A new apparatus for analysis of viscoelastic fluids by ultrasound radiation force

The acoustic radiation force has been used as the method to examine the physical properties of materials in several areas. Vibro-acoustography is an acoustic radiation force technique that is being used to perform analysis of mechanical properties of materials. In this application a focused acoustic modulated force excites target which vibrates at the frequency of modulation. The emitted sound is characteristic of the medium mechanical impedance and it is measured using a dedicated hydrophone. In this paper, we propose a modification of the vibro-acoustography (VAG) technique and apply a technique called vibro-acustomagnetography (VAMG) by replacing the hydrophone by a magnetic sensor with high sensitivity. In this case, the modulated acoustic radiation will be applied on a magnetized target immersed in the fluid under study. With this procedure, static and dynamic displacement of the magnetic target (ball) will be measured when acoustically excited. In this study, we used a magnetoresistive sensor with resolution of about nT for mounting the transducer to detect displacement of the magnetic target. The vibration of the target was induced by a non-contact force, using an ultrasonic beam modulated by two concentric beams generated by confocal piezoelectric elements with equal area and common focus to 7 cm. The target used was a magnetic sphere of NdFeB with a radius of 2.36 mm. The apparatus was evaluated through of measurements in water and oil. Viscoelastic parameters were estimated fitting the nonlinear response of the magnetic transducer function of frequency modulation.