Ultrasonic wave interactions with magnetic colloids

Originally invented as a method for moving spacecraft fuel in weightless conditions, magnetic colloids or ferrofluids are now used in applications as diverse as the dissipation of heat in the voice coils of a loudspeaker, and for the separation of scrap metal. It has been found that aqueous ferrofluids become unstable after a period of time and with dilution. Therefore, there is a need to characterize the colloidal fluid to study the effects of degradation. Additionally, due to the high cost of ferrofluids and the large volumes required for some applications, the fluid is recycled. It is therefore necessary to develop a system for quality assurance for the fluid reclamation process. Ultrasonic methods have been successfully utilized in the characterization of non-magnetic paniculate mixtures. This thesis is concerned with the characterization of magnetic colloids using the ultrasonic techniques developed within the Ultrasonics and Digital Signal Processing Laboratory at the University of Nottingham. A new test-cell has been designed and manufactured and software has been written for the ultrasonic spectroscopy of a number of different ferrofluids including a range of aqueous fluids and a hydrocarbon based fluid. Instrumentation has been developed to determine the ultrasonic properties of the water-based ferrofluids, with results presented in this thesis versus temperature, frequency, magnetic field and the effect of anisotropy. Experiments on dilutions of each of the fluids have been performed in an effort to determine the relative stability of the fluids. The experimental results have been compared with a combined scattering and hydrodynamic model (Allegra and Hawley 1972) and the ultrasonic anisotropy theory of Skumiel (1997). An on-line quality assurance process is proposed. (author)