Pavel Kuzhir

Magnetorheology of fiber suspensions. I. Experimental

This work reports a detailed study on the shear magnetorheology of suspensions of magnetic microfibers. The steady-state regime was investigated using a controlled-stress rheometer for different concentrations of particles and under the presence of a broad range of applied magnetic fields (up to 512kAm−1). The results were compared with those obtained for conventional magnetorheological fluids (suspensions of magnetic microspheres). It was found that the suspensions of magnetic fibers show an enhanced magnetorheological effect. We proposed the existence of field-dependent solid friction between fibers as the main physical reason for this enhancement. In order to ascertain the relevance of the interfiber solid friction, the microscopic structure of fiber suspensions was investigated using an optical microscope. In the absence of applied field, fibers form an entangled network with approximately isotropic orientation. Upon magnetic field application, the fiber network becomes deformed and approximately alig...

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 found that the experimental yield stress increases with the concentration of magnetite nanoparticles in the ferrofluid. Unfortunately, this behaviour is not obtained from calculations based on the chain model, which predict just the opposite trend.

New magnetorheological fluids based on magnetic fibers

In this work, we report the synthesis under the presence of external magnetic fields of two types of magnetic microfibers: (i) cobalt wires; and (ii) rigid filaments of chemically linked iron microspheres. Optical microscopy and scanning electron microscopy (SEM) were used for characterizing their shape and size. The effect of magnetic field intensity, mixing frequency and reagent concentration on the final size of the fibers was investigated. The particles were also characterized by means of energy dispersive X-ray spectroscopy (EDX). The magnetization curves of both the cobalt wires and the iron filaments were obtained and compared with those of cobalt spheres and iron spheres respectively. Finally, these magnetic fibers were used for the preparation of new smart composites by dispersing them in a liquid carrier. Interestingly, the suspensions of cobalt wires show a remarkable increase in the yield stress when compared to conventional (based on spherical particles) magnetorheological fluids.

Magnetorheology of fiber suspensions. II. Theory

This paper reports the first predictions of the yield stress of suspensions of non-Brownian magnetic fibers in the presence of uniform magnetic fields. The quasistatic regime of the shear deformation (before the flow onset) of the suspension is studied. Four different structures of the magnetic fiber suspensions are considered—column, zigzag, three-dimensional stochastic and near-planar stochastic structures—and the yield stress is attributed to the failure of the given structure at a critical strain. The main contributions to the yield stress are found to come from the restoring magnetic torque acting on each fiber and from the solid friction between fibers. The enhanced magnetorheological effect of magnetic fiber suspensions observed experimentally [M. T. Lopez-Lopez et al., J. Rheol. 53, 115–126 (2009)] is explained and quantified in terms of interfiber friction. Surprisingly, the dipolar magnetic interactions between fibers do not affect significantly the yield stress. The lowest yield stress is obtai...

Normal stresses in a shear flow of magnetorheological suspensions: Viscoelastic versus Maxwell stresses

This work reports an experimental and theoretical study on the normal force developed by suspensions of magnetic microparticles subjected to magnetic fields. Experimental values of the normal force were obtained using a rotational rheometer, for a broad range of particle concentration in the suspensions. Applied magnetic fields up to 343 kA/m were generated in the plate-plate measuring geometry. It was found that the normal force exhibited a high-value plateau at low shear, followed by a decrease as the suspensions started to flow and a final low-value plateau at high shear. These three regions in the normal force vs shear rate curve were well correlated with the microscopic regimes in the suspensions: field-aligned structures filling the gap, inclined structures still filling the gap, and structures non-filling the gap. The theoretical model developed is based on the equilibrium between hydrodynamic and magnetostatic torques and forces in a field-induced aggregate of particles subjected to shear. The stress tensor was obtained and the normal force calculated as the integral of the stress over the total surface of the rotational plate. A good correspondence among the theoretical and experimental values was obtained.

Yield stress in magnetorheological suspensions near the limit of maximum-packing fraction

This work deals with the magnetic field-induced static yield stress of magnetorheological (MR) suspensions with concentration near the limit of maximum-packing fraction. With this aim, homogeneous suspensions of iron microparticles with 50 vol.% concentration were prepared, and their yield stress measured as a function of the applied magnetic field. In view of the failure of existing models to predict, on the basis of realistic hypotheses, the values of the yield stress of highly concentrated MR suspensions, we developed a new model. Our model considers that field application induces body-centered tetragonal structures. Upon shearing, these structures deform in such a way that interparticle gaps appear between neighboring particles of the same chain, whereas the approach of particles of parallel chains ensures the mechanical stability of the whole multi-chain structure. Based on this hypothesis, and using finite element method simulations of interparticle magnetic interactions, our model is able to quanti...

Steady shear flow of magnetic fiber suspensions: theory and comparison with experiments

This paper is focused on the rheology of magnetic fiber suspensions in the presence of a magnetic field applied perpendicular to the flow. At low Mason numbers, Mn<0.1, the experimental flow curves show a steep initial section corresponding to the inclination and stretching of the gap-spanning aggregates formed upon magnetic field application. At higher Mason numbers, aggregates no longer stick to the walls and the flow curves reach a Bingham regime, with the dynamic yield stress growing with the magnetic field intensity. This yield stress appears to be about three times higher for the fiber suspensions than for the suspensions of spherical particles. Such difference, measured at relatively low magnetic field intensities, H0<30 kA/m, is explained in terms of the enhanced magnetic susceptibility of the aggregates composed of fibers compared to the aggregates composed of spherical particles. For weak magnetic fields, the forces of solid friction between fibers are expected to play a minor role on the stress...

Hydrodynamic bearing lubricated with magnetic fluids

This paper summarizes the work carried out in the development of hydrodynamic lubricated journal bearings with magnetic fluids. Two different fluids have been analyzed, one ferrofluid from FERROTEC APG s10n and one magnetorheological fluid from LORD Corp., MRF122-2ED. Theoretical analysis has been carried out with numerical solutions of Reynolds equation, based on apparent viscosity modulation for ferrofluid and Bingham model for MR fluid. To validate this model, one test bench has been designed, manufactured and set up, where preliminary results shown in this paper demonstrate that magnetic fluids can be used to develop active journal bearings.

Capillary ascension of magnetic fluids

Abstract The equilibrium of a magnetic fluid column inside a cylindrical capillary is investigated in the presence of the external uniform magnetic field. It is found that due to the fluid meniscus deformation, the surface pressure drop in the fluid decreases in the fields longitudinal and transverse to the capillary axis.

Synthesis and magnetorheology of suspensions of submicron-sized cobalt particles with tunable particle size

Different samples of cobalt powder were synthesized. Particle size and shape were characterized using electron microscopy and light scattering. These measurements showed that the synthesized powders consisted of monodisperse spheres with average diameters ranging between 63 and 760 nm. These powders were used for the preparation of magnetorheological (MR) fluids by dispersing them in silicone oil. The MR properties of these MR fluids were investigated. It was found that particle size did not have much influence on the MR response of MR fluids, for average particle diameters larger than 100 nm. On the other hand, the MR response decreased appreciably when the average particle diameter was diminished below 100 nm; a theory based on the change of the shape of the aggregates with the size of the particles could explain these observations.