Ferrofluid flow phenomena

Abstract

An investigation of ferrofluid experiments and analysis is presented in three parts: a characterization of ferrofluid properties, a study of ferrofluid flow in tubing and channel systems, and a study of ferrofluid free surface sheet flows. The characterization of ferrofluid samples is completed through analysis of magnetization curves measured with a vibrating sample magnetometer. Determination is made of the ferrofluid particle size range, saturation magnetization, low-field magnetic permeability, and magnetic volume fraction. The experimental results are well described by the Langevin theory of paramagnetism. A detailed discussion of the demagnetization factor within the ferrofluid sample is also included. Ferrofluid flow through circular tubing in a laminar regime is examined as a function of the applied magnetic field magnitude, direction, and frequency. Gradients within the applied magnetic field create a magnetic contribution to the pressure drop across a length of tubing. Experiments of ferrofluid flow through a rectangular channel with a free surface when driven by a rotating spatially uniform magnetic field exhibit an anti-symmetric flow profile across the channel width, with a net zero flow rate, consistent with theoretical work of previous research. The first known investigation of ferrofluid free surface sheet flows resulting from a ferrofluid jet impacting a small circular plate is presented. Two distinct magnetic field orientations relative to the incident jet and resulting sheet are examined, producing markedly different results. A magnetic field oriented perpendicular to the jet flow is found to deform the jet cross-section from circular toward an elliptical shape thereby causing the sheet to also change from circular to elliptical, but with the long axis of the sheet oriented perpendicularly to the long axis of the jet cross-section. In the case of a magnetic field applied everywhere perpendicular to the sheet flow a significant decrease in sheet radius is observed. The cause of the decrease in sheet radius is a magnetic field induced decrease in ferrofluid pressure as well as a magnetic field enhanced convective Kelvin-Helmholtz instability. A thorough theoretical development describes the observed phenomena.