Classical thermodynamics, heat transfer Definition

Soret effect

Builds on Temperature Entropy production

Definition

Let a binary fluid mixture have mass density $\rho$ , mass fraction $w$ of component $1$ and absolute temperature $T$ , all fields in space. Let $j$ be the diffusive mass flux of component $1$ , relative to the centre-of-mass velocity. In the linear regime $j = -\rho D \nabla w - \rho\, w (1 - w)\, D_T \nabla T,$ where $D > 0$ is the diffusion coefficient and $D_T$ is the thermal diffusion coefficient. The Soret effect is the second term, the mass flux driven by the temperature gradient. The ratio $S_T = D_T / D$ is the Soret coefficient.

Remarks

In a steady state with no mass flux, $j = 0$ gives $\nabla w = -w (1 - w)\, S_T \nabla T.$ So for $S_T > 0$ component $1$ gathers at the cold side, and for $S_T < 0$ at the warm side. Example: in a mixture with $w = 1/2$ and $S_T = 10^{-2}\,\mathrm{K}^{-1}$ , a temperature difference of $1$ K across the cell gives a mass fraction difference of about $2.5 \times 10^{-3}$ . Non-example: in an ideal mixture at uniform temperature, $\nabla T = 0$ , and the flux is pure Fickian diffusion with no Soret term. The flux and force pair of the Soret term enters the entropy production, and the reciprocal cross effect is the Dufour effect.

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Entropy production concept
Temperature concept
Soret effect definition

Tag	`WGC9ZX86X9`
URI	`m2://WGC9ZX86X9`
Kind	definition
Writing	W0 stub
MSC	80A19, 82C35, 82C70
Defines	Soret effect
Uses	Temperature, Entropy production
Changed	3 November 2026

Soret effect

Definition

Remarks

Reading path

Further reading

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