“We identified two key phenomena: the generation of secondary droplets above the oil layer and the formation of oil-encapsulated water (O-E-W) particles below the oil layer,” said author Shangtuo Qian.

The secondary droplets, predominantly from crown splash, numbered as high as nearly 600, with diameters less than a quarter than that of the water drop, and with velocities more than twice the initial water drop’s impact. Meanwhile, the oil volume of O-E-W drops reached up to nearly twice the impact drop volume.

“The simultaneous upward and downward transport mechanisms significantly enhance oil slick spread,” said Qian. “Oil-containing droplets contribute to oily aerosols in the atmosphere, while O-E-W particles create an easily overlooked pathway for putting oil in the water column.”

The team’s findings highlight the importance of addressing oil slicks in areas prone to heavy rainfall, and the researchers hope their work will inspire strategies for responding to oil spills.

“Radar-based forecasts could help prioritize cleanup efforts, enabling rapid mechanical recovery methods to minimize oil slick exposure before heavy rain intensifies the problem,” said Qian.

Source: “Water drop impact on oil layer floating on water pool,” by Qin Zeng, Shangtuo Qian, David Z. Zhu, Kan Kan, Jiangang Feng, and Hui Xu, Physics of Fluids (2025). The article can be accessed at https://doi.org/10.1063/5.0285833 .