Retention of oil droplets rising in a stratified fluid

Vertical density stratification within the ocean has been shown to be one of the main factors in trapping oil plumes and dispersed hydrocarbons. Beyond oil spills, the behavior of droplets or particles in stratification is of interest in many other areas of environmental fluid mechanics, with applications in the movement of marine snow and bubbles in the ocean, as well as the fate of pollutants, pollen, dust, or volcanic ashes in the atmosphere. However, the dynamics at the scale of a single droplet or particle in stratification are not well understood, and remain an active area of research.

This work aims to quantify and explain retention of an oil droplet rising through a transition between two homogeneous-density fluids. Using laboratory experiments, we examined droplet behavior for a range of droplet densities, droplet sizes, and ambient stratification profiles. A droplet is significantly slowed by its interaction with the density transition over a characteristic timescale, which coincides with the decay of a trailing column of entrained denser fluid. These dynamics are independent of the far-field nature of the droplet’s wake.

The timescale over which fluid is entrained and the droplet is delayed at the density transition was found to be dependent on the droplet Froude number. Significant retention only occurred for Fr < 1, suggesting that retention is primarily a function of the ratio of the buoyancy timescale (1/N) to the inertial timescale of the droplet (d/U), and that in this regime, trapping dynamics at the scale of a single droplet are dominated by the effects of stratification.