The acoustic method of Bogdanov is used for direct observation of the rotational viscosity of magnetic fluid. Originally this method was developed to measure the rotational viscosity of nematic liquid crystals in a rotating magnetic field by the phase diagram of the angular dependence of ultrasonic absorption. This method clearly shows the existence of rotational viscosity without additional processing of experimental data.
A stable internal structure in the magnetic fluids was obtained by a long exposure between the poles of a permanent magnet. The induced anisotropy of the relative ultrasonic absorption by the magnetic fluids has been studied in the wide range of magnetic field rotation frequencies f = 0.056 – 280 mHz. A qualitative change in the angular dependence of the relative ultrasonic absorption is observed in the transient frequency range from 0.14 mHz to 5.56 mHz. The explanation for this phenomenon is the partial destruction of the induced structure with an increase in the rotation frequency of the magnetic field. However, the induced anisotropy is retained even at moderate rotation frequencies. It is established that in moderate magnetic fields the magnetic field strength vector is parallel to the magnetization vector of the magnetic fluid and the frozen-in condition is fulfilled. Similar results were observed for both low-viscosity and high-viscosity magnetic fluids.