  Recent thermophoretic experiments on colloidal suspensions revived an old
debate, namely whether the Soret effect is properly described by thermostatics,
or necessarily requires non-equilibrium thermodynamics. Based on colloidal
transport theory and the entropy production of the related viscous flow, our
analysis leads to the conclusion that the equilibrium approach may work for
small ions, yet fails for colloidal particles and polymers. Regarding binary
molecular mixtures, our results shed some doubt on the validity of thermostatic
approaches that derive the Soret coefficient from equilibrium potentials.
