Eur. Phys. Lett. 111, 60013 (2015)

http://iopscience.iop.org/article/10.1209/0295-5075/111/60013

Slowing-down of non-equilibrium fluctuations in confinement

Cédric Giraudet, Henri Bataller, Yifei Sun, Aleksandar Donev, José Maria Ortiz de Zárate and Fabrizio Croccolo

Fluctuations in a fluid are strongly affected by the presence of a macroscopic gradient making them long-ranged and enhancing their amplitude. While small-scale fluctuations exhibit diffusive lifetimes, moderate-scale fluctuations live shorter because of gravity. In this letter we

explore fluctuations of even larger size, comparable to the extent of the system in the direction of the gradient, and find experimental evidence of a dramatic slowing-down of their dynamics. We recover diffusive behavior for these strongly confined fluctuations, but with a diffusion coefficient

that depends on the solutal Rayleigh number. Results from dynamic shadowgraph experiments are complemented by theoretical calculations and numerical simulations based on fluctuating hydrodynamics, and excellent agreement is found. Hence, the study of the dynamics of nonequilibrium

fluctuations allows to probe and measure the competition of physical processes such as diffusion, buoyancy and confinement, i.e. the ingredients included in the Rayleigh number, which is the control parameter of our system.