Study Shows Raindrops Push Oil Spill Pollutants into Air and Below Sea Surface

Splashes resulting from impact of a raindrop on a 30 micrometer oil slick (left) and a 400 micrometer oil slick (right). Splash behavior changes with increasing oil layer thickness, and more droplets are ejected as aerosol for the thicker oil slick. (Image credit:  David W. Murphy, Department of Mechanical Engineering, Johns Hopkins University)

(Click to enlarge) Splashes resulting from impact of a raindrop on a 30 micrometer oil slick (left) and a 400 micrometer oil slick (right). Splash behavior changes with increasing oil layer thickness, and more droplets are ejected as aerosol for the thicker oil slick. (Image credit: David W. Murphy, Department of Mechanical Engineering, Johns Hopkins University)

Scientists at Johns Hopkins University used high-speed imaging and digital holography in laboratory experiments to investigate the effects of raindrops falling on a simulated oil slick.

They observed an increase in the number of ejected droplets when the oil slick was introduced. The number of aerosolized oil droplets increased as the slick layer thickened, and slicks treated with dispersant released a subsurface plume of oil droplets in addition to aerosolizing drops. They published their findings in the Journal of Fluid Mechanics: Splash behaviour and oily marine aerosol production by raindrops impacting oil slicks.

This study extends previous research on how breaking waves eject water droplets into the air. The researchers examined the effects of raindrops on splash dynamics in varying oil layer thickness and oil properties. Using a tank filled partway with artificial seawater topped with Louisiana light sweet crude and Corexit 9500A mixtures, the team filmed splashes from simulated raindrops in slow motion (up to 20,000 frames per second). They analyzed the splash crown, subsurface cavity, and the size and distribution of airborne droplets.

Raindrops falling on oil and oil-dispersant layers resulted in oil-coated crowns that released fine oil droplets. The number of droplets increased by 56% at an oil layer thickness of 400 micrometers. Raindrops falling on this oil layer thickness produced two crowns, and the disintegration of the upper crown was responsible for the increase in aerosolized droplets. There was an additional 32% increase in droplets when dispersant was present. Thicker oil layers minimized the underlying water’s influence on splash behavior, preventing the raindrops from rupturing the oil-water interface and causing the crown to consist entirely of oil. Raindrops in all test cases resulted in subsurface cavities, but the cavities that formed in oil-dispersant mixtures released orders of magnitude more oil droplets into the water below than those without dispersant.

David Murphy, the lead author, explained the study’s implications: “When an oil spill creates a slick on the ocean surface, raindrops could potentially transfer that oil into the air where marine mammals and oil spill cleanup workers could breathe it in.” His team is continuing their work with other researchers at the Johns Hopkins School of Public Health and School of Medicine to study how raindrop splashes and breaking waves produce oily marine aerosol and how lung cell cultures react when exposed to these tiny oil droplets.

This video (above) shows the impact of raindrops on crude oil slicks of three different thicknesses (where h is the thickness). Researchers filmed these splash dynamics at 1000 frames per second; videos are played back at 5 frames per second for better visualization. Video credit: David W. Murphy, Department of Mechanical Engineering, Johns Hopkins University.

The study’s authors are David W. Murphy, Cheng Li, Vincent d’Albignac, David Morra, and Joseph Katz.

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This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Dispersion Research on Oil: Physics and Plankton Studies (DROPPS) consortium.

The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit http://gulfresearchinitiative.org/.

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