On the Breakup and Transport of Crude Oil by Surface Waves and Subsurface Plumes

The presentation summaries several laboratory studies aimed at characterizing interfacial phenomena affecting the breakup surface crude oil slicks and subsurface oil plumes. For crude oil slicks, where the breakup is dominated by turbulent shear, the droplet size distribution agrees with classical Weber number based Hinze scaling. Once entrained, the temporal evolution of the size distribution can be modeled by combining effects of turbulent diffusion and buoyant rise.  In contrast, mixing the crude oil with Corexit 9500 dispersant, which drastically reduces the oil-water interfacial tension, decreases the droplet sizes to the micron range in a phenomenon – tip streaming, that cannot be modeled based on turbulence length scales. Continued fragmentation of entrained droplets long after the wave breaking is also attributed to tip streaming. Aerosolization of oil is caused by the initial splash and by subsequent bubble bursting. Premixing the oil with dispersant increases the concentration of airborne nano-droplets by one to two orders of magnitude, raising health concern. In contrast, the dispersant causes a reduction in concentration of volatile organic compounds. Below the surface, fragmentation of a vertical buoyant oil jet is elucidated by refractive index matching. Compound droplets containing water droplets, some with smaller oil droplets, form regularly due to entrainment of water ligaments during the initial roll-up of K-H vortices. The fraction of compound droplets increases with droplet diameter, reaching 78% for 2mm droplets. While the oil droplets are deformed by the shear field, the interior interfaces remain spherical, consistent with prior studies of isolated compound droplets for relevant conditions. In the presence of cross flow, entrainment of small droplets into the core of the counter-rotating vortex pair defines the lower boundary of the plume while large droplets escape and define the upper boundary. Hence, dispersants alter the entire configuration of the plume, increasing the fraction of oil entrained into the vortex pair, and lowering the upper boundary of the plume.