INFLUENCE OF THERMODIFFUSIVE PARTICLE TRANSPORT ON THERMOMAGNETIC CONVECTION IN MAGNETIC FLUIDS

Thermomagnetic convection is driven by a temperature difference and a magnetic field applied to a layer of fluid. Its onset is characterized by the so-called Rayleigh number Ra. Since a temperature gradient also drives thermal diffusion in binary fluids characterized by the Soret coefficient S-T, the impact of the two transport phenomena on each other have to be investigated with the focus on the point of transition from enhanced to hindered or even suppressed convection. A linear stability analysis provides the functional context of Ra(ST). Suppression of the convective motion is thereby reached for S-T < -0.001, which implies a negative Soret coefficient. The zero magnetic field coefficient in ferrofluids is positive, but measurements of the magnetic Soret coefficient in an applied magnetic field prove that its sign is sensitive to the strength of the magnetic field. In fields higher than 75 kA/m, the Soret coefficient switches from positive to negative.