Experiments on the low-Reynolds-number settling of a sphere through a fluid interface

The low-Reynolds-number gravitational settling of a sphere through a fluid interface is investigated experimentally. By varying the viscosity ratio between the two fluids and the Bond number, two different modes of interfacial deformation are observed: a tailing mode and a film drainage mode. In the tailing mode, the interface deforms significantly as the sphere approaches, and the sphere becomes enveloped by a layer of the upper fluid. A tail forms, connecting the sphere to the bulk of the upper phase. In the film drainage mode, the interface deforms much less and the sphere impacts onto the interface, which either ruptures to form a contact line on the sphere or leaves a very thin wetting film. Additionally, two types of sinking profiles are observed: steady sinking, where the sphere velocity changes monotonically as it sinks, and stalled sinking, where the sphere's progress is inhibited by the interface, before it accelerates into the lower fluid. We present a regime diagram showing the different behaviors. Finally, the dependence of the sinking time on the Bond number and viscosity ratio is investigated. For the film drainage regime a simple scaling law is deduced; the tailing regime exhibits more complicated dynamics, possibly explained by a multistage sinking process.