Wave propagation in a stratified fluid ---From mesoscopic to macroscopic length scales.

A stratified fluid can be generated in different ways, either starting from the initial condition of a homogeneous binary mixture stressed by a temperature gradient or the one where a solid is made to dissolve in a fluid at the starting time. Both these cases can generate density profiles evolving with time and are thus subject to non-equilibrium (NE) conditions. We have performed experiments where we investigated the evolution of NE concentration fluctuations at different wavelengths and we have observed propagating modes occurring at the mesoscale. We have also performed experiments by introducing a macroscopic object able to levitate in a density gradient, a configuration where propagating waves are also observed, but at the macroscopic scale. Both phenomena are well described by the Brunt-V\"{a}is\"{a}l\"$a$ oscillations that take place in stratified fluids, such as the atmosphere and stars.