Dynamics of oil - water system
Understanding the physics of gravity-induced oil-water separation occurring underneath a surface slick after an oil spill is essential to predicting the slick dynamics and the efficacy of different treatment methodologies. When an oil droplet ascends towards an oil slick, it is stopped by a layer of water separating it from the slick. High-speed holographic imaging shows that as this water layer begins to break up and recede, it leaves behind a very thin continuous water film, as confirmed by planar laser-induced fluorescence. This thin film engulfs the droplet, preventing it from mixing with the bulk oil, for a duration that is 3-4 orders of magnitude longer than the droplet crossing process. The formation of this film has been confirmed for a series of oil and waters, including refractive index-matched sugar water and silicone oil, as well as millipore pure water with silicone oil and hexadecane. Hence, the thin film is an inherent property of the oil-water interface and does not depend on the presence of surfactants. After crossing, the water-coated droplet slowly spreads along the oil-water interface, causing the generation of surface kinks, where the film eventually breaks up into a cloud of sub micron droplets. This slow process is driven by electrostatic attraction between the film segments and the bulk water close to the water-oil interface. The time scales of the entire process, from crossing to the eventual mixing increase with the viscosity of the fluids involved, from seconds for 1 cSt oil to nearly one hour for 50 cSt oil. When multiple droplets cross the interface, they form a layer that does not mix with the bulk oil containing various segments of the thin films, presumably affecting the entire slick dynamics.