Thermodiffusive separation of ferrofluids in vertical column under the effect of a combined thermal and Soret driven convection

E. Blums

Institute of Physics, University of Latvia Salaspils-1, LV-2169 Latvia

Abstract
Separation measurements in thermodiffusion column allow investigating the Soret effect in multicomponent liquids. Usually the column theories assume small concentration differences and convective separation under the effect of a steady thermal convection. In present paper the thermodiffusion column theory is reconsidered for liquids of high Soret coefficients and of strongly different densities of solutes when the convection in channel is formed not only by a thermal but also by a solutal buoyancy force. The problem is considered approximately introducing the assumption of independence of the mean solute concentration gradient on vertical coordinate. It is shown that a simple analytical solution for calculation of Soret coefficients from the measured separation curves can be found only for the initial time region when a concentration boundary layer approximation is valid and for the asymptotic regime of a steady separation level. The obtained analytic dependencies are used to evaluate the Soret coefficient of surfacted magnetite and Mn-Zn ferrite nanoparticles in hydrocarbon based ferrofluids. The results well agree with independent measurements performed by using the forced Rayleigh scattering technique. Figs 7, Refs 23.