Dispersion Of Magnetic Particles Research Articles

Viscosity of magnetic particle suspension

The effect of microstructure on the viscosity of rheologically complex magnetic particle suspensions is investigated. The measurements of suspension viscosity are used to obtain the dependence of viscous energy dissipation on the microstructural states of magnetic particle dispersions as well as the microstructural effects which are related to magnetic particle orientation for both rod and plate-like particles. The empirical formulas from mean field theory and the Mooney equation, which are applicable for high concentrations of magnetic particles, are used to relate viscosity to particle concentration.

Evaluation of Polyurethane Having SO3Na Group as a Magnetic Binder II. Two-Step Dispersion of Magnetic Particles

Influence of magnetic paint formulations on a packing degree of magnetic particles was studied in the light of interactions of binder polymer and solvent. A method for producing magnetic paint is proposed to decrease the void fraction in magnetic layer. Dispersion in poor solvent gave the magnetic layer with rough surface and with strong interaction between magnetic particles and the binder polymer as well as the magnetic paint with much binder polymer adsorbed onto the magnetic particles. The effects of the dispersion in poor solvent persisted in the final magnetic paint, even though the solvent changed from a poor to good solvent in the second step. In this paper, the two-step dispersion process was investigated in which the first was the dispersion in poor solvent and the second was in good solvent. Binder polymer was added in each step. The decrease of void volume fraction in the magnetic layer was amplified by use of polyurethane with SO3Na group as a binder polymer in the two-step dispersion method. The decrease of void volume fraction improved the mechanical properties of the magnetic layer.

“Negative Viscosity” in a Magnetic Fluid

Fluids containing colloidal dispersions of magnetic particles exhibit new phenomena, including a reduction in viscosity under some conditions. In his Perspective, Rosensweig reviews the theoretical and experimental state of magnetic fluid studies.

Syntheses and Solution Properties of Novel Surfactants Containing Fluorocarbon Chains

Syntheses and solution properties of novel fluorinated surfactants with branched tail, especially on anionic surfactants having two polyfluoroalkyl chains and on anionic surfactants having different hydrophobic chains (hybrid surfactants), are reviewed.For example, when the fluorocarbon chain length in the surfactants possessing two polyfluoroalkyl chains increased, the Krafft points and the area of surfactant molecule at the air-water interface increased, the critical micelle concentration decreased, but the degrees of ionic dissociation of micelle were almost the same. Moreover, the flocculation and redispersion abilities of these surfactants for dispersed magnetic particles in water were enhanced by an increase in the chain length.The hybrid surfactants were found to have the ability to considerably lower surface tension, and they can emulsify a ternary-component system of hydrocarbon/water/perfluoropolyether oil. Furthermore, some of the hybrid surfactants shows anomalous solution properties and viscoelasticity due to the micellar structure changes with increasing its concentration.

Surfactants having polyfluoroalkyl chains. II. Syntheses of anionic surfactants having two polyfluoroalkyl chains including a trifluoromethyl group at each tail and their flocculation-redispersion ability for dispersed magnetite particles in water

Anionic surfactants having two polyfluoroalkyl chains per molecule, i.e. the sodium salt of bis(1 H, 1 H, 2 H,2 H-heptadeca-fluorodecyl)-2-sulfosuccinate, CF 3(CF 2) 7(CH 2) 2OCOCH 2CH(SO 3Na)COO(CH 2) 2(CF 2) 7CF 3, the sodium salt of bis(1 H, 1 H, 2 H, 2 H-tridecafluoro-octyl)-2-sulfosuccinate, CF 3(CF 2) 5(CH 2) 2OCOCH 2CH(SO 3Na)COO(CH 2) 2(CF 2) 5CF 3, and the sodium salt of bis(1 H, 1 H, 2 H, 2 H-nonafluorohexyl)-2-sulfosuccinate, CF 3(CF 2) 3(CH 2) 2OCOCH 2CH(SO 3Na)COO(CH 2) 2(CF 2) 3CF 3, have been prepared from maleic anhydride, the corresponding alcohols possessing a polyfluoroalkyl chain and sodium hydrogen sulfite. The flocculation and redispersion abilities of these surfactants for dispersed magnetic particles in water have been examined to investigate the effect of the chain length. It was found that this ability was enhanced by an increase in the chain length. The contact angles for water for pelleted surface-modified magnetite have been measured. In order to compare this ability and the contact angles, data for other fluorinated surfactant have been obtained. The Kraff point, the surface tension and the pNa of the aqueous surfactant solutions have also been measured.

Measurement Apparatus for Magnetic Susceptibility of Magnetic Fluid.

A magnetic fluid is a kind of multiphase fluid which consists of a stable colloidal dispersion of magnetic particles in a liquid carrier. The most basic factor governing its behavior is magnetic susceptibility. We describe the research on a measurement apparatus for magnetic susceptibility of a magnetic fluid by means of electromagnetic measurement. The apparatus is equipped with an electromagnet having a very low residual magnetization and magnetic field gradient adequate to secure a measurement of high accuracy. The measurement results of magnetic susceptibility with this apparatus are also reported for several kinds of magnetic fluids in various magnetic fields, temperatures and concentrations.

Molecular dynamics of magnetic particulate dispersions

A computational investigation using the method of molecular dynamics was undertaken to characterize the state of magnetic particle dispersions. The simulations revealed that the microstructure of spherical particulate dispersions consists of chainlike clusters resulting from magnetic dipole alignment. Acicular particles formed clusters such as dimers, chains, and rings. The effect of fluid viscosity on the dispersion quality and the response of the magnetic dispersions to an external DC magnetic field are also reported.

Rheological study of magnetic particle dispersion

The rheological properties of an acicular magnetic particle dispersion, as well as those of a platelet particle dispersion, were studied. The relative viscosity, reduced viscosity and yield value of the dispersion were redefined using the analogy of those of polymer solution to characterize dispersion rheology. The volume fraction of the solid component, which consists of the core particle and the polymer adsorbed layer, is a dominant factor in determining dispersion rheology, independent of the kind of solvent. The magnitude of interparticle interaction, which is indicated by the magnitude of relative viscosity or that of reduced viscosity, is notable when the volume fraction of the solid component is higher than 20%. The yield value rises at a lower volume fraction than do the relative viscosity or the reduced viscosity.

Polymer adsorption on a magnetic particle in a dispersion system

A magnetic particle dispersion was prepared by mixing the magnetic γ- Fe 2 O 3 particles or barium-ferrite particles and epoxy resin with solvent. The relationship between the mixing conditions and the adsorbed amount of the epoxy resin on the particle surface was investigated. It became clear that the polymer adsorption occurring during the mixing process required both large mechanical shear and high temperature. The large mechanical shear was necessary to break the coagulation of the particles and high temperature was necessary to cause chemical reaction between the polymer and the particle surface. The adsorbed polymer improved dispersibility of the particles in the dispersion, which was confirmed by transmission electron micrograph, viscosity measurement and surface roughness measurement of the coated films made from the dispersions.

Propagation of disturbances in a two-phase magnetic particle dispersion

The propagation of one-dimensional vertical perturbations of the magnetic particle concentration in a liquid-particle or gas-particle disperse medium in an external uniform magnetic field, oriented parallel or antiparallel to the force of gravity, is considered. The propagation of linear disturbances is analyzed. Linear stability criteria for layers of magnetized particles fluidized with a gas or liquid are found. The propagation of nonlinear long (kinematic) concentration waves in a medium containing dispersed magnetized particles is investigated. It is shown that in the kinematic wave approximation the propagation of the nonlinear concentration perturbations can be described by the Burgers equation. The formation of regions with a sharp change in concentration (“shock” fronts) is analyzed. The structure of the shock front, in particular its width, is found. It is shown that a magnetic field leads to broadening of the shock front, thus preventing the formation of concentration discontinuities and hence leading to the “smearing” of the boundaries of the inhomogeneities formed in the fluidized bed. This provides a basis for a qualitative explanation of the stabilization of developed nonuniform fluidized beds of magnetic particles in a magnetic field.

Deposition of Sn hydrosols on Co- gamma -Fe/sub 2/O/sub 3/ magnetic particles

An improved colloidal solution of tin hydrous oxide has been developed. This solution was found to readily deposit onto the low-wettability surface of Co- gamma -Fe/sub 2/O/sub 3/ particles to form a uniform layer of irremovable coating. Based upon the chemical characterization of the tin-sol-treated surface, the effectiveness of such treatments on magnetic oxide particles is described in terms of dispersion stability, dispersibility, rheological behavior, and other magnetic tape performance. A significant improvement in wetting, dispersion and flow behavior was observed for the 1% Sn coated magnetic particles upon dispersion in a polyurethane-based formulation. Additional magnetic tape data also revealed that surface treatments on magnetic particles facilitate the particle orientation by exposure to a 2000-Oe external magnetic field before dying, as evidenced by the high values of hysteresis squareness up to 0.8, indicating good dispersion of magnetic particles. >

Determination of structures in ferrofluids by small-angle neutron scattering

Stable colloidal dispersions of magnetic particles, or ferrofluids, form a technologically important class of magnetic fluids with novel structural features at the microscopic level. Small-angle neutron scattering (SANS) has proved to be ideally suited to the study of such structures. In addition to the techniques of neutron refractive index variation which have made SANS a tool of choice for studying colloids in general, the uniquely powerful methods of neutron polarisation analysis may be applied to ferrofluids, providing a selection rule for studying non-magnetic and magnetic structural features separately. This article considers the application of SANS to studies of dilute ferrofluids, after which extension of the techniques to concentrated (interacting) dispersions will be addressed. Advances in understanding binary colloidal mixtures in which at least one component is magnetic will then be considered. Finally, the use of ferrodispersion to generate ordered colloidal structures in ferrofluids will be discussed.

Stability control of magnetic fluids in the presence of foreign surfaces

The results of the interaction of highly dispersed magnetic particles with the surface of a foreign solid phase in different liquid media are given. On the basis of an analysis of experimental kinetic data, a phenomenological theory of the deposition process of magnetic particles on a foreign surface is developed. The dependences of the influence of the nature and composition of liquid media on the interaction of contacting solid phases in the system under investigation are established. A stability criterion for magnetic particles during interaction with a foreign surface under different conditions is proposed.

Effect of polymer adsorbed layer on magnetic particle dispersion

Experimental and theoretical investigations of dispersion properties of magnetic particles (acicular gamma -Fe/sub 2/O/sub 3/) with an adsorbed epoxy-resin layer are presented. The adsorbed polymer amount was 20-110 mg/g and the adsorbed layer thickness was 6-22 nm, depending on the preparation conditions. Dispersion of gamma -Fe/sub 2/O/sub 3/ particles was evaluated by the sedimentation rate and surface roughness of the coated film. It was clarified that dispersion was improved with increasing adsorption, increasing solvent/epoxy-resin solubility, and decreasing gamma -Fe/sub 2/O/sub 3/ particle size. Interaction energy, consisting of magnetic attraction, van der Waals attraction, and steric repulsion of the adsorbed layer between two cylindrical magnetic particles, was calculated. The interaction energy curves, which vary with the distance between two particles, had primary minimum, maximum, and secondary minimum. The maximum energy and the secondary minimum energy were strongly related to dispersion stability. Experimental results are explained by the energy curve shapes. >

SANS studies of ferrofluids

Abstract Stable colloidal dispersions of magnetic particles, or ferrofluids, form a technologically important class of magnetic fluids with novel structural features at the microscopic level. Small angle neutron scattering (SANS) has proved to be ideally suited to the study of these structures. Recent studies have concentrated on using ferrofluids as new dispersion media for non-magnetic colloids, such as macromolecules. This technique, known as ferrodispersion, is leading to new methods for creating ordered macromolecular structures in dispersed phases.

Viscoelasticity of F-actin measured with magnetic microparticles.

Dispersed submicroscopic magnetic particles were used to probe viscoelasticity for cytoplasm and purified components of cytoplasm. An externally applied magnetic field exerted force on particles in cells, in filamentous actin (F-actin) solutions, or in F-actin gels formed by the addition of the actin gelation factor, actin-binding protein (ABP). The particle response to magnetic torque can be related to the viscoelastic properties of the fluids. We compared data obtained on F-actin by the magnetic particle method with data obtained on F-actin by means of a sliding plane viscoelastometer. F-actin solutions had a significant elasticity, which increased by 20-fold when gels were formed by ABP addition. Both methods gave consistent results, but the dispersed magnetic particles indicated quantitatively greater rigidity than the viscoelastometer (two and six times greater for F-actin solutions and for F-actin plus ABP gels, respectively). These differences may be due to the fact that, compared with traditional microrheometers, dispersed particle measurements are less affected by long-range heterogeneity or domain-like structure. The magnetometric method was used to examine the mechanical properties of cytoplasm within intact macrophages; the application of the same magnetometric technique to both cells and well-defined, purified protein systems is a first step toward interpreting the results obtained for living cells in molecular terms. The magnetic particle probe system is an effective nonoptical technique for determining the motile and mechanical properties of cells in vitro and in vivo.

Degree and stability of magnetic dispersions: Sedimentation, rheological, and magnetic properties

There is a distinct correlation between the dispersion qualities of magnetic acicular particles as measured by slush grind rheology, sedimentation test, and coating magnetic properties. Sedimentation time measurement is a very sensitive tool to evaluate the degree of dispersion and dispersion stability. Sedimentation, rheology, and coating magnetic properties show that “steric stability” is essential to maintain the excellence of magnetic particle dispersion. Some surface active agents provide excellent pigment dispersion but poor stability with time, whereas other chemical compounds adsorbed on a particle surface can provide a very high degree of dispersion with excellent dispersion stability. Results of particle size evaluation from sedimentation measurements show five to six times improvement in the degree of dispersion due to suitable surface active agents.

Creep measurements on magnetic suspension

Rheological properties of disperse system of magnetic γ-Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> in silicone oil were investigated by the aid of creep experiments. The flow curve shows a typical pseudo-plastic flow pattern of which plastic viscosity is about 82.6 poise and apparent yield stress is about 1800 dyne/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . These phenomena are attributable to structural networks formed by dispersed magnetic particles. The suspension with denser network structure was thought to behave as an elastic solid at the stress under the true yield stress. However, an irreversible flow behavior has been observed in creep measurements at the stress which is smaller than the true yield stress. In the disperse system, the structural networks of dispersed particles depend on a flow history and their mechanical behavior seems to be a function of time. In the creep experiment at various magnitudes of shear stress measured immediately after cessation of shear flow, the density of structural networks seems to increase depending on the applied force at initial time region, however with increasing time, the irreversible flow becomes dominant in their creep behavior. An elastic behavior of interparticle network structure can be presumed by a creep recovery. The elastic modulus estimated from the relationship between the applying stress and the recoverable strain is about 1100 dyne/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Magnetic materials for recording: An overview with special emphasis on particles (invited)

Modern recording media—tapes and disks—are made using a broad variety of magnetic materials, each presenting a unique combination of strengths and weaknesses. Although recording surfaces can be made by the deposition of thin magnetic films on a support material, most commercially available media are made by coating the support material with a dispersion of magnetic particles in an organic binder. This approach has been used for decades and is capable of being extended to future applications. The particles that are dispersed in such coatings can be of many compositions. The general requirements placed on magnetic recording materials are outlined, and the relative merits of the most significant candidates are described. Of the important particulate materials, the cobalt-modified iron oxides are treated most fully, because of their economic as well as technical interest.

Agglomeration of magnetic fine particles in fluid dispersion

Agglomeration of γ-Fe2O3 acicular magnetic particles in concentrations of approximately 0.005% by weight in water was studied by measuring light transmission through the dispersions while they were subjected to a stepwise rotating magnetic field of 15-Oe magnitude. Transmitted light intensity varies synchronously with rotation, having its minimum when particles are aligned with their long axes parallel to the light and its maximum when particles are perpendicular to the beam. With time, the minimum light level diminishes, whereas the maximum light condition remains constant. This behavior is attributed to the formation and growth of particle chains. The chaining idea is supported by the observation that the response time of particles to the changing field increases with time and by other optical effects which are observed in high-speed rotation of the field. The effect of milling time on particle mobility is also observed. The observations are discussed in terms of a model of the attraction and attachment behavior of dilutely dispersed magnetic particles in a fluid.