Investigation of structural changes in oil-based magnetic fluids by surface acoustic waves

Abstract

Surface acoustic waves (SAWs) are used for the first time to investigate structural changes in transformer oil-based magnetic fluids under magnetic field. The measurement of the attenuation of SAW propagating along the interface between magnetic fluid and the piezoelectric substrate is showed as an effective tool to study such processes in the case of magnetic fluids when the obviously utilized longitudinal acoustic waves due to the higher viscosity are not available. The magnetic particles FeO.Fe2O3 in several different volume concentrations, from 0.05% to 3.5%, were added to the transformer oil MOL. Both linear increasing and jumped changed magnetic field were applied to study structural changes. The interaction between the magnetic field and magnetic moments of nanoparticles can lead to the aggregation of magnetic nanoparticles and following chain or cluster formations that have an influence on the SAW attenuation. The concentration of magnetic nanoparticles and temperature of magnetic fluid have a decisive influence on structural changes due to the changes of viscosity and thermal motion with the increasing temperature that both affect the process of cluster formation. The measurement of the acoustic anisotropy and applying the theoretical predictions gives additional useful information about the structure of nanoparticles formations in investigated magnetic fluids. Obtained results are compared and discussed.