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Abstract

Thermal convection in ferrofluids, the colloidal suspension of ferromagnetic particles, is investigated theoretically. Ferrofluids are treated as binary liquid mixtures with weak solutal diffusivity but large separation ratio. Due to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces are dominant and the concentration dynamics cannot be disregarded for thermal convection. In principle, convective motion sets in at Rayleigh numbers well below the critical threshold for single-component liquids. But only far above this (hypothetical) threshold the growth dynamics of the amplitude is fast enough to be detectable and a nonlinear analysis demonstrates that there it quickly saturates in a state of stationary convective motion.