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Convection Effects in Diffusion Experiments

Convection effects are a well known problem in diffusion experiments on low viscous liquid samples. The convection induced motion of the molecules can sometimes override the effect of the diffusive, Brownian motion, which makes diffusion measurements impossible. Particularly bad in laboratory practice are the cases, where convective motion obscures the pure diffusion effect while being not recognised and thus leading to wrong diffusion coefficients.

The reason for the convection is a temperature gradient in the probe. The temperature gradient leads to an inverted density distribution in the sample, which itself leads to compensation flow currents. This effect is particularly strong in probes using water cooling for the gradient system. The cool inner surface of the gradient system couples via radiation and air convection to the sample. The temperature controlling gas is cooled during its way up.