Do thermal diffusion and Dufour coefficients satisfy Onsager's reciprocity relation?

TL;DR

This paper questions the common assumption that thermal diffusion and Dufour coefficients in liquids always satisfy Onsager's reciprocity, providing examples where this may not hold and highlighting unresolved issues in molecular mixtures.

Contribution

It challenges the general validity of Onsager's reciprocity for thermal diffusion and Dufour coefficients, using physical examples and discussing experimental uncertainties.

Findings

In liquids, $D_T$ and $D_F$ do not always satisfy Onsager's reciprocity.

Micellar solutions and colloidal suspensions show cases where reciprocity fails.

Experimental and simulation data for molecular mixtures are inconclusive.

Abstract

It is commonly admitted that in liquids the thermal diffusion and Dufour coefficients $D_{T}$\ and $D_{F}$ satisfy Onsager's reciprocity. From their relation to the cross-coefficients of the phenomenological equations, we are led to the conclusion that this is not the case in general. As illustrative and physically relevant examples, we discuss micellar solutions and colloidal suspensions, where $D_{T}$ arises from chemical reactions or viscous effects but is not related to the Dufour coefficient $D_{F}$. The situation is less clear for binary molecular mixtures; available experimental and simulation data do not settle the question whether $D_{T}$\ and $D_{F}$ are reciprocal coefficients.