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, larger-scale
fluctuations live shorter because of gravity, as theoretically and
experimentally well-known. We explore here 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 in their dynamics. We
recover diffusive behaviour 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. The study of the dynamics of non-equilibrium
fluctuations allows to probe and measure the competition of physical processes
such as diffusion, buoyancy and confinement.Comment: Includes see Supplementary Material. Submitted to PR
