Effects of oil properties on the aerosolization of crude oil-dispersant slicks during bubble bursting

This study examines the effect of dispersant and oil properties on the aerosolization of fresh and weathered crude oil slicks by bursting of a plume of ~0.7 mm bubbles. A Scanning Mobility Particle Sizer measures the size distribution of aerosols in the 20-400 nm range in a clean air chamber. The 500 μm thick slicks contain oils of varying origin, viscosity, interfacial tension, and weathering state. Test are performed with and without pre-mixed dispersant (Corexit 9500A), which reduces the oil-seawater interfacial tension by two orders of magnitude at a dispersant-to-oil ratio of 1:25. The nano-aerosol concentration decreases for slicks without dispersant, but increases upon introduction of dispersant. For most cases, the concentration increases with decreasing Capillary or Morton numbers. To explain the origin of nano-droplets, we show that prior to bubble injection, even mild agitation of the oil-dispersant interface generates a subsurface cloud of nano-droplets that diffuses away from the interface. This process appears to be caused by thermal capillary instability when the interfacial tension is low enough to increase the thermal length scale to a few nm. Only bubbles larger than 0.5 mm, which form film droplets upon bursting, contribute to the increase in nano-aerosols.