Acoustic studies of magnetic fluids

Abstract

This report is a summary of the acoustic studies of magnetic fluids performed by the author and his co-workers. A first investigation applied acoustic spectroscopy methods with the nonmagnetized samples of magnetic fluid. The ultrasonic frequency was varied in the range 3–50 MHz. The velocity of the ultrasound was shown to decreased nonlinearly with the growth of the magnetic particle concentration. The elastic properties are defined mainly by the type of base fluid. In high-concentrated magnetic fluid, ultrasound velocity dispersion was found. The knowledge of frequency dependence of the absorption coefficient made it possible to estimate the applicability of known mechanisms of ultrasonic energy dissipation for the magnetic fluids. A second investigation was of the magnetic field influence upon the acoustic properties of the magnetic fluids. The external uniform magnetic field caused the formation of the needlelike structures. It leads to the breakdown of rotational invariance, and the magnetic fluid becomes acoustically anisotropic. Examination of the angular dependence of ultrasound attenuation indicates that it is the same as the nematic liquid crystal. Current theories are shown not to predict the observed experimental data. The electromagnetic effect accompanying the propagation of the ultrasound waves through a magnetic fluid was examined. This effect can be used for the determination of ultrasound velocity and of the average size of particles of magnetic fluids.