Magnetorheology in rotating magnetic fields

We studied the flow of ferrofluid induced by rotating magnetic fileds. This flow was observed for first time in 1967 by Moskowitz and Rosensweig [Appl. Phys. Lett., 10, pag. 301 (1967)] and since then researchers has attempted to explain the phenomenon through theories such as spin diffusion, magnetic tangential surface stresses, and others that claim the impossibility in to obtain ferrofluid flow through a uniform rotating magnetic field. These theories have been tested using experimental observations of tracer particles on the free surface of the container because of the difficulty in obtaining bulk velocity profiles by traditional methods. Nevertheless, as is demonstrated herein such surface velocity profiles are inadequate for the assessment of bulk flow theories as they misrepresent the bulk flow of the fluid and as such can be the cause of confusion.; The principal contribution of the thesis is to provide experimental evidence that allows clarification of the mechanism responsible for the observed flow. To this end we focused on the flow of ferrofluid induced by a rotating magnetic field in a cylindrical container and between two coaxial cylinders. In order to obtain bulk flow measurements we used the ultrasound velocity profile method. Bulk flow measurements taken in the cylindrical container show the fluid co-rotating 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. Our experimental measurements show qualitative agreement with an extension of the spin diffusion theory of Zaitsev and Shliomis [ J. Appl. Mech. Tech. Phys. 10, pag. 696 (1969)], obtained using the regular perturbation method. 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 eta' = 5.8 x 10-10 kg ms-1 is several orders of magnitude higher than that obtained from dimensional analysis.; We present the first measurements of bulk flow for ferrofluid in the annular gap between two stationary coaxial cylinders. These results contrast with current theories which only predict flow when one of the cylinders is free to rotate. Qualitative comparison of the experimental results with the predictions of the spin diffusion theory for the annular flow of ferrofluid in the limit of low fields were found in good agreement with velocity profiles obtained for the kerosene based ferrofluid.