Volume entrained in the wake of a disc intruding into an oil-water interface

TL;DR

This study experimentally investigates the volume of fluid entrained across an oil-water interface by a moving disc, revealing universal behavior influenced by gravity and vortex dynamics, with implications for understanding fluid entrainment phenomena.

Contribution

It demonstrates the universal behavior of fluid entrainment in oil-water interfaces and highlights the role of vortices beyond potential flow predictions.

Findings

Gravity influences interface deformation due to density differences.

Universal behavior exists within a specific time window.

Vortex dynamics are key to explaining deviations from potential flow.

Abstract

An object moving through a plane interface into a fluid deforms the interface in such a way that fluid from one side of the interface is entrained into the other side, a phenomenon known as Darwin's drift. We investigate this phenomenon experimentally using a disc which is started exactly at the interface of two immiscible fluids, namely oil and water. First, we observe that due to the density difference between the two fluids the deformation of the interface is influenced by gravity, and show that there exits a time window of universal behavior. Secondly, we show by comparing with boundary integral simulations that, even though the deformation is universal, our results cannot be fully explained by potential flow solutions. We attribute this difference to the starting vortex, which is created in the wake of the disc. Universal behavior is preserved, however, because the size and strength of the vortex shows the same universality as the potential flow solution.