Influence of particle size on the rheological properties of magnetorheological suspensions

Magnetorheological suspensions with different particle sizes including bimodal particle size distributions have been rheologically investigated. The results show a continuous decrease in the base viscosity without magnetic field when increasing the ratio of coarse to fine particles, which is in accordance with the observed trend of sedimentation stability of the samples. However, measurements of the shear stress with a rising magnetic flux density demonstrate higher values for MR suspensions with special mixtures of coarse and fine particles indicating a strengthening of the particle interactions in the magnetic field. The change of the viscoelastic properties of the MR suspensions in dependence on the field strength and shear load (amplitude) have been studied with oscillatory experiments. As expected, at low field strengths, the viscous behavior dominates and, at high field strengths, the elastic behavior, but differences in the crossover field strengths between the two regimes occur for the different particle size distributions. The shear load variation also shows differences between the various MR suspensions indicating the highest yield stress for the sample with only fine particles without magnetic field, but the lowest yield stress at moderate field strengths