Investigating molecular diffusion in liquids with a Cartesian diver

We propose a variation of the classic Cartesian diver experiment in which the diver moves within a fluid which is density stratified rather than uniform. At variance with the traditional setup, the diver reaches a stable equilibrium at a depth where its density matches that of the surrounding fluid under a given external pressure. By modifying the applied pressure, the diver’s density changes, causing it to shift to a new equilibrium depth. When exposed to a sudden pressure pulse, its density fluctuates, leading to oscillations driven by a restoring force, with the oscillation frequency determined by the density gradient. We show that if the experiment is prepared by putting in contact two miscible liquids with different density and the diver is put at the interface of the liquids, the device can be used to monitor the variation of the density gradient along the diffusion process. Experimental data can be compared with those theoretically predicted by the diffusion equation.