Splash formation and cavity dynamics of sphere entry through a viscous liquid resting on the water

The splash formation and cavity dynamics during the entry of an object into a single-phase liquid system have been investigated considerably in the past, but only a few studies focus on such phenomena in a two-layer liquid system. In the current study, we present an experimental investigation of the entry of a falling sphere into a liquid system consisting of a deep pool of water and a thin layer of dimethicone resting on the water surface. To highlight and support the present findings, experiments of the same sphere entering water were performed and direct comparison were made. As anticipated, the upper-layer viscous liquid introduces a significant change in the splash formation and the differences in behavior are characterized by layered films and wave-like instability structures. In the experiments of the two-layer liquid system, observations show that the early sealing of the splash and the generation of a larger-scale splash fingers are strongly associated with large Froude number, Fr = V0(gD)-1/2 (where V0 is the impact velocity at the still free surface, g is the gravitational acceleration and D is the diameter of the sphere). In addition, the dimensionless cavity depth, the pinch-off depth and the pull-away cavity length behind the sphere are found to be in linear relationship with the Froude number for both water-entry cases and water-dimethicone-entry cases. One of the most interesting results is that the pinch-off time is almost independent of the Froude number. The highly viscous liquid in the upper layer is shown to be critical in determining the cavity dynamics.