Behavior of oil droplets under the influence of counter-rotating vortices in a Langmuir cell tank

Langmuir circulation (LC), a form of wind shear-driven turbulence in the upper ocean layer, can enhance vertical mixing of positively or negatively buoyant particles like pollutants and organisms in the water column. During oil spills, LC can facilitate underwater entrapment and dispersion of oil droplets generated from crude oil slicks and facilitate oil/sediment interaction, especially in shallow waters. However, experimental knowledge on the influence of LC on the behavior of submerged oil droplets is limited. Using a 1×0.2×0.5 m³ Langmuir tank facility that can repeatably inject an oil jet in a counter-rotating vortex pair, we study the flow and resulting oil droplet behavior. The flow field in the Langmuir tank is characterized using 2D particle image velocimetry. The central downwelling region has maximum velocity of 14 to 26 mm/s and turbulence kinetic energy of 6.52E-5 to 1.16E-4 (m/s)². The spatiotemporal oil droplet size distribution is characterized using multiple high-speed cameras viewing different regions of the tank and a droplet detection algorithm. Tests using dyed vegetable oil showed that oil droplets with ~500 µm diameter can be retained underwater for more than 10 min after injection and that retained oil droplet size positively correlates with LC strength.