New phenomena not previously documented in the available literature have been experimentally observed subsequent to the entry of falling steel spheres into a stratified system of a shallow layer of sunflower oil above a deep pool of water. Further experiments on similar sphere entries into sunflower oil demonstrated that these phenomena arose mainly as a result of the interaction between the surface of the spheres and the sunflower oil. It should be noted that the sunflower oil layer in the aforementioned two-layer system was relatively very thin compared to the dimensions of the spheres. Therefore, the experiments showed the substantial influence both the upper layer liquid and the surface conditions of the solid body could potentially have on the phenomena and cavity dynamics that arise as a result of solid entries into stratified two-layer systems of immiscible liquids.

We experimentally study the cavities forming in the wake of rigid spheres when submerging into a stratified, two-layer system of immiscible, quiescent liquids comprising a thin layer of oil above a deep pool of water. The results obtained for our two-layer system are compared with data from the literature for the corresponding type of cavities formed when spheres enter a homogeneous liquid that is not covered by an oil layer. The discussion and the data analysis reveal that the oil coating acquired by the spheres while propagating through the thin oil layer, before entering the pool of water underneath, substantially affects qualitative and quantitative aspects of the dynamics associated with the cavity formation. In particular, we observe the formation of a ripple-like pattern on the cavity walls which is not known to exist when spheres enter a homogeneous liquid. The data analysis suggests that the ripple patterns form as a consequence of a two-dimensional instability arising due to the shear between the oil layer coating the spheres and the ambient water.

A new phenomenon associated with the entry of solid spheres into a stratified, two-layer, highly viscous silicone oil-water system had been observed. Following the passage of solid spheres through the oil-water interface, the emergence of an air cavity from within the highly viscous oil thread connecting the spheres to the interface was observed. In addition, the data analysis show that a relatively thin layer of highly viscous upper-layer liquid could absorb significant impact while also significantly increase the drag experienced by relatively larger spheres moving in the less viscous lower-layer liquid.

The jet formation and deep seal phenomena following the inclined oil entry of rotating steel spheres were experimentally investigated. The results were compared with those obtained from vertical and non-rotating oil entry of the same spheres. It had been observed that the jet formation could be classified into two processes. First, a thin primary jet was formed immediately after deep seal. Second, the same jet became significantly thicker following the complete collapse of the air cavity at the oil surface. The inclined oil jet would gradually turn toward the vertical plane, while the angle between the primary jet and the quiescent oil surface was found to decrease when the Reynolds number of the spheres increased. The deep seal time was also independent of both linear and angular sphere velocities, while the vertical deep seal displacement increased with the Froude number.