Ultrasonic velocity anisotropy in ferrofluids under the influence of a magnetic field

The propagation of ultrasonic waves in a water-base ferrofluid has been investigated by pulse-echo and continuous wave (CW) methods. The CW methods proved to be well suited for studying the velocity changes (Δv/v) due to a magnetic field. The magnitude of Δv/v was about 10 percent for fields as low as 125 gauss. In order to determine the velocity anisotropy, Δv/v was studied as a function of the relative angle (ϑ) between the magnetic field and the propagation vector. The anisotropy in Δv/v did not show a sin22ϑ dependence as the theory predicts. Also the magnitude of Δv/v was larger than the theoretical value by a factor of 103.