Abstract
A ferrofluid, which is a suspension of magnetic particles in matrix fluid, is known as one of the intelligent fluids. The particles form clusters under the magnetic fields. As a result, the rheological properties are very complex. The main cause is a lot of types of aggregate structures. In this study, we have studied the relation between rheological properties and aggregate structures for oscillatory shear flows by Stokesian dynamics simulations. The following results are obtained. The aggregation is of layered structures when magnetic forces are dominant, while it is of the inclined structures when shear forces are dominant. In the latter case, the phase angle of the stress due to the magnetic force is delayed relative to that of the strain, and the magnetic force has a negative contribution to the viscosity. It is found that the origin of this negative viscosity contribution is the interactions between particles in the same cluster in the low strain region, and the interactions between clusters in the large strain region.
