Flow-induced structures in magnetorheological suspensions

In this paper we compare the structures obtained in suspensions of micronic magnetic particles in the presence of a magnetic field and of a shear flow. We present experimental results which show that, when the shear forces are much lower than the dipolar magnetic forces, then the usual structure made of cylindrical aggregates rearrange in a well-defined striped pattern whose period can be predicted by introducing an hydrodynamic pressure. If instead of an alternating shear flow we use a steady shear flow, then we observe a jump of stress at some critical shear rate which is related to the onset of a new layered pattern. We demonstrate that, if instead of a steady shear flow in a constant field, we use a rotating field we observe the same kind of layered pattern. The origin of these layered patterns is shown to be due to the average attractive dipolar interaction in the plane of rotation of the field.