On the measurement of positive Soret coefficients

In this paper, we report Soret coefficients obtained by two independent methods and for two different systems(water-ethanol containing respectively 60.88 and 50 wt % of water). The Soret coefficient is defined by S-T = D-T/D where D-T is the thermal diffusion coefficient, and D is the isothermal diffusion coefficient. In the first method, D-T is determined by a 5-point sampling process in a thermogravitational column and D by the well-known OEC technique (open-ended capillary). The ratio of the experimental values of D-T and D gives the expected value of S-T. A totally new method consists of velocity measurements of a transient natural convective state: indeed, the Soret effect induces modifications of density gradients and therefore of the buoyancy responsible for free or natural convection. The modifications of the convective amplitudes are thus an indirect way to have access to the Soret coefficient. The velocity measurements were obtained by LDV. The comparison between the two techniques shows a deviation of the order of 4%, which is quite satisfactory. The agreement with the literature is also excellent.