(316p) Experimental and Theoretical Study of Suspensions of Magnetic Nanoparticles in an Annular Gap Subjected to a Rotating Magnetic Field

The flow of a ferrofluid, a suspension of magnetic nanoparticles in a Newtonian fluid, between two concentric cylinders subjected to a rotating magnetic field has been investigated by various researchers for the past thirty years. The consensus in the literature, based on analysis of the ferrohydrodynamic equations, is that when both cylinders are held fixed there should be no flow in the bulk of the annular gap. Experimental observations of a velocity profile at the ferrofluid-air interface are ascribed to interfacial magnetic stresses, rather than to a bulk flow. Direct measurements of the bulk velocity profile have been lacking because the opaque nature of the ferrofluid precludes usage of standard flow visualization methods. We have applied an asymptotic solution scheme to the ferrohydrodynamic equations and shown that bulk flow is indeed expected when higher order terms of the solution are taken into account. The analysis predicts bulk flow of ferrofluid in the direction of field rotation. Predictions for the torque on the inner cylinder were found to be in agreement with experimental measurements of torque for three different aspect ratios of inner to outer cylinder radii with various dilutions of a water based ferrofluid. An ultrasound velocity profile measurements technique was used to obtain the first measurements of the bulk flow of the ferrofluid in the annular gap.