Effects of Nonmagnetizable Particles on the Structure of Magnetorheological Fluids

Background: Addition of nonmagnetizable particles to a magnetorheological (MR) suspension causes an increase in the magnetic field-induced yield stress. The goals of this paper are to determine how the suspension microstructure changes when the nonmagnetizable particles are added, and whether or not such changes can cause the changes in rheological properties. Methods: We employed particle-level simulations to investigate the structural and rheological properties of MR suspensions containing a mixture of magnetizable and nonmagnetizable spheres. We demonstrate that nonmagnetizable spheres cause the yield stress to increase in monolayers and three-dimensional simulations, as is observed in three-dimensional experiments. We examine the role of nonmagnetizable spheres in the suspension structure for monolayer and three-dimensional suspensions. Structure measures examined included the fluctuations in volume fraction, the pair distribution functions, and the eigenvalue ratio of the mass moment tensor of clusters. Results: The nonmagnetizable spheres cause only minor changes to the microstructure of the magnetizable sphere component of the suspensions. Furthermore, the nonmagnetizable spheres cause structural changes to monolayers that differ from the changes in microstructure of three-dimensional suspensions. Conclusion: The results suggest that the small structure changes caused by the addition of nonmagnetizable particles do not cause the increase in yield stress.