On the effect of particle porosity and roughness in magnetorheology

Abstract

We report a study on the mechanical properties of magnetorheological (MR) fluids prepared with porous iron particles with rough surfaces. These particles were obtained by reducing a magnetite precursor in a H2 atmosphere at 400 °C. Small-amplitude dynamic oscillatory and steady shear flow measurements were carried out in the presence of external magnetic fields. Results were compared with those obtained for MR fluids prepared with conventional solid carbonyl iron particles of comparable size. We found significant differences between the rheology of both types of suspensions, and, more importantly, we found that simple available models can predict quantitatively those differences as long as the average density of the particles is known and is used to calculate their effective volume magnetization and the real volume fraction of the MR fluids prepared with them. By doing so, we obtained for both the porous iron suspensions and the solid iron suspensions a single master curve of the dimensionless storage modulus at saturation [G'sat/(μoMs2)] as a function of volume fraction (φ), and a good collapse of the viscosity versus Mason number curves as well as of the yield stress versus applied field curves. Particle porosity is thus an important factor in MR fluids and should be considered in their design and modeling. Finally, the porous iron suspensions also exhibited an atypical thickening behavior that was not observed in the solid iron analogues and that we tentatively ascribe to the rougher surface of the porous particles.