Onsager's reciprocity relations for the Soret and Dufour effects

Abstract

The Onsager reciprocity relations for the Soret and Dufour effects are shown to hold true in a binary mixture of inert dilute gases only if the diffusive force is appropriately chosen. This statement follows from the solution of the Boltzmann equations for the mixture via the Chapman-Enskog method to first order in Knudsen's parameter. We also show that if the gradients in the chemical potentials are taken as diffusive forces, the linear relations hold true, but Onsager's relations are not obeyed.