Ferrofluid flow and torque measurements in rotating magnetic fields

Abstract

The purpose of this research is to provide data from ferrofluid flow and torque measurements in uniform and nonuniform rotating magnetic fields that can be compared to theoretical analyses in order to fully understand observed paradoxical ferrofluid behavior. In the presence of rotating magnetic fields, ferrofluid particles will rotate to try to align their magnetic moment with the field but because of the fluid viscosity, magnetization M will lag behind the rotating H field, thereby resulting in a body torque on the ferrofluid. The viscous torque from this fluid flow is measured using a Couette viscometer as a function of magnetic field amplitude, frequency, and direction of rotation. The first three sets of experiments measure this torque on the outer wall of a Lexan spindle that is attached to a viscometer, functioning as a torque meter. The spindle is immersed in a beaker of ferrofluid centered inside a 2-pole or 4-pole motor stator winding, creating uniform or nonuniform rotating magnetic fields, respectively. The spindle rotates at a constant speed up to 100 rpm or is stationary in these measurements. Anomalous behaviors such as zero and negative magnetoviscosity are demonstrated and discussed. The next set of experiments measure the magnetic torque on the inner wall of a hollow spindle attached to the torque meter and filled completely with ferrofluid so that there is no free surface. The spindle is centered inside the motor stator windings and exposed to clockwise (CW) or counterclockwise (CCW) rotating magnetic fields. The last set of experiments measures the surface spin rate of a small floating plastic ball placed on the ferrofluid surface at a fixed location as a function of magnetic field parameters and radial position on the surface. When the rotating magnetic fields induce ferrofluid flows, the ball spins in the opposite direction to magnetic field rotation and this spin rate is determined using frame-by-frame video analysis.