Effect of surface irregularities on sphere settling through a liquid-liquid interface

In this study, we investigate experimentally how surface irregularities will affect the gravitational settling of a sphere through an interface of two immiscible fluids. We consider different configurations of sharp spikes of varying length scale and radii of curvature uniformly distributed over the sphere surface. Spiky spheres of different size and density fall through a layer of silicone oil at their terminal velocity and approach an interface above an aqueous solution. Depending on the solid-liquid density ratio and the Bond number, a smooth sphere may either float or sink, entraining and trapping a volume of lighter silicone oil into the heavier aqueous solution. It is hypothesized that for an appropriate range of length scale and radius of curvature(sharpness), the spikes will break the thin film of oil surrounding the sphere and induce sinking. To investigate the dynamics in detail, the sphere motion is quantified using high-speed imaging. The parameter space includes the Bond number (based on liquid-liquid density difference and the sphere radius) in the range of 0.2-4 and fluid-fluid viscosity ratios in the range of 0.05-15.