Soret coefficient of nanoparticles in ferrofluids in the presence of a magnetic field

Experiments on a nonstationary separation of nanometer-sized Mn_0.5Zn_0.5Fe₂O particles of hydrocarbon-based ferrocolloids in a flat vertical thermal diffusion column are performed. By using a modified separation theory which accounts for a one-dimensional mixed (thermal and concentration) convection in the column, the Soret coefficient of lyophilized nanoparticles from the separation curves are calculated. It is shown that in a zero magnetic field particles are transferring toward decreasing temperatures. The thermal diffusion ratio α_T reaches a value α_T≈+20. A significant influence of a uniform magnetic field B on particle separation is observed. If B is oriented along the temperature gradient ∇T, a strong decrease in thermal diffusion coefficient takes place whereas the transversal field B⊥∇T causes an intensification of particle thermophoretic transfer. Both effects qualitatively well agree with theoretical predictions based on a hydrodynamic theory of particle thermomagnetophoretic motion.