0000000000315727
AUTHOR
Georges Bossis
Effects of the magnetodipolar interactions in the alternating magnetic fields
A macroscopic theory for the description of the structure formation in a system of magnetic dipoles under the action of a high-frequency rotating field is presented. Continuum equations for the effective magnetic field strength describing particle interactions are derived. It is shown that, contrary to the case of a constant magnetic field, where the demagnetizing field arising from a concentration fluctuation stabilizes the system with respect to the phase separation, the same concentration fluctuation will be amplified in the case of a rotating magnetic field, leading to the formation of a layered structure. The extensions of the model necessary for the description of the formation of the…
Yield behavior of magnetorheological suspensions
The rheology of suspensions containing magnetic particles of micrometer size is strongly modified by the application of a magnetic field. We first describe a model experiment where the suspension is made of spheres of millimeter size and we show that the experimental dependence of the yield stress is well predicted by a non-affine model where the chains of particles break in the middle. Then we compare these predictions with some experimental results obtained on suspensions of carbonyl iron; it is shown that this model does not apply in this case. We propose a mechanism where after some friction on the walls the aggregates begin to rotate and break at a strain smaller than unity, due to the…
MAGNETIC INTERACTIONS OF CHAINS FORMED BY FERROMAGNETIC SPHERES
General properties of sphere-chain and chain-chain structures are analysed for homogeneously magnetized spheres. For field aligned structures the local maximum of the magnetic force is observed at H0/MS ≈ 0.5. The magnetic saturation effects are accounted by the Frölich-Kennelly law. Outlines of the approximate methodology of multi-chain interaction are given together with the test results.
Magnetorheology for suspensions of solid particles dispersed in ferrofluids
In this work, the magnetorheological properties of suspensions of micron-sized iron particles dispersed in magnetite ferrofluids were studied. With this aim, the flow properties of the suspensions in the steady-state regime were investigated using a commercial magnetorheometer with a parallel-plate measuring cell. The effect of both magnetite and iron concentration on the magnitude of the yield stress was studied for a broad range of magnetic fields. In addition, the experimental values of the yield stress were compared with the predictions from the chain model. With this purpose the values of the yield stress were obtained by means of finite element calculations. Interestingly, it was foun…
Magnetorheology: Fluids, Structures and Rheology
Magnetorheological suspensions are complex fluids which show a transition from a liquid behavior to a solid one upon application of a magnetic field. This transition is due to the the attractive dipolar forces between the particles which have been magnetized by the applied field. The formation of a network of particles or aggregates throughout the suspension is the basic phenomena which is responsible for the strength of the solid phase. In this paper we shall give an overview on the fluids and their properties and we shall especially emphasize the interplay between magnetic forces which are responsible for the gelling of the suspension and on the other hand of hydrodynamic and thermal forc…
Permeability measurements in cobalt ferrite and carbonyl iron powders and suspensions
Abstract Magnetic permeability data of cobalt ferrite and carbonyl iron suspensions are discussed. Using an induction method, the relative differential permeability, μr,dif, was measured as a function of the internal magnetic field for different volume fractions of the solid phase. In the case of cobalt ferrite suspensions, the μr,dif−H curve was obtained for a first increasing ramp of magnetic field (data series “1”), a decreasing ramp (data series “2”), and the second increasing one (data series “3”). Series “1” showed a maximum in the μr,dif−H trend that did not appear in series “2” and “3”. Furthermore, the data in series “2” and “3” are always below those in series “1”. The latter beha…
Measurement of elastic forces between iron colloidal particles in a nematic liquid crystal.
The forces that arise between two iron particles in a nematic liquid crystal with a strong homeotropic anchoring were studied. For the first time, the short range repulsive force resulting from the presence of a hedgehog defect between two particles was precisely determined thanks to application of a small magnetic field and observation of the equilibrium position resulting from the balance between the elastic and magnetic forces. Above a given threshold force, the particles stuck together whereas the hedgehog defect was expelled and transformed into a Saturn ring located between the particles. The attractive part of the interparticle force was determined with the same method on the entire …
Experimental and Theoretical Study of the Field Induced Phase Separation in Electro- and Magnetorheological Suspensions
We present here the study of field induced phase separation in E.R. and M.R. fluids. Two thermodynamic models — one for the formation of chais of particles and the other for phase separation are presented and compared with experimental results obtained with two kinds of suspensions. One was made of silica particles in silicone oil and the other was made of magnetic polystyrene particles in water. In the presence of a flow the phase separation occurs with the dense phase forming a regular pattern of stripes. The dependence of the period of these stripes on the intensity of the magnetic field is well reproduced by the same kind of thermodynamic model if we add the effect of normal stresses i…
Flow-induced structures in magnetorheological suspensions
We have measured the yield stress of a magnetic suspension as a function of the external field. To explain the existence of this yield stress, we report a comparison between two models, one based on isolated chains of particles and the other taking into account that the structure is formed by aggregates of particles.