SHEAR BANDED STRUCTURES AND NEMATIC TO ISOTROPIC TRANSITION IN MR FLUIDS

Abstract

The experimental shear stress versus shear rate curves of MR fluids are analysed with the help of a chain model. Two critical shear rates are found, one corresponding to the onset of agregate breaking and the other corresponding to the total disappearance of aggregates. Above this second critical shear rate we observe an abrupt jump of stress and the onset of a layered stripe pattern. This novel shear induced transition can be observed by using a cone-plate geometry but is usually smeared out in a plate-plate geometry. Still in a plate geometry we also have observed a similar phase separation in ER fluids but without a clear rheological signature. We show that this phase separation can be explained by the transition from a nematic like order induced by the field to an isotropic state which is obtained when the shearing hydrodynamic forces on a pair of particles overcome the magnetic or electrostatic forces. The critical shear stress predicted on this basis is in good agreement with the experimental results. We find a similar layered pattern in a rotating magnetic field. The connection between the two situations is explained.