Victor V. Sokolov
Dept. of Phys., Moscow Instrument-Making Inst., 20 Stromynka St., Moscow 107076, Russia
This report is a summary of the acoustic studies of magnetic fluids which were performed by the author and his co-workers during the period from 1985 to the present. One direction of these investigations consisted of the use of acoustic spectroscopy methods with the nonmagnetized samples of magnetic fluid. The ultrasonic frequency was varied in the range 3--50 MHz. It is shown that the velocity of the ultrasound is 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, the ultrasound velocity dispersion was found. The knowledge of frequency dependencies of the absorption coefficient made it possible to estimate the applicability of known mechanisms of ultrasonic energy dissipation for the magnetic fluids. The second direction consisted of the investigation 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 the rotational invariance, and the magnetic fluid becomes acoustically anisotropic. Examination of the angular dependence of ultrasound attenuation indicates that one 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. It is shown that this effect can be used for the determination of ultrasound velocity and the average size of particles of magnetic fluids.
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