Rheodielectric study of transformer-oil-based ferrofluids

The rheological properties of nonpolar ferrofluids have been often studied under the action of an external magnetic or electric field. In this paper, we approach the problem conversely, and we aim to answer whether the ferrofluid shear flow affects the dielectric response. The ferrofluids are based on transformer oil and iron oxide nanoparticles stabilized with oleic acid. Basic physical characterization of the ferrofluids with three different nanoparticle concentrations is followed by experimental investigation of flow curves. Newtonian behavior of the ferrofluids is confirmed. The key experiment of this study consists of complex dielectric permittivity measurements in the frequency range from 20 Hz to 100 kHz at three different temperatures. In these measurements, the ferrofluids are sandwiched between two disk electrodes of a modular rheometer under the shear rates of 0, 10, 100, 500, and 1000s^{-1}. It is found that the low-frequency dielectric spectra of the ferrofluids exhibit a remarkable conductivity contribution leading to electrode polarization. Additional measurements of electrical conductivity and qualitative analysis of charge mobility in the ferrofluids are provided to understand the low-frequency dielectric relaxation. The results reveal that the slow dielectric relaxation is shear rate independent. The low-frequency dielectric spectra measured under the various shear rates exhibit constant behavior. The electric charge migrates across the shear flow velocity in the shear rate direction and the given rheological conditions do not affect its behavior. It is concluded that the electrical forces dominate the charge motion in the ferrofluids over the shear forces. The finding of the stable dielectric response of ferrofluids under the shear flow conditions supports the application of ferrofluids as dielectric media in electrical equipment with the presence of forced or natural convection.