Propagating modes in a binary liquid mixture under thermal stress

8 pags., 4 figs.
Nonequilibrium temperature and concentration fluctuations inside a binary liquid mixture under the action of a temperature gradient relax back to equilibrium either due to conduction and diffusion at large wave numbers, or due to the quenching determined by gravity at small wave numbers. We investigate the dynamics of nonequilibrium fluctuations in a binary liquid mixture of polystyrene and toluene heated from above under stationary conditions in a thermodiffusion experiment. We show that the strong gravitational stabilization at small wave numbers determines the appearance of propagating modes of nonequilibrium fluctuations as detected through the structure function of shadowgraph images. The propagating modes are the combined effect of temperature and velocity nonequilibrium fluctuations induced by the buoyancy force. The experimental results are in good agreement with a fluctuating hydrodynamics theroretical model including the coupling of fluctuations of velocity, temperature and concentration.
We gratefully acknowledge support from the European Space Agency (ESA) and the Centre National d’Etudes Spatiales (CNES) of France, specifically for supporting the acquisition of the fast camera that made it possible to observe the propagating mode described in this paper. F.C., L.G.F., and H.B. would like to thank E2S-UPPA for financial support. The research at the Complutense University was supported by Grant No. ESP2017-83544-C3-2-P of the Spanish Agencia Estatal de Investigación.