Effects of demagnetizing factors on transient motion of ferrofluid in a uniform rotating magnetic field

Abstract

The mechanisms that lead to bulk flow within a ferrofluid-filled container subjected to a rotating uniform magnetic field are experimentally studied. There are two prevailing theories: spin diffusion theory and flow due to non-uniformities in magnetic field within the ferrofluid due to nonuniform demagnetizing factors. This research sought to confirm previous measurements that indicated demagnetizing factors are the primary cause of bulk ferrofluid flow. Flattened spherical containers of various volumes, and thus different demagnetizing factors, were filled with EFH1 oil-based ferrofluid and subjected to a uniform rotating magnetic field of varying conditions (rotation direction and field strength). The shapes and magnitudes of the velocity profiles measured by an ultrasound velocimeter system differed between containers, indicating that demagnetizing factors did affect flow. The complicated flows within the flattened spheres that affected both the shape and magnitude of the flow velocity prevented a direct magnitude comparison between profiles but the flows differed enough to safely conclude that spatial non-uniformities within the fluid likely caused the bulk flow of fluid in the uniform rotating magnetic field.