Understanding Ferrofluid Spin-Up Flows in Rotating Uniform Magnetic Fields

Ferrofluid spin-up flow has been studied in finite height cylinders and spheres subjected to a uniform rotating magnetic field. Ultrasound measurements in finite height cylinders with a free surface show the presence of surface driven flows as well as bulk flows. In finite height cylinders with a cover, only bulk flows exist. Ultrasound measurements in fully filled spheres of ferrofluid give negligible measureable flow. The bulk spin-up flow has been attributed to the demagnetizing non-uniform magnetic field associated with the shape of the fluid, the nonuniform distribution of fluid magnetization, and to spin-diffusion. COMSOL Multiphysics simulations help confirm that the real mechanism of bulk spin-up flow is due to non-uniform magnetic properties of the ferrofluid, either imposed externally or associated with the shape of the ferrofluid.