Cavity formation in the wake of falling spheres submerging into a stratified two-layer system of immiscible liquids

Abstract

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.

Inertial settling of a sphere through an interface. Part 1. From sphere flotation to wake fragmentation

“…In the deep-seal configuration, ripples due to the acoustic disturbance generated by the pinch-off have been observed to develop at the cavity surface (Grumstrup, Keller & Belmonte 2007). A similar phenomenon but with a different origin was recently reported by Tan et al (2016), who considered the impact of spheres coated with a thin oil film (obtained by using a two-layer configuration where the sphere first crosses an oil layer). In that case, ripples form before the cavity snaps, owing to an instability due to the shear at the oil-water interface.…”

Effect of ambient gas on cavity formation for sphere impacts on liquids

“…Mansoor et al also studied superhydrophobic spheres in detail and used a splash-guard mechanism to eliminate the phenomenon known as surface seal [17]. Related studies have considered projectiles with varying aspect ratios and impacts on a two-phase fluid [18][19][20].…”