Diffusion of particles or molecules in a fluid is an essential manifestation of thermal energy. It is seen in the familiar brownian motion of dust particles in a fluid or a gas, and it ensures the mixing of different molecules in a fluid. So mixing, at the shortest length scales, results from diffusion rather than convection. This is behind a standard method for measuring molecular diffusion coefficients: a sharp concentration gradient is established between two fluids, and the decay of this gradient as the two fluids mix determines the diffusion coefficient of one fluid in the second. Observers look in a direction perpendicular to the gradient (that is, with the interface edge-on), and the results are interpreted assuming a smooth relaxation of the concentration gradient. But that may not be valid: on page 262of this issue1, a team from Milan report unexpected spatial fluctuations in the concentration of two fluids mixing by diffusion.
The authors, Vailati and Giglio, take the unorthodox approach of looking along the direction of the gradient, so that no large-scale variation from the main concentration profile is seen. Rather than the expected uniform variation in concentration, they observe huge spatial fluctuations of the concentration perpendicular to the gradient. These fluctuations develop rapidly, and persist until the gradient has been completely relaxed. It is only by looking from this new perspective that they can observe these surprising fluctuations.
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