Abstract
Measurements of magnetic-field-induced ferrofluid flow and torque in uniform rotating magnetic fields are presented and compared to theoretical analyses in order to understand observed paradoxical behavior. The viscous torque from this fluid flow is measured using a cylindrical Couette viscometer, as a function of magnetic field amplitude, frequency, and direction of rotation. The first set of experiments measures the torque on the outer wall of a polycarbonate spindle that is attached to a viscometer, which functions as a torque meter. The spindle is immersed in the ferrofluid, which is centered in the gap of a three-phase AC 2-pole motor stator winding. Anomalous behavior, such as negative effective viscosity, is demonstrated and discussed. The second set of experiments measures the viscous 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. These measurements show that magnetic fluid effects arise even in the absence of free surfaces. These observations are then shown to agree with a recently derived analysis of spin-up flow in ferrofluids.
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