Effect of particle volume fraction on the velocity of sound in magnetorheological fluids

In this work the velocity of sound in magnetorheological fluids as a function of the particle volume fraction is presented. The influence of the magnetic field on the sound speed and on the material microstructure is also analyzed. It is shown that particles in suspension interact to form complex microstructures which depend on the volume fraction. The range of particle volume fractions studied goes from 1% up to 10%. In the absence of magnetic field, there is a decrease in the velocity of sound as the particle volume fraction is increased, which agrees with the predictions of theoretical models. In an applied magnetic field, the microstructure passes from a suspension to an ordered structure, resulting in an increase velocity of sound. For low volume fractions a model of fiber suspensions predicts the microstructure formed in accordance to the experimental velocity of sound measured. On the other hand, for higher volume fractions, the microstructure can be considered as a porous material and the increase of sound velocity can be qualitatively explained from this theoretical point of view. These results are compared to microstructure images obtained using optical methods.