Thermodiffusion in magnetic fluids

Investigations were made to determine the Soret coefficient of magnetic particles in a ferrofluid under the influence of a magnetic field. This so-called magnetic Soret effect was theoretically predicted to be two to three orders of magnitude smaller than the conventional Soret effect. In contrast, former experiments have qualitatively shown that the magnetic Soret effect is much higher than the theoretical predictions. However, in those experiments the influence of buoyancy and magnetic driven convection disturbed the measurement significantly. Thus, it is still an open question how strong the magnetic Soret effect can be. Therefore, an experimental setup was developed which minimizes parasitic effects, simplifying the analysis of the experimental results. These results provide quantitative measures of the magnetic field dependence of the Soret effect in suspensions of magnetic nanoparticles. It is shown that the magnetic Soret effect can even be higher than the conventional one and that its strength as well as its direction depend on the magnetic field strength and its relative alignment to the temperature gradient in the fluid.