Impact of superhydrophobic sphere onto a pool covered by oil layer

Abstract

We experimentally investigate the impact of a millimetric superhydrophobic sphere on a water pool covered by a thin oil layer, with the aim of seeking the critical conditions for sphere entrapment at the interfaces. The interfacial tension and viscosity of the thin oil layer are found to have a significant effect on the fate of the impacting spheres that are denser than the liquids: sinking or floating. For the oil layer of low viscosity, the impact dynamic is dominated by the capillary force, and the sphere experiences more or less uniform acceleration after the impact, which is similar to a sphere impacting onto a pure water pool. For the oil layer of relatively high viscosity, the viscous dissipation inside the thin oil layer greatly hinders the descending of the sphere, and thus, it is the viscosity of the oil layer that dictates the acceleration process of the spheres at the early stage of impact. At the late stage, the sphere moves very slowly under water (particularly at the onset of sinking), and the competition between the oil–water interfacial tension and buoyancy determines whether the sphere would eventually sink or float. We then conduct the theoretical analysis of the dynamic processes of the impacting sphere and give the theoretical predictions of the respective critical conditions, which agree well with the experimental observations.