Sphere entry through an oil lens floating on water

The low speed entry of a sphere onto a two-phase fluid consisting of an oil lens floating on a water surface is examined in experiments with spheres of different radii, densities and materials. Oil coats both the leading edge of the sphere as it penetrates the free surface of the two-phase fluid and the wall of the air-entraining cavity that forms behind the descending sphere. Spheres with lower inertia form smooth cavities whereas spheres with higher inertia develop a three-dimensional crumpled morphology along the cavity wall due to a shear-induced instability between the oil layer and surrounding water near the sphere front. Despite these different dynamics the sphere depth at collapse, either by deep seal or rupture, scales logarithmically with sphere mass for all of the spheres examined. We also observe a new phenomena: as air evacuates the necked region of the cavity, the oil coating the cavity forms an oil filament that tethers the two disjoint air cavities together before eventually breaking up into satellite drops. We find that the oil lens at the free surface is critical to forming an air-entraining cavity; with no oil lens only a small air pocket forms as the water completely wets the spheres either sealing the free surface or forming a quasi-static cavity.