Role of a layer of porous medium in the thermodiffusion dynamics of a liquid mixture

This paper is addressing the challenge related to understanding transient mass transfer associated with a thermal gradient through a liquid mixture containing a layer of a porous medium saturated with liquid. Digital optical interferometry in combination with tomography was used to unveil the difference in the mechanism of mass transfer caused by the Soret effect in a free liquid and a porous medium and, especially, at their border. With continuous monitoring of mass transfer throughout the system, it was found that at the beginning of the experiment (the first 24 h) the temporal evolution of the concentration difference between the interfaces of the porous layer looked as if the inverse and strong component separation occurred in the porous medium. It was evidenced that different characteristic times of the thermodiffusion process in free liquids and a porous medium were essential for the formation of the apparent inverse separation. This study not only provides physical insights into the mass transfer dynamics, but also opens up the possibility to measure several transport coefficients from a single experiment. We have obtained the Soret, diffusion and thermodiffusion coefficients of the THN-nC(12) binary mixture as well as tortuosity and thermal conductivity of a saturated porous medium. The Soret coefficients, determined in the porous layer and in the compound system using the mean concentration in free volumes are similar, and about 10% higher than the benchmark value (Platten et al., 2003), but coincide with the value reported in literature for measurements in porous media (Platten and Costeseque, 2004). (C) 2019 Elsevier Ltd. All rights reserved.