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2576963 
Journal Article 
Spin-up flow of ferrofluids: Asymptotic theory and experimental measurements 
Chaves, A; Zahn, M; Rinaldi, C 
2008 
1 
Physics of Fluids
ISSN: 1070-6631 
20 
5 
WOS:000256304800010 
We treat the flow of ferrofluid in a cylindrical container subjected to a uniform rotating magnetic field, commonly referred to as spin-up flow. A review of theoretical and experimental results published since the phenomenon was first observed in 1967 shows that the experimental data from surface observations of tracer particles are inadequate for the assessment of bulk flow theories. We present direct measurements of the bulk flow by using the ultrasound velocity profile method, and torque measurements for water and kerosene based ferrofluids, showing the fluid corotating with the field in a rigid-body-like fashion throughout most of the bulk region of the container, except near the air-fluid interface, where it was observed to counter-rotate. We obtain an extension of the spin diffusion theory of Zaitsev and Shliomis, using the regular perturbation method. The solution is rigorously valid for alpha(K) << root 3/2, where alpha(K) is the Langevin parameter evaluated by using the applied field magnitude, and provides a means for obtaining successively higher contributions of the nonlinearity of the equilibrium magnetization response and the spin-magnetization coupling in the magnetization relaxation equation. Because of limitations in the sensitivity of our apparatus, experiments were carried out under conditions for which alpha similar to 1. Still, under such conditions the predictions of the analysis are in good qualitative agreement with the experimental observations. An estimate of the spin viscosity is obtained from comparison of flow measurements and theoretical results of the extrapolated wall velocity from the regular perturbation method. The estimated value lies in the range of 10(-8)-10(-12) kg m s(-1) and is several orders of magnitude higher than that obtained from dimensional analysis of a suspension of noninteracting particles in a Newtonian fluid. (C) 2008 American Institute of Physics.