Ultrasonic investigations of hydrogels containing barium ferrite particles

We have attempted to establish a systematic and accurate method for evaluating the sound velocity of polymer gels containing magnetic particles using 2.25- and 10-MHz ultrasonic waves at 295.5 K. The sound velocity of the gels gradually decreased with the increasing volume fraction of particles. The experimental data were well fit by the Harker and Temple model that is known for the coupled-phase model of suspensions. The effect of the magnetization direction on the gel modulus has also been discussed by means of the elastic theory. The longitudinal modulus of magnetic gels after magnetization was higher than that before magnetization. Moreover, it was clear that the longitudinal modulus increased and decreased when the strain direction was perpendicular and parallel to the magnetization, respectively. The observed anisotropy in the modulus at these frequencies would be ascribed to the magnetic interaction between magnetic particles in the range of similar to100 mum.