On the notion of thermophoretic velocity
Résumé
The phenomenology of thermal diffusion of a particle in a solvent involves the Soret coefficient of the particle/solvent mixture. It is usually considered that the migration velocity of the particle in the solvent is proportional to the Soret coefficient. We show here that this view is wrong because ordinary diffusion contributes to migration if the solvent is not homogeneous. We examine several examples (Na Cl in water, polystyrene in ethylbenzene, maghemite nanoparticules) to show that this contribution can be strong enough to change the sign of the Soret coefficient and vitiate the interpretation of experimental data, the measurement of the Soret coefficient, the comparison of experiment with theory, and inter-experimental comparisons.
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