O. Volkova

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 forces which contribute to break this gel and allow the suspension to flow. The combination of these three forces gives rise to a very rich rheology whose many aspects are still not understood.

Magnetorheology of magnetic holes compared to magnetic particles

We compare the rheological behavior in a shear flow of two types of suspension of magnetic particles in the presence of a magnetic field. The first suspension is made of silica particles in a ferrofluid and the second one is made of carbonyl iron particles in silicone oil. The permeability curves of these two suspensions have been measured for different volume fractions as a function of the magnetic field in order to characterize the magnetic interactions. We show for both cases the existence of two different yield stresses: one associated with the solid friction of the particles on the plates of the rheometer and the second one with the rupture of the aggregates. This second yield stress presents a maximum with the volume fraction for the suspension of magnetic holes but grows faster than linearly with the volume fraction for the suspension of carbonyl iron. These features are explained by theoretical models based, respectively, on the deformation of aggregates of macroscopic size and on ruptures between...

Flow of magnetorheological fluid through porous media

Flow of a magneto-rheological (MR) fluid through different types of porous media (bundle of cylinders, packed beds of magnetic and non-magnetic spheres and cylinders) is considered, both theoretically and experimentally. The theory is based on averaging the magnetic and rheological properties of MR fluid in tortuous channels making different angles between local field and local velocity. A comparison of the pressure drop through porous beds and spiral channels is analyzed and practical recommendations are developed. It is shown that the mean yield stress of Bingham MR fluid (as well as the pressure drop, ΔP) depends on the mutual orientation of the external magnetic field and the main axis of the flow. This theory is tested against our experimental results and is shown to well predict the pressure drop obtained in different porous media.

Rheology of a gypsum suspension in the presence of different superplasticizers

In this work, the rheological properties of suspensions of micron-sized gypsum particles dispersed in water were studied in the presence of different fluidizer molecules. The yield stress and the shear moduli were measured versus the volume fraction in the presence of these molecules. Using the same polyelectrolyte with different molecular weights, the dependence of yield stress versus the gyration radius of the polymer was investigated; also different sizes of the gypsum particles allowed to check the size dependence of the yield stress. A particular attention was brought to the change of the thickness of the polymer layer with the volume fraction. From a model, which relates the steric interaction between the two polymer layers to the yield stress and shear modulus, an important compression of the polymer layer with the volume fraction was found. At higher volume fractions a dynamic jamming transition was observed at a critical volume fraction of 0.485 which does not depend on the presence of the fluidi...

Effect of the orientation of the magnetic field on the flow of magnetorheological fluid. II. Cylindrical channel

We consider the flow of a magnetorheological (MR) fluid in a cylindrical channel in the presence of a magnetic field inclined relatively to the channel axis (Oz). The stress tensor in the MR fluid is derived by assuming that chain structures are located in the planes parallel to the velocity and the magnetic field (plane Oxz). We show that the velocity field has two orthogonal axes of symmetry, Ox and Oy, in the plane perpendicular to Oz and that the plug zone can be approximated by an ellipsoid of major axis Ox. The plug zone is not defined in the usual way since inside the plug each line perpendicular to the Oy axis is moving at a different velocity. The experiments in a cylindrical capillary at high Mason numbers are compared for different angles between the flow and the field to theoretical predictions. We still recover Bingham behavior and all the pressure versus flow rate curves obtained at different field angles gather on a single straight line when represented against an average normalized yield s...

Granular rheology in zero gravity

We present an experimental investigation on the rheological behaviour of model granular media made of nearly elastic spherical particles. The experiments are performed in a cylindrical Couette geometry and the experimental device is placed inside an aeroplane undergoing parabolic flights to cancel the effect of gravity. The corresponding curves, shear stress versus shear rate, are presented, and a comparison with existing theories is proposed. The quadratic dependence on the shear rate is clearly shown, and the behaviour as a function of the solid volume fraction of particles exhibits a power law function. It is shown that theoretical predictions overestimate the experimental results. We observe, at intermediate volume fractions, the formation of rings of particles regularly spaced along the height of the cell. The differences observed between experimental results and theoretical predictions are discussed and related to the structures formed in the granular medium submitted to the external shear.

Functionalized microfibers for field-responsive materials and biological applications

Fiber-like particles are mostly used as basic components of electrorheological and magnetorheological suspensions because of their lower sedimentation rate and higher yield stress. The theoretical approach of the yield stress mainly rests on the interaction between two spherical particles. We analyze the interactions between fibers, taking into account the contribution of friction between fibers. Comparing experimental results to the model we conclude that it is not possible to clearly assess what is the part due to the friction in the increase of the yield stress which, on the other hand, can be explained by the increase of magnetization of the particles due to their lower demagnetization factor. We also show in an experiment between two nickel fibers that the rupture force between the fibers is very sensitive to the shape of their extremity. Besides the particles made of the same material, the core-shell particles made of different materials can also present advantages. We compare electrorheological suspensions made of pure polyaniline to those made of sepiolite and coated with polyaniline. We show that the much higher yield stress obtained with the hybrid particles is essentially due to a higher aspect ratio of sepiolite fibers.

Modelling of magnetic fluid support

One kind of elastic magnetic fluid support representing the magnetic fluid drop with permanent magnet inside is investigated experimentally and numerically. The dependencies between the magneto static force in support and the geometrical parameters and properties of the magnet and the magnetic fluid are established.

Active control of rod vibrations using magnetic fluids

The use of magnetic fluids in controlling rod vibrations is investigated. A prototype of ferrofluid vibration damper is designed and experimentally set up based on the principle of anti-resonance. The efficiency of this damping system is verified in experiments and well explained with classical equations of motion. The improvement of the present system towards active control of rod vibration is also discussed.

Discontinuous shear thickening in the presence of polymers adsorbed on the surface of calcium carbonate particles

In 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.