Oil droplet collisions with marine snow: effect of manipulating droplet size

Solid particle aggregates in the ocean, such as marine snow, can scavenge oil droplets as they are transported in the ocean, resulting in the removal of oil from the water column. Often, chemical dispersant is applied to oil spills to manipulate the oil droplet size; we study how such manipulations affect the rate at which oil is removed from the water column by collision with marine snow. We model the collision process using the particle pair methodology. Three dominant collision mechanisms are considered for particle pairs in the ocean environment: turbulent shear, differential settling and Brownian motion. A comparison of the removal rate of oil from the water column for large and small droplets size is conducted at constant volume fraction. The results of the study show that, for a constant volume of oil, droplet size does alter the amount of oil removed from the water column during collisions with marine snow, and that a greater amount of oil is removed when the droplets are large. This finding holds regardless of which collision mechanism is considered. Of the three mechanisms, differential settling results in the largest constant-volume removal rate (since oil droplets rise while marine floc settle downward) while Brownian diffusion results in the lowest removal rate. These finding suggest that using chemical dispersant on deep-sea oil spills to reduce droplet size will reduce the total volume of oil that becomes attached to marine snow and reduce the amount removed from the water column by this mechanism.