Observations of ferrofluid flow under a uniform rotating magnetic field in a spherical cavity

• Published in: Journal of applied physics 2012-04, Vol.111 (7), p.07B313-07B313-3
• Main Authors: Torres-Díaz, Isaac, Rinaldi, Carlos, Khushrushahi, Shahriar, Zahn, Markus
• Format: Article
• Language: English
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/1.3675153

Flow of a ferrofluid in spherical and cylindrical geometries were measured under the influence of a uniform rotating magnetic field produced by two perpendicular spherical coils, a so-called fluxball, excited by quadrature currents. Using an ultrasound velocity profile technique and a commercial oil based ferrofluid (EFH1, Ferrotec) we observed rotational flow around the z -axis. In comparison, the radial component of the flow was found to be negligible. Results show that the magnitude of the azimuthal velocity profile increases as the applied magnetic field amplitude increases. This behavior is also observed for ferrofluid in a cylindrical container placed inside the fluxball cavity and inside a two-pole stator winding. These results indicate that inhomogeneities in the magnetic field produced by slots and finite height of the stator winding used in prior experiments are not the source of previously observed flows produced by a two pole stator winding. The experiments reported here either point to the existence of non-uniform demagnetizing magnetic fields due to the finite height of the cylindrical container, the existence of couple stresses and spin viscosity in ferrofluids, or to the need to develop alternate governing and constitutive equations capable of describing the experimental observations.