Abstract:

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 262 of this issue, a team from Milan report unexpected spatial fluctuations in the concentration of two fluids mixing by diffusion.

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