0000000000160868
AUTHOR
S. Lacis
DIRECT NUMERICAL SIMULATION OF MOTION OF FERROMAGNETIC PARTICLES IN MAGNETORHEOLOGICAL SUSPENSION
ABSTRACT Results simulation of magnetorheological suspension at particle level are reported. The present approach accounts for a better description of hydrodynamic interaction between close spheres. Development of lamellar structures similar to those obtained by other researchers in Poiseuille flow is observed in shear flow. Studies of single layer lamellar structures reveal presence of short chains and more complex aggregates.
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…
Kinetics of doublet formation in bicomponent magnetic suspensions: The role of the magnetic permeability anisotropy
Micron-sized particles (microbeads) dispersed in a suspension of magnetic nanoparticles, i.e., ferrofluids, can be assembled into different types of structures upon application of an externalmagnetic field. This paper is devoted to theoretical modeling of a relative motion of a pair of microbeads (either soft ferromagnetic or diamagnetic) in the ferrofluid under the action of applied uniform magnetic field which induces magnetic moments in the microbeads making them attracting to each other. The model is based on a point-dipole approximation for the magnetic interactions between microbeads mediated by the ferrofluid; however, the ferrofluid is considered to possess an anisotropic magnetic p…
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…
Direct numerical simulation of MR suspension: The role of viscous and magnetic interactions between particles
A numerical method is developed with aim to simulate the magnetorheological (MR) suspension taking into account realistic magnetic forces. The MR suspension is described by spherical particles with nonlinear magnetic properties suspended in a shear flow. Inertia effects, Brownian motion and buoyancy forces are neglected. The hydrodynamic interaction between close particles is taken into account approximately. Results of some test simulations are presented.
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 …