When milk is poured into tea, swirling clouds emerge as the liquids mix, owing to the difference in densities of the two fluids. Systems like this, in which a dense fluid is placed on top of a less dense one, exhibit the Rayleigh–Taylor instability, which also leads to certain types of cloud formation and even the beautiful structures of nebulae. Writing in Nature, Benjamin Apffel and colleagues present an experiment that can bring a dynamic equilibrium to this instability. By using simple vertical oscillations, they levitate a volume of liquid and float a boat upside down under the fluid (pictured).
The experiment starts with a large bubble of air formed within a fluid. According to the Archimedes principle, an air bubble should float to the top of a liquid. However, vertically shaking the container can turn the Archimedes principle on its head and cause the bubble to sink. Using carefully calibrated oscillations, Apffel et al. pushed the bubble down and enlarged it to fill the width of the container. This led to a band of levitating liquid that has both an upper and a lower interface with air. The lighter air under the heavy liquid should lead to the Rayleigh–Taylor instability kicking in to mix the fluids, but the vertical oscillations hold the system in equilibrium.
The balance of the gravitational and buoyancy forces at both interfaces are identical but the potential energies are not the same. At the upper interface, the potential energy forms a minimum so that a boat can float comfortably as expected. In theory, the potential energy at the lower interface, where a denser fluid is above a less-dense fluid, would be maximum. However, Apffel et al. find that the vertical oscillations that allow the liquid to levitate above the air, cause the potential energy at the lower interface to be a minimum, so that a boat can float — upside down. This upside-down buoyancy effect is an unexpected feature of stabilizing the Rayleigh–Taylor instability. Harnessing this effect could lead to new experimental configurations to study other interface effects.
Apffel, B. et al. Floating under a levitating liquid. Nature 585, 48–52 (2020)