Magnetic fluid flow phenomena in DC and rotating magnetic fields

Abstract

An investigation of magnetic fluid experiments and analysis is presented in three parts: a study of magnetic field induced torques in magnetorheological fluids, a characterization and quantitative measurement of properties relating to the transition of a ferrofluid drop from a continuous phase into a discrete phase and also into a spiral flow, and a study of magnetic field induced ferrofluid flow reversals. The torque exerted on a spindle filled with magnetorheological fluid (MR fluid) and placed inside a uniform rotating magnetic field is measured with varying rotating magnetic field amplitude and frequency, total fluid volume, and MR fluid volume ratio. When compared to similar ferrofluid torque measurements where the torque increased with rotating magnetic field frequency, the torque frequency dependence of the MR fluid decreases with increasing magnetic field frequency. A simple analysis determines the dependence of the magnetic body torque on particle size to describe the different behavior between the ferrofluid and MR fluid. When a fluorocarbon based ferrofluid is contained between two glass plates separated by a small gap (Hele-Shaw cell) and excited by an applied uniform rotating magnetic field first and then a DC axial magnetic field, a phase like transition occurs that transforms the ferrofluid drop from a continuous phase to a discrete phase. Considering the dominant energy in the configuration to be contributed from the magnetostatic energy of the DC magnetic field and interfacial surface energy, a calculus of minimization of free energy is performed to determine the number of smaller ferrofluid drops that will result from the transition and the threshold axial magnetic field for the transition to occur.

(cont.) When the order of the applied magnetic fields is reversed, the DC axial magnetic field is applied first causing the ferrofluid droplet to form the labyrinth instability. The rotating magnetic field is then applied creating a spiral formation. Experiments are conducted for varying Hele-Shaw cell separation gap, and rotating magnetic field amplitude and frequency. Measurements were consistent with our model. A cylindrical vessel is filled with a water-based ferrofluid and excited by a uniform rotating magnetic field that induces a counter-rotating circular flow in the vessel. A DC axial magnetic field is slowly raised to change the curvature of the fluid surface and results in a change in the ferrofluid flow direction to co-rotating with the applied magnetic field. Measurements are taken of the threshold axial magnetic field that results in the change of flow direction for varying rotating magnetic field direction, amplitude, and frequency. An analysis is included that describes the change in flow direction due to surface curvature.