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Above a critical shear rate we observe in suspensions polarized by an external field an abrupt jump of stress and the onset of a layered stripe pattern. This novel shear-induced transition can be systematically found by using appropriated geometry. We show that it can be explained by the transition from a nematiclike 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.[S0031-9007(98)08119-8] PACS numbers: 82.70.Kj, 83.80.Gv Electrorheological (ER) and magnetorheological (MR) fluids are suspensions of highly polarizable particles in a nonconducting oil. In the presence of an electric or of a magnetic field the attractive dipolar interaction in the direction of the field induces the formation of a solid network of particles which can sustain a stress without flowing. This fundamental change of rheology being electronically controlled, these fluids are very attractive for applications in active damping and many others fields [1][2][3]. For the sake of simplicity, the rheology of these fluids is very often characterized by a Bingham law: t t s 1 h ᠨ g; although a Casson law or other power laws can often better describe their rheological behavior [4,5].Actually, a good model should take into account the shear rate dependence of the average length of the transient aggregates by using a balance between the hydrodynamic and dipolar forces and some attempts have been done in this direction at low volume fractions [6,7].In order to obtain accurate rheological measurements on magnetic fluids, we have used a cone-plate geometry which has the advantage-relative to the more usual plate-plate geometry-of a constant shear rate inside the cell. In this geometry we have found a jump of stress at a critical shear rate, and we have observed that this jump of stress was related to the onset of a layered structure. To account for these unexpected results we propose a novel mechanism of phase separation which couples the disappearance of oriented chains of particles to the onset of attractive forces in the plane defined by the velocity and the field.Samples.-We have prepared fluids which are made of spherical particles with a rather good monodispersity. For MR fluids the magnetic particles are made of polystyrene containing magnetite inclusions. These particles are manufactured by Rhone-Poulenc for protein separation. They have an average diameter of 0.5 mm and a standard deviation of 10% measured by light scattering and are suspended in a mixture of water and glycerol in order to increase the zero field viscosity of the suspension. The permeability of the particles as a function of the magnetic field has been obtained from a measurement of the magnetization at a volume fraction of 4.7% and by using the Maxwell-Garnett theory [8], which is well adapted for low volume fraction. The ER fluid we have used is made fr...

International audienceIn the presence of dispersant molecules currently used in cement industry and based on polyethylene oxide (PEO), we found a strong discontinuous shear thickening (DST) at high volume fraction in suspensions of calcium carbonate particles. The transition was reversible and the critical shear rate and shear stress for which this instability appears are reported versus the volume fraction of particles. A model of repulsive forces between polymers, taking into account the thickness of the polymer layer and the density of adsorption on the surface of the particles, can explain the differences of critical stresses observed between these three dispersant molecules. In particular, it explains why a small polymer densely adsorbed can be more efficient to repel the transition at higher stress than a larger molecule less densely adsorbed. Above the transition, we find that the suspension presents a special kind of stick-slip instability with even the presence of a negative shear rate under constant applied stress. A model is proposed which well predicts this regime by taking into account both the inertia of the apparatus and the viscoelasticity of the suspension

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