The Ludwig–Soret effect and stochastic processes

New general stochastic models of thermodiffusion are proposed, which include velocity-dependent friction coefficient and noise tensor. These coefficients are computed exactly from the Boltzmann equation for the particular case of thermodiffusion in dilute gas mixtures. The Soret coefficients predicted by the new thermodiffusion models are computed via a Chapman–Enskog expansion and compared favorably to predictions of earlier models. In particular, the new models can accommodate for Soret coefficients of both signs, as observed experimentally.