Characterizing ferrofluid spin-up flow in rotating uniform magnetic fields

Abstract

A ferrofluid is a collection of nanoscale ferromagnetic particles with a stabilizing surfactant in a liquid to form a colloid. The dynamic behavior of ferrofluids in the presence of magnetic fields has long been an area of research interest. A particular area of interest deals with the "spin-up" mechanisms of ferrofluids, which describe how a container of ferrofluid comes to a steady state of bulk flow when subjected to a uniform rotating external magnetic field. There are two prevailing theories that attempt to explain the spin-up mechanisms of ferrofluids: spin diffusion theory, and the presence of non-uniformities in the magnetic field, due to "demagnetizing factors" introduced by the shape of the container. This research attempts to confirm previous measurements indicating that non-uniformities in the magnetic field are the primary cause of ferrofluid bulk flow. Partial spheres and cylindrical containers of different volumes -- and thus different demagnetizing factors -- were filled with Ferrotec EFH1 oil-based ferrofluid and subjected to an external uniform rotating magnetic field for various parameters of rotation direction and magnetic field. Ferrofluid bulk flow was measured using ultrasound velocimetry, and the magnitudes and shapes of the velocity profiles were compared. Despite the complicated flows observed within the containers, enough of a trend was established to safely conclude that demagnetizing factors are often the primary cause of ferrofluid bulk flow.