Our new paper entitled Soret separation and thermo-osmosis in porous media has just been published in the topical collection “Thermal non-equilibrium phenomena in fluid mixtures” in the European Physics Journal E.
When a temperature difference is applied over a porous medium soaked with a fluid mixture, two effects may be observed, a component separation (the Ludwig-Soret effect, thermodiffusion) and a pressure difference due to thermo-osmosis. In the work presented in the paper, we study both effects using non-equilibrium thermodynamics and molecular dynamics. We derive expressions for the two characteristic parameters, the Soret coefficient and the thermo-osmotic coefficient in terms of phenomenological transport coefficients, and we show how they are related. Numerical values for these coefficients are obtained for a two-component fluid in a solid matrix where both fluid and solid are Lennard-Jones/spline particles.
We find that both effects depend strongly on the porosity of the medium and weakly on the interactions between the fluid components and the matrix. The Soret coefficient depends strongly on whether the fluid is sampled from inside the porous medium or from bulk phases outside, which must be considered in experimental measurements using packed columns. If we use a methane/decane mixture in bulk as an example, our results for the Soret coefficient give that a temperature difference of 10 K will separate the mixture to about 49.5/50.5 and give no pressure difference. In a reservoir with 30% porosity, the separation will be 49.8/50.2, whereas the pressure difference will be about 15 bar. Thermo-osmotic pressures with this order or magnitude have been observed in frost-heave experiments. A picture with some results can be shown below.
The main responsible person behind the work was Bjørn Hafskjold.