Giraudet, CédricBataller, HenriSun, YifeiDonev, AleksanderOrtiz De Zárate Leira, José MaríaCroccolo, Fabrizio2023-06-182023-06-182015-090295-507510.1209/0295-5075/111/60013https://hdl.handle.net/20.500.14352/24282© European Physical Society. We acknowledge fruitful discussions with ALBERTO VAILATI, DORIANO BROGIOLI, ROBERTO CERBINO and JAN SENGERS. JMOZ acknowledges support from the UCM-Santander Research Grant PR6/13-18867 during a sabbatical leave in Anglet. AD was supported in part by the U.S. National Science Foundation under grant DMS-1115341 and the Office of Science of the U.S. Department of Energy through Early Career award No. DE-SC0008271.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 non-equilibrium 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.engjournal articlehttp://dx.doi.org/10.1209/0295-5075/111/60013http://iopscience.iop.org/http://arxiv.org/pdf/1410.6524open access536Liquid-mixturesFree diffusionGravityFluidsSoretMicrogravityShadowgraphScatteringSchlierenTermodinámica2213 Termodinámica