Slowing-down of non-equilibrium concentration fluctuations in confinement

TL;DR

This paper investigates large-scale non-equilibrium concentration fluctuations in confined fluids, revealing a dramatic slowing-down of their dynamics and a diffusion coefficient influenced by the solutal Rayleigh number, through experiments, theory, and simulations.

Contribution

It provides experimental evidence and theoretical understanding of the slowed dynamics of large-scale fluctuations under confinement, extending knowledge of non-equilibrium fluctuation behavior.

Findings

Large-scale fluctuations exhibit diffusive behavior with a confinement-dependent diffusion coefficient.

A dramatic slowing-down of fluctuation dynamics is observed in strongly confined systems.

Excellent agreement between experiments, theory, and simulations confirms the findings.

Abstract

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.