Ultrasound Frequency Analysis of a Magnetic Fluid in Low-Intensity External Magnetic Field

Abstract

This work deals with an interaction of a magnetic fluid of a dielectric nature with a magnetic field by means of ultrasound waves measurements and analysis. Ultrasound analysis is known as a non-destructive inspection tool often used in technical diagnostics, moreover, it has numerous applications in medicine and biology, too. We report the low-frequency ultrasound analysis of a dielectric magnetic fluid in a low-intensity external static magnetic field. The studied magnetic fluid was composed of a transformer oil and dispersed magnetite nanoparticles coated with oleic acid. Experiments were carried out by using an ultrasonic testing cell. The cell was exposed to a magnetic field of 50 mT in both parallel and perpendicular direction to the waves propagation. A through-transmission mode measurement was applied, where two fixed narrow-band transducers with completely shielded crystal for maximum RFI/EMI immunity (Physical Acoustic R15I-AST, the resonant frequency 150 kHz) served as a transmitter and a receiver. In this way we carried out the measurement of the frequency-dependent ultrasonic response to a rectangle calibrating signal of 5 µs pulse width. Digitized signals were recorded for further analysis. We present the frequency domain analysis of the low-frequency ultrasound in magnetic fluid. The frequency spectrum in magnetic fluid colloidal system was calculated by the Fourier transformation method. Results show that there is a frequency shift in the amplitude-frequency spectrum caused by the step-up magnetic field. The higher the magnetic field, the higher the frequency of the peaks. The effect of particle aggregation in magnetic field on the ultrasound wave propagation is discussed in the paper.