The Enhanced MR Effect of Dense Magnetic Particles Suspensions Consisting of Superfine α-Fe Particles

It is reported that magnetorheological (MR) effect was enhanced when superfineα-Fe particles and other nanosize particles were added to suspensions of dense micron magnetic particles. The effect of adding superfine particles on dynamics shear stress, sedimentation stability and structure of solidified MR of magnetic suspensions were studied. The experiment showed considerable increase of shear stress and much stability of sedimentation when the suspension consisting of superfine particles. The enhanced MR effect by superfine particles dealt with the properties, weight ratio and scale of superfine particles.     

Phase Transitions in Electro- and Magnetorheological Fluids

The model of internal structural phase transformations occurring in electro- and magnetorheological fluids under the action of external electric (magnetic) field is presented. The model describes both types of experimentally observed transformations such as the formation of linear chain clusters and bulk dense phases composed of suspension particles. The main qualitative result of this work is the conclusion that the appearance of chain clusters precedes the suspension separation into bulk phases with different particle densities and considerably affects the phase equilibrium diagrams. This conclusion agrees with the results of known experiments.     

Fabrication of multiwalled carbon nanotube-wrapped magnetic carbonyl iron microspheres and their magnetorheology

Magnetorheological (MR) properties and dispersion stability of magnetic carbonyl iron (CI) microspheres were examined and found to be enhanced by fabricating a dense nest composed of multiwalled carbon nanotubes (MWCNTs) on the surface of CI particles in this study. The coating process is achieved by using 4-aminobenzoic acid as a grafting agent via self-assembly mechanism under sonication in which the MWCNTs were adopted as the coating material because MWCNTs possess similar density with polymer but better magnetic properties due to the iron catalyst originally included within the walls. The coating thickness and morphology of the MWCNTs nest were found to be related with the sonication duration. The influence of the coating layers on the magnetic properties and MR performance (yield stress behavior, shear viscosity) were examined using a vibrating sample magnetometer and rotational rheometer. Sedimentation rates of the fabricated MWCNT/CI suspension and pure CI suspension were also investigated.     

An experimental method for the measurement of the stability of concentrated magnetic fluids

In this paper we present a device and method suited to the experimental determination of the sedimentation rate of concentrated suspensions of magnetic particles. The method is based on the measurement of the inductance of one or more sensing coils located at specified positions around a test tube containing the suspension. Such measurement is made possible by the determination of the resonant frequency of a parallel LC circuit in which L is the inductance of the sensing coil and C is the capacity of a capacitor chosen in such a way that the resonant frequency is easily measured. Upon calibration it is possible to relate the resonant frequency to the volume fraction of the particles at the coil location. The method is applied in the present work to the evaluation of the sedimentation kinetics of iron suspensions in base fluids of viscosities ranging from 0.3 to 100 mPa s and volume fractions of solids between 2.5 and 25%. Both if a single coil is used and if a set of three coils at different positions are employed, it is possible to detect the rate of accumulation of particles at the bottom of the container as well as a phenomenon of buoyancy of the largest particles brought about by the hydrostatic push of a dense fluid consisting of the smallest particles in the supporting liquid.     

Photochemical decoration of silver nanoparticles on magnetic microspheres as substrates for the detection of adenine by surface-enhanced Raman scattering

In this work, silver nanoparticles (AgNPs) decorated magnetic microspheres (MMs) are prepared as surface-enhanced Raman scattering (SERS) substrate for the analysis of adenine in aqueous solutions. To prepare these substrates, magnetic particles were first synthesized by coprecipitation of Fe(II) and Fe(III) with ammonium hydroxide. A thin layer of cross-linked polymer was formed on these magnetic particles by polymerization through suspension of magnetic particles into a solution of divinyl benzene/methyl methacrylate. The resulted polymer protected magnetic particles are round in shape with a size of 80 μm in diameter. To form AgNPs on these MMs, photochemical reduction method was employed and the factors in photochemical reduction method were studied and optimized for the preparation of highly sensitive and stable AgNPs on MMs substrates (abbreviated as AgMMs substrates). By dispersing the AgMMs in aqueous samples, cylindrical magnet was used to attract the AgMMs for SERS detections. The observed enhancement factor of AgMMs reached 7 orders in magnitude for detection of adenine with a detection limit approaching to few hundreds of nanomolar.     

聚乙二醇包裹羰基铁核壳粒子的制备及水基磁流变液的性能

以羰基铁粉(CI)为原料用共溶胶-凝胶反应制备CI/聚乙二醇核壳复合粒子，并将其与水组成了磁流变液. 用SEM、TEM、FT-IR和VSM表征了核壳复合粒子的微观结构和静磁特性，并测试了水基磁流变液的性能. 结果表明，核壳复合粒子表面有SiO_x和聚乙二醇的包覆层，它有较好的亲水性和优良的软磁特性，用它组成的水基磁流变液具有抗沉降性优良、零场粘度低、磁流变效应显著等特点.     

Superior magnetic monodisperse particles for direct purification of immunoglobulin G under magnetic field

Monodisperse magnetic acrylate based particles (5.0 µm in diameter) containing histidine were synthesized using a modified suspension polymerization method for the purification of immunoglobulin G from human plasma in a magnetically stabilized fluidized bed. N-methacryloyl-(L)-histidine methyl ester (MAH) was used as pseudo-specific ligand/co-monomer. MAH content of the magnetic particles was calculated as 55.3 µmol MAH/g polymer using elemental analysis. Immunoglobulin G binding amount of the magnetic particles decreased with increase of the flow-rate. The maximum immunoglobulin G binding was observed at pH 7.4 (phosphate buffer). Immunoglobulin G binding amount onto the magnetic poly(ethylene glycol dimethacrylate) [mPEGDMA] particles was found to be almost negligible due to the hydrophilic polymer structure. High binding values were obtained from aqueous solutions (1646 mg/g). Higher immunoglobulin G binding was observed when human plasma was used (2169 mg/g). Purity of the separated immunoglobulin G from human plasma was found to be 87%. Magnetic PEGDMAH particles could be used many times without significant loss in protein binding amount.     

Effect of Surfactants on the Particle Capture Mechanism in an Electromagnetic Filter

In this article, effects of the parameters of the filtration process and surfactants on the effectiveness of the separation of particles have been investigated. Water samples containing corrosion particles (rust) of low concentrations were mixed with a typical domestic type of detergent passed through an electromagnetic filter. It was found that the capturing capacity of the particles in the filter increased as the external magnetic field, filter length and the detergent concentration of the suspension increase, but the increase in diameter of the filter elements gave an opposite effect. Furthermore, it was recorded that the pH value of the suspension changes the efficiency of magnetic filter.     

α-Fe粉体浓悬浮体系的制备及磁流变效应

报道使用价格低廉的还原铁粉制备磁性粒子浓悬浮体系,对体系磁流变效应进行研究,并研究了磁性颗粒尺寸和氧化物对悬浮体系力学性质及沉降稳定性的影响.     

无机-高分子磁性复合粒子的制备与表征

详细地研究了乳化剂用量对种子乳液聚合反应的影响,并成功地制备出粒径约300nm的苯乙烯/丙烯酸共聚小球。另外,还报道使用化学共沉淀法使无机粒子与高分子球复合,制备出高分子球为核,无机粒子为壳层的磁性复合粒子,使用XRD、TEM等手段对此复合粒子进行了表征。同时,进一步研究了这种复合粒子悬浮液的悬浮性能以及粘度随磁场的变化情况。     

Branched networks by directed assembly of shape anisotropic magnetic particles

The directed assembly of shape anisotropic magnetic particles into targeted macrostructures requires judicious particle design. We present a framework to understand the self-assembly of magnetic non-Brownian H-shaped particles and the formation of branched networks under an applied magnetic field. A finite element integration (FEI) method is developed to identify the preferred particle orientation (relative to the applied field) at different values of the geometric parameters defining H shapes, and used to construct a phase diagram to generalize the results. Theoretical predictions are validated by comparing with experiments performed using magnetic hydrogels synthesized using stop-flow lithography (SFL). We demonstrate the ability of H-shaped particles to form chains parallel to the field that can thicken in a direction orthogonal to the field, and in some cases with branching. The assembly of a suspension containing H-shaped particles, or rods, or a combination of both, is reported.     

Influence of Magnetic Field and Temperature on the Synthesis and Colloidal and Chemical Properties of Fe3O4

A magnetic field and temperature was shown to affect disperse composition, electrophoretic behavior, and aggregation stability of synthesized magnetite particles. When synthesis proceeds, a process of the nucleation of ultramicroscopic Fe₃O₄ particles prevails over a process of their aggregation. Here an electrostatic factor of stability of magnetite suspension plays an important role. An application of the magnetic field decreases the aggregation stability and increases the sedimentation rate of product particles.     

Numerical simulation of lateral interactions between flexible dipole chains in liquids

The molecular dynamics method is employed to study the effect of the flexibility of chains of particles in a magnetic suspension on their lateral interactions and the formation of columnar structures in a uniform magnetic field. It is shown that the chain flexibility substantially influences the character of the lateral interactions, whereas the critical distance at which the chains coalesce increases with their length according to a power law with an exponent of 0.05 versus 0.002 typical of rigid chains.     

Soft magnetic carbonyl iron microsphere dispersed in grease and its rheological characteristics under magnetic field

Magnetorheological (MR) grease, comprised of a suspension of soft magnetic carbonyl iron (CI) microspherical particles dispersed in a grease medium, was fabricated by a mechanical stirring method. As potential medium oil for MR system, shear viscosity of the pure grease was measured as a function of temperature. Its MR characteristics were investigated using a rotational rheometer under an external magnetic field. Flow curve responses (shear stress and shear viscosity), yield stress, and elasticity were investigated using various magnetic field strengths ranging from 0 to 342 kA/m. It was confirmed that MR grease has a yield stress under no external magnetic field due to the inherent property of grease. In addition, CI based MR grease exhibited a characteristic of a Bingham fluid.     

Advantages and limitations of the synchrotron based transmission X-ray microscopy in the study of the clay aggregate structure in aqueous suspensions

This paper reports new application of new transmission X-ray microscopy powered by a synchrotron source for the study of aqueous based clay suspensions. This paper delineates the advantages and limitations of this method. The tested transmission X-ray microscopy (TXM) technique has shown good agreement with the cryo-stage SEM technique. The spacial resolution of this TXM technique is 60 nm and clay particles with diameter below 500 nm are clearly visible and their pseudohexagonal symmetry is recognizable in detail. It is clearly demonstrated the methodology of implementing TXM to study aqueous based clay suspensions that are close to approximately 60 nm tomographic resolution. The technique enables us to study discrete structure of clay suspensions in water and within aggregates. This has never been previously possible. Larger crystals, more compact aggregates and less colloidal fraction present in kaolinite from Georgia has impact on faster settling and gelling in denser suspension than for Birdwood kaolinite in which colloidal particles create gel-like networking in less dense aqueous suspension.     

Influence of magnetic interactions between clusters on particle orientational characteristics and viscosity of a colloidal dispersion composed of ferromagnetic spherocylinder particles: analysis by means of mean field approximation for a simple shear flow

We have theoretically investigated the particle orientational distribution and viscosity of a dense colloidal dispersion composed of ferromagnetic spherocylinder particles under an applied magnetic field. The mean field approximation has been applied to take into account the magnetic interactions of the particle of interest with the other ones that belong to the neighboring clusters, besides those that belong to its own cluster. The basic equation of the orientational distribution function, which is an integrodifferential equation, has approximately been solved by Galerkin's method and the method of successive approximation. Some of the main results obtained here are summarized as follows. Even when the magnetic interaction between particles is of the order of the thermal energy, the effect of particle-particle interactions on the orientational distribution comes to appear more significant with increasing volumetric fraction of particles; the orientational distribution function exhibits a sharper peak in the direction nearer to the magnetic field one as the volumetric fraction increases. Such a significant inclination of the particle in the field direction induces the large increase in viscosity in the range of larger values of the volumetric fraction. The above-mentioned characteristics of the orientational distribution and viscosity come to appear more significantly when the influence of the applied magnetic field is not so strong compared with that of magnetic particle-particle interactions.     

Frequency-dependent electrical conductivity of concentrated dispersions of spherical colloidal particles

This paper outlines the application of a self-consistent cell-model theory of electrokinetics to the problem of determining the electrical conductivity of a dense suspension of spherical colloidal particles. Numerical solutions of the standard electrokinetic equations, subject to self-consistent boundary conditions, are implemented in formulas for the electrical conductivity appropriate to the particle-averaged cell model of the suspension. Results of calculations as a function of frequency, zeta potential, volume fraction, and electrolyte composition, are presented and discussed.     

On the Theory of Phase Transitions in Magnetorheological Suspensions

The “condensation” of the particles of magnetorheological suspensions (MRSs) under the action of an external magnetic field is theoretically studied. The analysis demonstrated that the formation of rather long linear chain aggregates preceded the condensation of particles into the dense phase. The phase transition of the “gas-liquid” type proceeded in a particle ensemble via the “condensation” of such chains caused by their magnetic interaction. In thin MRS layers oriented perpendicular to the external magnetic field, the scenario of phase transition differs from that observed in systems with infinite volume. After the completion of the phase transition, the ensemble of dense domains discretely distributed in dilute dispersion is formed in thin layers rather than two simple connected phases with different particle concentrations, as it takes place in infinite media. Under fairly strong external magnetic fields, the transition of the “gas-solid” type occurs in a system of particles.     

Rheomagnetic properties of mixed magnetic particle suspensions

Single-domain magnetic particles are the essential ingredient of magnetic tapes, particulate recording disks and magnetic stripes. The particles are single-domain γ-Fe₂O₃, CrO₂ or barium ferrite, and non-magnetic α-Fe₂O₃ mixture. Each of these particles has intrinsic coercivity, which should be matched with the magnetic field strength of the writing element of a particular device. In this study a magnetic inductance measurement with low field strength was employed to obtain the magnetic permeability of suspensions containing two of the particle types mixed together as a function of composition and volume fraction of particles. The bulk magnetic property B is a linear combination of the contributions from each particle type such that the “excess” inductance is L − L_s = Σφ_iB_i where φ_i is the volume fraction and B_i, is the magnetic property of particle type i. For the non-magnetic α-Fe₂O₃, B_i = 0. This allows the formulation of mixed particle suspensions to obtain a desired property for custom-designed magnetic particle coatings. However, mixing magnetic particle types will broaden or produce a bimodal switching field distribution. This may affect the squareness of the magnetic hysteresis loop. These properties should be taken into account for the design of a practical magnetic coating with mixed particle suspension. Another requirement of the magnetic particle suspensions is that they remain well dispersed, even though strong magnetic forces between the particles promote flocculation. An extension of the inductance measurement technique is employed to study the flocculation of a suspension containing magnetic γ-Fe₂O₃ and non-magnetic α-Fe₂O₃. The presence of the α-Fe₂O₃ decreases the flocculation state of the suspension. Thus the suspension stability is enhanced by incorporating a small amount of non-magnetic particles in addition to surfactant.     

Structural transformations in magnetic suspensions

Structural transformations in dispersions of micron-sized iron particles suspended in a magnetite ferrofluid (the colloidal suspension of ferromagnetic nanoparticles in nonmagnetic liquid) are theoretically considered. An attempt is made to explain the tendency of iron particles to form doublets and longer chain aggregates with finite distance between particles in external magnetic field observed in recent experiments; in colloidal ferrofluid, micron-sized iron particles approach one another to finite distance that is equal approximately to the particle diameter. At moderate magnetic fields, minimal distance between approached particles is nearly independent of the strength of magnetic field. In ordinary magnetorheological dispersions, which are suspensions of magnetizing micron-sized particles in nonmagnetic liquid, the approach of particles practically does not occur up to their physical contact.