The low speed vertical entry of a sphere onto a two-phase fluid consisting of an oil lens floating on a water surface is examined in experiments using spheres with 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. We also observe a new phenomena: as air evacuates the necked region of the cavity, the oil coating the cavity forms an oil filament tethering the upper and lower air cavities together before eventually breaking up into satellite drops. For the experimental conditions in this study, 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.
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