Transient stratified force on particle crossing a density interface

What is about really? it is about the most peculiar phenomenon we have encountered - a small particle sinks in a dense liquid pretty slow - buoyancy and viscosity work against it. If we drop a particle in a lighter fluid - it will sink faster, as its density is higher in respect to that of the fluid and therefore gravity wins. But, when the particle crosses from one liquid to another (without differences in surface tension, viscosity, etc.) a peculiar thing happens - some light fluid that is surrounding the particle, crosses the interface together with the particle and starts acting as a parachute - pulling the particle upwards, back to the original liquid layer. This is a transient process, i.e. it is only until all the light fluid leaves the particle and returns to its original layer. But this time is crucial - the particle slows so much so the force balance changes and then particle can stop. It can even levitate, i.e. jump back upwards, against the direction of the steady state forces that are supposed to bring it down to the bottom. We were amazed to see it in the lab. We could not believe it is true. We compared it with theory and found that the theory cannot explain what we see - the parameters are not right. So we had to dig deeper. And we are still working on it. The most exciting part is that it should work in turbulent environment as well. Let's see. What is it about?Oil spills create millions of small oil droplets that float up to the surface because oil is lighter as compared to the sea water. If the droplets can reach the surface, we could clean them up. Unfortunately, if the oil droplet on the way upwards meets a density interface of a warmer/less salty sea water body on top of a colder/more salty sea water, they will slow down so much by the force we present here that they will simply stop and collect in large concentrations. This changes the situation for the natural habitat and physical processes (heating, mixing, etc.) Why is it important?Understanding how fast particles (solid particles, microplastics, oil droplets) cross interfaces between liquids of different densities is important for natural, environmental and engineering flows. Marine snow, swimming plankton, mixture of oil and water just a few to mention.