A Cartesian diver to study oscillations and internal gravity waves in a stratified fluid

We propose a variation of the well-known Cartesian diver experiment where, instead of moving in a uniform fluid, the diver floats in a fluid stratified in density. In contrast to the original experiment, for a given external pressure the diver can stop in a stable equilibrium position within the fluid, at the depth where the surrounding density matches its own. By varying the applied pressure, the density of the diver changes and it moves until it reaches a new stable equilibrium condition at a different depth. When a sudden pressure pulse is applied, the diver, pushed off its equilibrium position, starts oscillating due to a restoring force that depends on the density gradient. The oscillations produce internal gravity waves that are typical of stratified fluids, when a portion of them is displaced and transmits its motion to the surrounding fluid. Although they are extremely difficult to observe, gravity waves are particularly interesting, as they typically occur in the atmosphere and in the stars. We propose a simple experiment and suggest a way to make the internal gravity waves visible. The experiment can be realized by students with easy-to-find household objects and used to improve their understanding of many concepts and laws of hydrodynamics, but also to introduce them to complex phenomena of general interest.