Magnetorheology: Fluids, Structures and Rheology

Computation of stagnation coating flow of electro-conductive ternary Williamson hybrid [Formula: see text] nanofluid with a Cattaneo-Christov heat flux model and magnetic induction

Modern smart coating systems are increasingly exploiting functional materials which combine multiple features including rheology, electromagnetic properties and nanotechnological capabilities and provide a range of advantages in diverse operations including medical, energy and transport designs (aerospace, marine, automotive). The simulation of the industrial synthesis of these multi-faceted coatings (including stagnation flow deposition processes) requires advanced mathematical models which can address multiple effects simultaneously. Inspired by these requests, this study investigates the interconnected magnetohydrodynamic non-Newtonian movement and thermal transfer in the Hiemenz plane's stagnation flow. Additionally, it explores the application of a transverse static magnetic field to a ternary hybrid nanofluid coating through theoretical and numerical analysis. The base fluid (polymeric) considered is engine-oil (EO) doped with graphene [Formula: see text], gold [Formula: see text] and Cobalt oxide [Formula: see text] nanoparticles. The model includes the integration of non-linear radiation, heat source, convective wall heating, and magnetic induction effects. For non-Newtonian characteristics, the Williamson model is utilized, while the Rosseland diffusion flux model is used for radiative transfer. Additionally, a non-Fourier Cattaneo-Christov heat flux model is utilized to include thermal relaxation effects. The governing partial differential conservation equations for mass, momentum, energy and magnetic induction are rendered into a system of coupled self-similar and non-linear ordinary differential equations (ODEs) with boundary restrictions using appropriate scaling transformations. The dimensionless boundary value problem that arises is solved using the bvp4c built-in function in MATLAB software, which employs the fourth-order Runge-Kutta (RK-4) method. An extensive examination is conducted to evaluate the impact of essential control parameters on the velocity [Formula: see text], induced magnetic field stream function gradient [Formula: see text] and temperature [Formula: see text] is conducted. The relative performance of ternary, hybrid binary and unitary nanofluids for all transport characteristics is evaluated. The inclusion of verification of the MATLAB solutions with prior studies is incorporated. Fluid velocity is observed to be minimized for the ternary [Formula: see text]-[Formula: see text]-[Formula: see text] nanofluid whereas the velocity is maximized for the unitary cobalt oxide [Formula: see text] nanofluid with increasing magnetic parameter ([Formula: see text] Temperatures are elevated with increment in thermal radiation parameter (Rd). Streamlines are strongly modified in local regions with greater viscoelasticity i.e. higher Weissenberg number [Formula: see text]. Dimensionless skin friction is significantly greater for the ternary hybrid [Formula: see text]-[Formula: see text]-[Formula: see text] nanofluid compared with binary hybrid or unitary nanofluid cases.

Sedimentation Stability of Magnetorheological Fluids: The State of the Art and Challenging Issues

Among the many factors causing particle sedimentation, three principal ingredients are heavily involved: magnetic particles, a carrier liquid (base oil), and additives (surfactant). Therefore, many works have been carried out to improve the sedimentation stability of magnetorheological fluids (MRFs) by adopting the three methods. In the particle modification stage, the weight concentration, size distribution, particle shape, coated materials, and combinations of different sizes of the particles have been proposed, while for the modification of the carrier liquid, several works on the density increment, wettability control, and the use of natural oils, lubricant oil, grease, and ethyl- and butyl-acetate oils have been undertaken. Recently, in certain recipes to improve sedimentation stability, some additives such as aluminum stearate were used to increase the redispersibility of the aggregated particles. In addition, several works using more than two recipes modifying both the particles and base oils are being actively carried out to achieve higher sedimentation stability. This review article comprehensively introduces and discuses the recipes to improve sedimentation stability from the aspects of the three ingredients. A few conceptual methodologies to prevent the sedimentation occurring via a bottle's storage on the shelves of the application systems are also presented, since, to the author's knowledge, there has not been a report on this issue. These are challenging works to be explored and developed for successful application systems' MRFs.

Formulation of Magneto-Responsive Hydrogels from Dually Cross-Linked Polysaccharides: Synthesis, Tuning and Evaluation of Rheological Properties

Smart hydrogels based on natural polymers present an opportunity to fabricate responsive scaffolds that provide an immediate and reversible reaction to a given stimulus. Modulation of mechanical characteristics is especially interesting in myocyte cultivation, and can be achieved by magnetically controlled stiffening. Here, hyaluronan hydrogels with carbonyl iron particles as a magnetic filler are prepared in a low-toxicity process. Desired mechanical behaviour is achieved using a combination of two cross-linking routes—dynamic Schiff base linkages and ionic cross-linking. We found that gelation time is greatly affected by polymer chain conformation. This factor can surpass the influence of the number of reactive sites, shortening gelation from 5 h to 20 min. Ionic cross-linking efficiency increased with the number of carboxyl groups and led to the storage modulus reaching 103 Pa compared to 101 Pa–102 Pa for gels cross-linked with only Schiff bases. Furthermore, the ability of magnetic particles to induce significant stiffening of the hydrogel through the magnetorheological effect is confirmed, as a 103-times higher storage modulus is achieved in an external magnetic field of 842 kA·m−1. Finally, cytotoxicity testing confirms the ability to produce hydrogels that provide over 75% relative cell viability. Therefore, dual cross-linked hyaluronan-based magneto-responsive hydrogels present a potential material for on-demand mechanically tunable scaffolds usable in myocyte cultivation.

Ferrofluids and bio-ferrofluids: looking back and stepping forward

Ferrofluids investigated along for about five decades are ultrastable colloidal suspensions of magnetic nanoparticles, which manifest simultaneously fluid and magnetic properties. Their magnetically controllable and tunable feature proved to be from the beginning an extremely fertile ground for a wide range of engineering applications. More recently, biocompatible ferrofluids attracted huge interest and produced a considerable increase of the applicative potential in nanomedicine, biotechnology and environmental protection. This paper offers a brief overview of the most relevant early results and a comprehensive description of recent achievements in ferrofluid synthesis, advanced characterization, as well as the governing equations of ferrohydrodynamics, the most important interfacial phenomena and the flow properties. Finally, it provides an overview of recent advances in tunable and adaptive multifunctional materials derived from ferrofluids and a detailed presentation of the recent progress of applications in the field of sensors and actuators, ferrofluid-driven assembly and manipulation, droplet technology, including droplet generation and control, mechanical actuation, liquid computing and robotics.

High performance magnetorheological fluids: very high magnetization FeCo-Fe3O4 nanoclusters in a ferrofluid carrier

High magnetization Fe₃O₄/OA-FeCo/Al₂O₃ nanocomposite magnetic clusters have been obtained using a modified oil-in-water miniemulsion method. These nanocomposite clusters dispersed in a ferrofluid carrier result in a magnetorheological fluid with improved characteristics. The magnetic clusters have a magnetic core consisting of a mixture of magnetite nanoparticles of about 6 nm average size, stabilized with oleic acid (Fe₃O₄/OA) and FeCo/Al₂O₃ particles of about 50 nm average size, compactly packed in the form of spherical clusters with a diameter distribution in the range 100-300 nm and a hydrophilic coating of sodium lauryl sulphate surfactant. The surface chemical composition of the Fe₃O₄/OA-FeCo/Al₂O₃ clusters investigated by XPS indicates the presence of the Co²⁺ and Co³⁺ oxidation states of cobalt and the components of Fe²⁺ and Fe³⁺ characteristic to both an enhanced oxidation state at the surface of the FeCo particles and to the presence of magnetic nanoparticles of spinel structure which are decorating the supporting FeCo. This specific decorating morphology is also indicated by TEM images. Advanced characterization of the Fe₃O₄/OA-FeCo/Al₂O₃ magnetic clusters has been performed using Mössbauer spectroscopy and magnetization measurements at various temperatures between 6 K and 200 K. The unexpected formation of Co ferrite decorating nanoparticles was supported by Mössbauer spectroscopy. The dispersion of magnetic clusters in the ferrofluid carrier highly influences the flow properties in the absence of the field (shear thinning for low and moderate shear rates) and especially in applied magnetic field, when significant magnetoviscous effect and shear thinning was observed for the whole range of shear rate values. Detailed analysis of the magnetorheological behavior of the nanocomposite magnetic clusters dispersed in a ferrofluid carrier evidence significantly higher normalized dynamic yield stress values in comparison with the magnetite nanocluster suspensions of the same mass concentration, a promising result for this new type of nanocomposite magnetorheological fluid.

The role of thermal diffusion, particle clusters, hydrodynamic and magnetic forces on the flow behaviour of magneto-polymer composites

In this paper, we study the shear-induced flow of magneto-polymer composites, consisting of dispersions of magnetic particles in solutions of polymers, as a competition between the colloidal forces amid particles and their bulk transport induced by the hydrodynamic forces. For this aim, we analyse the role of different experimental parameters. Firstly, by using only solutions of a well-known anionic polymer (sodium alginate), we provoke a moderate hindering of particle movement, but keeping the liquid-like state of the samples. On the contrary, a gel-like behaviour is conferred to the samples when a cationic polymer (chitosan) is additionally added, which further reduces the particle movement. We analyse the effect of an applied magnetic field, which is opposed to particle transport by hydrodynamic forces, by inducing magnetic attraction between the particles. We perform the analysis under both stationary and oscillatory shear. We show that by using dimensionless numbers the differences between samples and experimental conditions are emphasized. In all cases, as expected, the transport of particles driven by bulk hydrodynamic forces dominates at high values of the shear rate. This article is part of the theme issue 'Transport phenomena in complex systems (part 1)'.

Internal structures and mechanical properties of magnetic gels and suspensions

We present results of theoretical and computer study of linear chain-like and complicated labyrinth structures in magnetic gels and suspensions as well as effect of these internal structures on macroscopic elastic properties of the composites. Our results show that at a certain threshold deformation, the structures experience a rupture which provokes a fall down of the macroscopic elastic stress, induced by the deformation. This effect is detected for both shear and tensile deformations. The results of calculations are compared with experimental data’s.

Compressions of magnetorheological fluids under instantaneous magnetic field and constant area

Compressions of magnetorheological (MR) fluids have been carried out under instantaneous magnetic fields. The yield strength of the MR fluid in compressive mode has been derived by assuming that it was a transformed shear flow in Bi-visous model. The compressive stresses have experimentally studied under different magnetic fields, different initial gap distances and different compressive velocities. The nominal yield shear stresses of the compressed MR fluid under different influential factors have been calculated. The compressive stress increased in a power law as the applied magnetic field increased, while it decreased as the initial gap distance and the compressive velocity increased. With the increase of magnetic field, the difference between the nominal yield shear stress curves increased, and the exponents of the power law increased with the increase of the magnetic field strengths. A larger initial gap distance and a lower compressive velocity resulted in a higher nominal yield shear stress under the same instantaneous magnetic field. The achieved results of the nominal yield shear stress with magnetic field seemed to deviate from the prediction of dipole model, and the chain structure aggregation effect, the sealing effect and the friction effect by compression should be considered.

Magneto-mechanical properties of elastic hybrid composites

The paper gives an overview of tunable elastic magnetic composites based on silicon rubber matrix highly filled with a magnetic soft and hard filler. The magnetic soft phase, which is represented by iron microparticles, allows active control of the physical properties of the composites, while the magnetically hard phase (e.g. neodymium–iron–boron alloy microparticles) is mainly responsible for passive adjustment of the composite. The control is performed by the application of an external magnetic field in situ, and passive adjustment is performed by means of pre-magnetization in order to change material remanent magnetization, i.e. the initial state. The potential and limits of active control and passive tuning of these composites in terms of their magneto-mechanical behavior are presented and discussed.

Magnetoactive elastomers for magnetically tunable vibrating sensor systems

Magnetoactive elastomers (MAEs) are a special type of smart materials consisting of an elastic matrix with embedded microsized particles that are made of ferromagnetic materials with high or low coercivity. Due to their composition, such elastomers possess unique magnetic field-dependent material properties. The present paper compiles the results of investigations on MAEs towards an approach of their potential application as vibrating sensor elements with adaptable sensitivity. Starting with the model-based and experimental studies of the free vibrational behavior displayed by cantilevers made of MAEs, it is shown that the first bending eigenfrequency of the cantilevers depends strongly on the strength of an applied uniform magnetic field. The investigations of the forced vibration response of MAE beams subjected to in-plane kinematic excitation confirm the possibility of active magnetic control of the amplitude-frequency characteristics. With change of the uniform field strength, the MAE beam reveals different steady-state responses for the same excitation, and the resonance may occur at various ranges of the excitation frequency. Nonlinear dependencies of the amplification ratio on the excitation frequency are obtained for different magnitudes of the applied field. Furthermore, it is shown that the steady-state vibrations of MAE beams can be detected based on the magnetic field distortion. The field difference, which is measured simultaneously on the sides of a vibrating MAE beam, provides a signal with the same frequency as the excitation and an amplitude proportional to the amplitude of resulting vibrations. The presented prototype of the MAE-based vibrating unit with the field-controlled “configuration” can be implemented for realization of acceleration sensor systems with adaptable sensitivity. The ongoing research on MAEs is oriented to the use of other geometrical forms along with beams, e.g. two-dimensional structures such as membranes.

Targeted patterning of magnetic microparticles in a polymer composite

Structured and polymerized in a uniform external magnetic field, polymer composites based on magnetic soft microparticles are considered. Variations of magnetic field parameters and material composition provide a possibility of targeted micro-structural patterning of these composites. The influences of parameter variations on the resulting internal micro-structure of the low concentrated specimens are evaluated and visualized using optical microscopy and microcomputed tomography. The experimental findings are discussed in order to provide advanced possibilities of controlled patterning of soft magnetic materials. It is experimentally demonstrated that the final three-dimensional morphology of composite structure is determined mainly by the concentration of magnetic powder. The intensity of the applied magnetic field influences the rate of structuring of particles in initially viscous media and, therefore, may provide a potential opportunity to obtain non-ergodic microstructures when the matrix is polymerized before the particles have completed the structuring process. The results obtained can serve as a basis for further development of the engineering method of targeted patterning. The method is intended to obtain a material with the desired microstructure by selecting specific parameters of external stimuli and components of the composite. This article is part of the theme issue 'Patterns in soft and biological matters'.

Macroscopic two-fluid effects in magnetorheological fluids

We investigate macroscopic two-fluid effects in magnetorheological fluids generalizing a one-fluid model studied before. In the bulk of the paper we use a model in which the carrier fluid, with density ρ_{1}, moves with velocity v_{1}, while the magnetic component (density ρ_{2}) and, therefore, the magnetization and the magnetic-field-induced relaxing strain field move with velocity v_{2}. In the framework of macroscopic dynamics we find, in particular, reversible dynamic and dissipative cross-coupling terms between the magnetization and the velocity difference. Experiments to detect some of these cross-coupling terms are suggested. We also compare the results of the two-fluid model presented here with two-fluid models available for electrorheological fluids. In two appendices we discuss the simplifying assumptions made to arrive at the model used in this paper and we also outline how to detect potential deviations from this model.

Recovery and characterization of nickel particles by chemical reduction method from wastes generated in electroless industry

The discarded solutions by the chemical nickel industry have high amounts of nickel, this is why they are considered hazardous wastes for the health and the environment. On the other side, Ni particles can have potential applications in the developing of magnetorheological fluids currently being used to improve the performance of mechanical devices. The present study raises the treatment of a residual effluent from a chemical nickel industry by applying a chemical precipitation which uses sodium hypophosphite as a reducer, and varying the order of the reagents involved in the conditioning of the reaction with respect to the reducer. The recovered solids were studied using different material characterization techniques to recognize the chemical composition (X-Ray Fluorescence, Inductively Coupled Plasma Mass Spectrometry), crystallinity and morphology (X-Ray Diffraction, Scanning Electron Microscope), surface charge and size distribution (Dynamic Light Scattering). By the chemical reduction treatment it was possible to decrease the amount of nickel in the residual between 97.25% and 99.50%, obtaining Ni particles that were then tested to be used in magnetic fluids. To this purpose a suspension was prepared by mixing the Ni particles with silicone oil in a constant solid/liquid ratio, and the rheological behavior of this suspension was evaluated as a function of the magnetic field and the deformation applied, revealing an interesting magnetorheological behavior.

Zero-Field and Field-Induced Interactions between Multicore Magnetic Nanoparticles

In this paper, the Langevin dynamics simulation method is used to study magnetic interactions between a pair of multicore magnetic nanoparticles subjected to a uniform magnetic field. Multicore nanoparticles are modelled as spherical rigid clusters of single-domain superparamagnetic cores coupled via dipole-dipole interactions. It is shown that the magnetic force between two well-separated clusters in a strong applied field can be accurately described within the induced point-dipole approximation. However, this approximation also assumes that there are no interactions between clusters in the zero-field limit. On the contrary, simulations indicate the existence of a relatively small attractive magnetic force between clusters, even in the absence of an applied field. It is shown that this force is a direct superparamagnetic analog of the van der Waals interaction between a pair of dielectric spheres.

Continuum model of magnetic field induced viscoelasticity in magnetorheological fluids

An effective macroscopic model of magnetorheological fluids in the viscoelastic regime is proposed. Under the application of an external magnetic field, columns of magnetizable particles are formed in these systems. The columns are responsible for solidlike properties, such as the existence of elastic shear modulus and yield stress, and are captured by the strain field, while magnetic properties are described by the magnetization. We investigate the interplay of these variables when static shear or normal pressure is imposed in the presence of the external magnetic field. By assuming a relaxing strain field, we calculate the flow curves, i.e., the shear stress as a function of the imposed shear rate, for different values of the applied magnetic field. Focusing on the small amplitude oscillatory shear, we study the complex shear modulus, i.e., the storage and the loss moduli, as a function of the frequency. We demonstrate that already such a minimal model is capable of furnishing many of the key physical features of these systems, such as yield stress, enhancement of the shear yield stress by pressure, threshold behavior in the spirit of the frequently employed Bingham law, and several features in the frequency dependence of storage and loss moduli.

Discontinuous shear thickening in concentrated suspensions

The flow of concentrated suspensions of solid particles can be suddenly blocked by the formation of a percolated network of frictional contacts above a critical value of the applied stress. Suspensions of magnetic particles coated with a superplastifier molecule were shown to produce a strong jamming transition. We find that, for these suspensions with an abrupt discontinuous shear thickening, a model using the divergence of the viscosity at a volume fraction that depends on the applied stress does not well describe the observed behaviour both below and above the critical stress. At a constant applied stress above the critical one, we have a stick-slip behaviour of the shear rate whose period can be predicted and scaled as the square root of the relaxation time of the frictional contacts. The application of a small magnetic field allows us to continuously decrease the critical shear rate, and it appears that the yield stress induced by the magnetic field does not contribute to the jamming transition. Finally, it is shown that this jamming transition also appears in the extrusion of a suspension through a die, but with a much slower dynamics than in the case of stress imposed on a rotational geometry. This article is part of the theme issue 'Heterogeneous materials: metastable and non-ergodic internal structures'.

Separation of two attractive ferromagnetic ellipsoidal particles by hydrodynamic interactions under alternating magnetic field

In applications where magnetic particles are used to detect and dose targeted molecules, it is of major importance to prevent particle clustering and aggregation during the capture stage in order to maximize the capture rate. Elongated ferromagnetic particles can be more interesting than spherical ones due to their large magnetic moment, which facilitates their separation by magnets or the detection by optical measurement of their orientation relaxation time. Under alternating magnetic field, the rotational dynamics of elongated ferromagnetic particles results from the balance between magnetic torque that tends to align the particle axis with the field direction and viscous torque. As for their translational motion, it results from a competition between direct magnetic particle-particle interactions and solvent-flow-mediated hydrodynamic interactions. Due to particle anisotropy, this may lead to intricate translation-rotation couplings. Using numerical simulations and theoretical modeling of the system, we show that two ellipsoidal magnetic particles, initially in a head-to-tail attractive configuration resulting from their remnant magnetization, can repel each other due to hydrodynamic interactions when alternating field is operated. The separation takes place in a range of low frequencies f_{c1}<f<f_{c2}. The upper frequency limit f_{c2}τ_{r}≈0.04 (where τ_{r} is the rotation time scale) depends weakly on the ratio of magnetic field to particle magnetization strength, whereas f_{c1} tends to zero when this ratio increases.

Novel reversible and switchable electrolytes based on magneto-rheology

Replacing organic liquid electrolytes with solid electrolytes has led to a new perspective on batteries, enabling high-energy battery chemistry with intrinsically safe cell designs. However, most solid/gel electrolytes are easily deformed; under extreme deformation, leakage and/or short-circuiting can occur. Here, we report a novel magneto-rheological electrolyte (MR electrolyte) that responds to changes in an external magnetic field; the electrolyte exhibits low viscosity in the absence of a magnetic field and increased viscosity or a solid-like phase in the presence of a magnetic field. This change from a liquid to solid does not significantly change the conductivity of the MR electrolyte. This work introduces a new class of magnetically sensitive solid electrolytes that can enhance impact resistance and prevent leakage from electronic devices through reversible active switching of their mechanical properties.

The influence of hydrodynamic diameter and core composition on the magnetoviscous effect of biocompatible ferrofluids

Suspensions of magnetic nanoparticles have received increasing interest in the biomedical field. While these ferrofluids are already used for magnetic resonance imaging, emerging research on cancer treatment focuses, for example, on employing the particles as drug carriers, or using them in magnetic hyperthermia to destroy diseased cells by heating of the particles. To enable safe and effective applications, an understanding of the flow behaviour of the ferrofluids is essential. Regarding the applications mentioned above, in which flow phenomena play an important role, viscosity under the influence of an external magnetic field is of special interest. In this respect, the magnetoviscous effect (MVE) leading to an increasing viscosity if an external magnetic field of a certain strength is applied, is well-known for singlecore ferrofluids used in the engineering context. In the biomedical context, multicore ferrofluids are preferred in order to avoid remanence magnetization and to enable a deposition of the particles by the organism without complications. This study focuses on a comparison of the MVE for three ferrofluids whose composition is identical except in relation to their hydrodynamic diameter and core composition-one of the fluids contains singlecore particles, while the other two feature multicore particles. This enables confident conclusions about the influence of those parameters on flow behaviour under the influence of a magnetic field. The strong effects found for two of the fluids should be taken into account, both in future investigations and in the potential use of such ferrofluids, as well as in manufacturing, in relation to the optimization of flow behaviour.

Description and performance of a fully automatic device for the study of the sedimentation of magnetic suspensions

In this paper we describe an experimental setup for the automatic determination of the sedimentation behavior of magnetic suspensions (i.e., disperse systems consisting on ferro- or ferri-magnetic particles in a suitable fluid) of arbitrary volume fraction of solids. The device is based on the evaluation of the inductance of a thin coil surrounding the test tube containing the sample. The inductance L is evaluated from the measurement of the resonant frequency of a parallel LC circuit constructed with the coil and a capacitor of known capacitance. The coil can be moved vertically along the tube at specified steps and time intervals, and from the knowledge of L as a function of the vertical position and time, one can get an image of the particle concentration profiles at given instants of time. The performance of the device is tested against suspensions of spherical iron particles in the micrometer size range dispersed in silicone oil, with various initial concentrations of solids. The sedimentation profiles are then compared with the predictions of existing models for the settling of disperse systems of non-interacting particles.

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.

Magnetic field induced inversion in the effect of particle size on powder cohesiveness

Experimental measurements are reported on the tensile yield stress of magnetofluidized beds of fine magnetic powders operated in the cross-flow configuration. In the absence of externally applied magnetic field the yield stress of the powder depends on particle size as expected, i.e., it increases as bead size is decreased. This trend is however inverted when an external magnetic field is applied. It is suggested that the average orientation of interparticle contacts relative to the direction of the field as affected by particle size plays a relevant role on the magnetic yield stress of these systems.

Preparation and characterization of iron-based magnetorheological fluids stabilized by addition of organoclay particles

Suspensions of micrometer-sized iron particles (10 vol %) dispersed in kerosene and stabilized by addition of organoclay particles were prepared. The magnetization curves of these suspensions were measured, and their sedimentation and redispersion behaviors were analyzed as a function of clay concentration by means of optical and rheological methods. Furthermore, their magnetorheological properties were investigated using a controlled rate magnetorheometer and the effect of clay concentration on these properties was also analyzed. These experiments showed that the addition of clay slows down iron particle settling and eases the redispersion of the iron-based suspensions without masking their magnetorheological properties. Two mechanisms were found to be involved in this behavior: (i) the formation of a clay gel network and (ii) the presence of heterogeneous iron-clay adhesion.

Oscillatory squeeze flow of suspensions of magnetic polymerized chains

We report a rheological study of suspensions of non-Brownian chain-like magnetic particles in the presence of magnetic fields. These particles have been synthesized using spherical iron particles by linking them with a polymer and are called polymerized chains. We have shown that, in oscillatory squeeze mode, the suspensions of such chain-like particles develop yield stress several times higher than that of conventional magnetorheological fluids based on spherical iron particles. This is explained in terms of solid friction between polymerized chains, which form entangled aggregates in the presence of a magnetic field. For the suspension of spherical particles, the squeezing force increases with the magnetic field intensity at low magnetic fields, but decreases dramatically at higher fields because of cavitation or air entrainment. Such a decrease in transmitted force does not take place in suspensions of polymerized chains, at least for fields smaller than 30 kA m(-1), which could make these suspensions preferable for application in squeeze-film dampers.

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 range of separation distances; we found that the equilibrium distance between two particles was r = 1.19 +/- 0.05 <d> (<d> was the average diameter of the pair of particles).

Preparation and sedimentation behavior in magnetic fields of magnetite-covered clay particles

This work is devoted to the preparation of magnetite-covered clay particles in aqueous medium. For this purpose, magnetite nanoparticles were synthesized by a coprecipitation method. These magnetic particles are adhered to sodium montmorillonite (NaMt) particles in aqueous suspensions of both materials, by appropriate control of the electrolyte concentrations. The best condition to produce such heteroaggregation corresponds to acid pH and approximately 1 mol/L ionic strength, when the electrokinetic potentials (zeta-potential) of both NaMt and Fe3O4 particles have high enough and opposite sign, as demonstrated from electrophoresis measurements. When a layer of magnetite re-covers the clay particles, the application of an external magnetic field induces a magnetic moment in clay-magnetite particles parallel to the external magnetic flux density. The sedimentation behavior of such magnetic particles is studied in the absence or presence of an external magnetic field in a vertical direction. The whole sedimentation behavior is also strongly affected by the formation of big flocculi in the suspensions under the action of internal colloidal interactions. van der Waals and dipole-dipole magnetic attractions between magnetite-covered clay particles dominate the flocculation processes. The different relative orientation of the clay-magnetite particles (edge-to-edge, face-to-edge, and face-to-face) are discussed in order to predict the most favored flocculi configuration.