A ferrofluid emulsion, subjected to a slowly increasing magnetic field, exhibits a complicated structural behavior: a gas of Brownian particles changes to columnar solid structures due to induced dipole interaction. Two transition (intermediate) structural regimes are observed: (i) randomly distributed chains and particles and (ii) distinct thin columns and randomly distributed chains and particles. Three structural transition magnetic fields are found, one marking each structural transition, from the initial to the final structural regime. A structural diagram of the structural transition magnetic fields, H(C), versus particle volume fractions, straight phi, is constructed experimentally. Theoretical models of scaling calculations, based upon the dominant magnetic interaction in each structural regime, give the three structural transition magnetic-field relations as H(C1) proportional to straight phi(-1/2), H(C2) proportional to straight phi(-1/4), and H(C3) proportional to (straight phi(gamma)/G2)exp(piG/straight phi((gamma/2))), where gamma=0.39 and G=0.29 for our sample. The final end shape of columns and the relative position between columns show that the end-end repulsion between chains is important in the structural formation.
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...
A transition from a hexagonal to a layered pattern is observed when a magnetic suspension structured by a magnetic field is submitted to an oscillating shear flow. This transition occurs at a well-defined strain ␥ o ϭ0.15, which is found to be independent of the cell thickness, the intensity of the magnetic field, and the Péclet number. This layered pattern is stable in the absence of the flow if it has been formed at a Péclet number higher than unity. In this domain the period of the stripes increases with the intensity of the magnetic field and decreases with the initial volume fraction of magnetic particles. These features are explained by a model based on the minimization of the magnetic energy and on the equilibrium of osmotic, hydrodynamic, and magnetic pressures.
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