The anisotropy of transformer oil-based magnetic fluids studied by acoustic spectroscopy

The anisotropy in transformer oil-based magnetic fluids upon the effect of an external magnetic field and temperature were studied by acoustic spectroscopy. When a magnetic field is increased, the interaction between the external magnetic field and the magnetic moments of the nanoparticles occurs, leads to the aggregation of magnetic nanoparticles and following clusters formation. However, the temperature of magnetic fluids has also very important influence on the structural changes because of the mechanism of thermal motion that acts against the cluster creation. In present the anisotropy of the transformer oil-based magnetic fluids can be described by two theories. Taketomi theory assumes existence of spherical clusters. These cluster form long chains, aligned in magnetic field direction. Later Shliomis proposed similar theory, in which these chains are formed by nanoparticles. A comparison of the experimental results with the predictions of the Taketomi theory allowed a determination of the cluster radius and the number density of the colloidal particles. These data allowed us to draw conclusions about the changes in the parameters describing the structure of the ferrofluid at different temperatures. On the basis of the measurements performed, it was possible to establish the proportion of the acoustic wave used for excitation of the translational and rotational degrees of freedom.