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.     

Small-angle light scattering study of quasi-two-dimensional structures of magnetic particles

We studied quasi-two-dimensional magnetic domain structures using a small-angle light scattering technique. The structures are formed when magnetic particles in an aqueous suspension between two parallel glass plates are assembled and oriented in a magnetic field H perpendicular to the plates. Three different structure regions can be identified, and they are bounded by two critical field lines H_c1 (φ) and H_c2(φ). Below H_c1 (φ) the system consists of single particles; between H_c1 (φ) and H_C2 (φ), the particles assemble into elongated disordered domains; and above H_c2 (φ), the domains develop a strong in-plane correlation. In the strong field limit H H_c2, the structure factor of the magnetic fluid resembles that of a two-dimensional liquid. However, long lasting metastability and hysteresis observed in the experiment suggest that the system is essentially locked in a glassy state.     

Effect of Particle Size Distribution on the Rheology of Dispersed Systems

The rheological properties of aqueous polystyrene latex dispersions from three synthetic batches, with nearly the same z-average particle sizes, 400 nm, but varying degrees of polydispersity, 0.085, 0.301, and 0.485, respectively, were systematically investigated using steady-state shear and oscillatory shear measurements. The particles were sized with photon correlation spectroscopy and transmission electron microscopy and were stabilized sterically with PEO–PPO–PEO triblock copolymer (Synperonic F127). Results from steady-state shear measurements show that the viscosities of the systems exhibit shear-thinning behavior at high solid fractions. However, the degree of shear thinning depends on the breadth of particle size distribution, with the narrowest distribution suspension exhibiting the highest degree of shear thinning. The Herschel–Bulkley relationship best describes the flow curves. The relative viscosities as a function of volume fraction data were compared, and it was found that the broadest distribution suspension had the lowest viscosity for a given volume fraction. In addition, the data were fitted to the Krieger–Dougherty equation for hard spheres. A reasonable agreement of theory with experiment is observed, particularly and surprisingly for the very broad distribution. However, when the contribution to the volume due to the adsorbed polymer layer is considered, the agreement between experiment and theory becomes closer for all the suspensions, although the agreement for the broad distribution suspension is now worse. Fitting the Dougherty–Krieger theory to the experimental data based on our experimental maximum packing fractions gives very good agreement for all the systems studied. From oscillatory shear measurements, the moduli were obtained as a function of frequency at various latex volume fractions. The results show general change of the dispersions from viscous (G" > G′) at low volume fractions (0.25–0.30) to moderately elastic (G′ > G") at moderately high volume fractions (0.41–0.45). The change at this concentration level is likely due to some compression and interpenetration of the stabilizing polymer chain at the periphery, indicating the dominance of the interparticle forces. Overall, the very broad distribution was found to have the lowest elastic modulus for a given volume fraction.     

Effect of surface charge of carbonate-containing aluminum hydroxide on particle interactions in aqueous suspensions

The area under the curve (AUC) of the modified Lorentzian power spectrum as determined by fiber optic Doppler anemometry (FODA) is directly related to the number of freely diffusing particles in suspension. The initiation of particle interactions is characterized by deviation from a linear volume fraction-AUC relationship. The AUC approaches zero when an extensive particle network forms which dampens the random motion of particles. The rheological behavior supports this conclusion as a yield value was first noted when the AUC approached 0. The ability of AUC to monitor particle interactions was confirmed by a decrease in AUC when a latex dispersion was flocculated by NaCl, CaCl₂, or AlCl₃. Both the volume fraction at which particle interactions were initiated, φ_d, and the volume fraction at which particle random motion was dampened, φ_c, decreased when the pH of an aluminum hydroxide suspension was adjusted closer to the point of zero charge. The AUC was useful in studying the disaggregation of an aluminum hydroxide suspension by shear. Shear was found to also produce aggregation under specific conditions of surface charge and volume fraction. The reversibility of shear-induced changes in particle interactions was studied and the conditions of surface charge and volume fraction which control the reversibility of the particle structures were defined.     

Electrochemical deposition and properties of PbO2 + Co3O4 composites

Electrolysis of suspensions of Co₃O₄ particles in Pb²⁺-containing electrolytes has been used for depositing PbO₂ + Co₃O₄ composite layers on Ni rotating dise anodes. A sufficiently high angular speed of the electrode is necessary to obtain layers of homogeneous thickness and Co₃O₄ concentration. The volume fraction of Co₃O₄ particles in the deposit α reaches a limiting value of ca. 0.1 when the volume fraction of particles in suspension C exceeds 0.008. The current density j has little effect on α as long as it is in the range 1 to 20 mA cm⁻²; if j increases further, α decreases.PbO₂ + Co₃O₄ composite layers have been studied as electrode materials for the oxygen evolution reaction (mainly in NaOH solution). The overpotential and Tafel slope decrease upon increasing α. At a fixed potential, j is roughly proportional to OH⁻ concentration. The PbO₂ + Co₃O₄ electrode performance is fairly stable at 25°C but declines with time at higher temperature.     

Rheological properties of filled gels. Influence of filler matrix interaction

The dynamic moduli of gels filled with particles have been studied as a function of the volume fraction of dispersed particles _f (0–0.4) and of the way in which they interact with the gel matrix. Two gels of different nature were studied, viz. polyvinyl alcohol (PVA) — Congo red gels (a so-called rubber gel) and casein gels made by acidification of skimmed milk. Emulsion droplets stabilized by different macromolecules have been used as dispersed particles. If there was no interaction between the macromolecules adsorbed on the particles and the gel matrix, both the filled PVA and the filled casein gels showed a small decrease in the elastic moduli with _f , approaching the behaviour theoretically predicted for foams. In the case of interaction, the results for filled PVA gels roughly fitted the theoretical predictions, if the deformability of the emulsion droplets and the formation of an intermediate layer between the dispersed particles and the gel matrix was taken into account. The increase in the elastic moduli of the acid milk gels with _f was much greater than expected and was probably due to aggregation of the dispersed particles during gelation.     

Giant colloidal crystals of fluorine-containing polymer spheres in exhaustively deionized aqueous suspensions and in the presence of sodium chloride

 Gigantic colloidal single crystals (2–6 mm) are formed for fluorine-containing polymer spheres (120–210 nm in diameter) in exhaustively deionized aqueous suspensions. The spheres used are poly(tetrafluoroethylene) (PTFEA and PTFEB), copolymer of tetrafluoroethylene and perfluorovinylether (PFA) and copolymer of tetrafluoroethylene and perfluoropropylene (PTP). The phase diagrams of these spheres are obtained in the deionized suspensions and also in the presence of sodium chloride for PFA. The critical sphere concentrations of crystal melting (φ _c) for these spheres are around 0.0006 in volume fraction, which are close to, but slightly larger than, those of monodispersed polystyrene spheres (φ _c ≈ 0.00015) and colloidal silica spheres(φ _c = 0.0002–0.0004) reported previously. The crystals are largest when the sphere concentrations are a bit higher than the φ _c value and their size decreases as the sphere concentration increases. Reflection spectra are taken in sedimentation equilibrium as a function of the height from the bottom of the suspension. The static elastic modulus is estimated to be 10.8 and 28.7 Pa for PTFEA and PTP spheres at the sphere concentrations 0.00325 and 0.00322 in volume fraction, respectively. Received: 27 October 1999 Accepted in revised form: 16 November 1999     

Investigations into the formation and characterization of microemulsions. II. Light scattering conductivity and viscosity studies of microemulsions

Microemulsions formed with water—xylene—sodium alkyl benzene sulfonate (NaDBS)-hexanol have been investigated using time-average light scattering conductivity and viscosity measurements at 25°C. Four different concentrations of NaDBS were used, namely, 5, 10.9, 15, and 20 wt% and the molar ratio of n-hexanol:NaDBS was kept constant at 3.4, 3.24, 3.4, and 3.4, respectively. The light scattering results showed that at low volume fractions of water, φ_H2O, the Rayleigh ratio, R₉₀ increased slowly with an increase in φ_H2O but above a critical volume fraction of water, φ_H2O^S, R₉₀ increased almost linearly with increase of φ_H2O, reached a maximum at another critical water volume fraction, φ_H2O^C, above which R₉₀ decreased with further increase in φ_H2O. The results were interpreted qualitatively in terms of the possible aggregate units formed. A quantitative analysis of the light scattering data was carried out using a procedure based on the use of a hard sphere model for particle interactions. Using this approach, the water droplet radius R was calculated as a function of φ_H2O for the four systems investigated. The results showed a linear increase of R with increase in φ_H2O. Approximate values of the water radii were also calculated from the interfacial area and these were found to be in full agreement with the radii obtained from light scattering measurements. The conductance κ showed a non-monotonic variation as the water concentration was increased. A maximum in the κ-φ_H2O was observed at a critical volume fraction of water φ_H2O^S above which κ decreased and then remained almost constant over a range of φ_H2O values. The conductance then sharply increased at another φ_H2O value, namely φ_H2O^C. This φ_H2O^C value was reduced with increase in NaDBS concentration. The conductance results indicate structural changes in the system as the water concentration increases and the transition observed correlate with those obtained from light scattering. Moreover, the low κ values found and the non-monotonic variation of κ with φ_H2O are indicative of the presence of definitive water cores with an external surfactant film which acts as a barrier for ion transport. Viscosity results showed the behavior normally encountered with concentrated dispersions, the relative viscosity η_r increasing exponentially with φ_H2O. The viscosity data were fitted to the Mooney equation. The results showed an increase in the Einstein coefficient with increase in NaDBS concentration indicating an increase in the hydrodynamic volume of droplets. This was attributed to the increase of the ratio of the surfactant layer thickness to the droplet core radius as the NaDBS concentration is increased.     

Elasticity of the crystal-like,amorphous solid-like,and liquid-like structures in deionized suspensions of colloidal silica spheres

Elastic moduli (G) of deionized suspensions of colloidal silica spheres (diameters=45–192 nm; standard deviations of sphere size divided by the mean diameter=0.05–0.21; surface charge densities=0.33–0.94 C/cm²; volume fractions of sphere ()=0.019–0.21) have been determined by the reflection spectrum measurements in a sedimentation equilibrium and the rotatory viscometry. TheG-values are 56–460 Pa (=0.019–0.051), 330–890 Pa (0.06–0.09), and 100 Pa (0.025) for the crystal-like, amorphous solid-like, and liquid-like structures, respectively. TheG-values seem to be in the order amorphous solid-like > crystal-like > liquid-like at the same sphere concentration. The suspension structures and their elastic moduli are highly influenced by the sizes and their monodispersities of the spheres. These results support that the electrostatic intersphere repulsion and the long Debye-screening length around the spheres are important for the appearance of the suspension structures.     

Structural, electronic and elastic properties of MAX phases M2GaN (M = Ti, V and Cr)

Based on first-principles total energy calculations, we have investigated the systematic trends for structural, electronic and elastic properties of the MAX phases M₂GaN depending on the type of M transition metal (M are Ti, V and Cr). The optimized zero pressure geometrical parameters: the two unit cell lengths (a, c), the internal coordinate z and the bulk modulus are calculated. The results for the lattice constants are in agreement with the available experimental data. The band structures show that all studied materials are electrical conductors. The analysis of the site-projected l-decomposed density of states shows that bonding is due to M d-N p and M d-Ga p hybridizations. The elastic constants are calculated using the static finite strain technique. The shear modulus C₄₄, which is directly related to the hardness, reaches its maximum when the valence electron concentration is in the range 10.5–11.0. The isotropic elastic moduli, namely, bulk modulus (B), shear modulus (G), Young's modulus (E) and Poisson's ratio (σ) are calculated in framework of the Voigt–Reuss–Hill approximation for ideal polycrystalline M₂GaN aggregates. We estimated the Debye temperature of M₂GaN from the average sound velocity. This is the first quantitative theoretical prediction of the electronic structures, and elastic constants and related properties for Ti₂GaN, V₂GaN and Cr₂GaN compounds that require experimental confirmation.     

Concentration-dependence of faradaic currents and conductivity in a polystyrene sulfonic latex suspension

The conductivity, κ, in a suspension of polystyrene sulfonic latex without supporting electrolyte showed a linear dependence on the volume fraction, v_f, of the latex for v_f<0.03 with a finite intercept. In contrast, this deviated upward from the linear line for v_f>0.03. These variations were qualitatively consistent with the dependence of the voltammetric reduction current of H⁺ on v_f without supporting electrolyte. The current values were only a few percent of the theoretical diffusion-controlled current that could be observed in the suspension with supporting electrolyte. This fact indicates the electrostatic immobilization of the hydrogen ions by sulfonic latex particles. A plot of the current against κ at common values of v_f showed that the current for v_f>0.07 was smaller than the value predicted from the conductivity. This can be explained in terms of a combination of the increase in electrostatically unbounded H⁺ estimated by conductance measurements and electric migration in which the electrochemical depletion of [H⁺] also causes the depletion of the latex.     

Effect of pH and Salt on Rheological Properties of Aerosil Suspensions

Ionic strength and pH will influence the zeta potential of suspended particles, and consequently particle interactions and rheological properties as well. In this study the rheological properties and aggregation behaviour of Aerosil particles dispersed in aqueous solutions with various pH and salt concentration were studied. The potential energy was estimated by the DLVO theory and short range hydration forces and compared to the experimentally determined zeta potential. The strongest attraction between particles occurs at the isoelectric point (pH 4) and resulted in large aggregates, which gave relatively higher values of viscosity, yield stress, moduli, and shear thinning effects. The relative viscosity as a function of volume fraction was fitted to the Krieger and Dougherty model for all the suspensions. Oscillation measurements showed that the suspensions display elastic behaviour at low pH and viscous behavior at high pH. Furthermore, suspensions with high salt content had higher storage moduli. A power law dependency of storage moduli with volume fraction could be used to indicate the interaction strength between particles.     

Semi-empirically derived petrophysical and thermodynamical coefficients of permselective shales—Implications on ore mineralization

Phenomenological coefficients of shale–electrolyte systems may offer a glimpse into probable matrix-permeability and solute-exclusionary relationships. Shales from unexposed Upper Cretaceous Period Mancos Shale and from Permian Period Abo Formation were cut into thin wafers, placed in custom built osmometers and a chemical potential applied across them giving rise to induced osmotic flow. This in turn spawned matrix unique constants namely mechanical filtration coefficient L_P (m³ N⁻¹ s⁻¹), diffusional mobility per unit osmotic pressure L_PD (m³ N⁻¹ s⁻¹), osmotic flow coefficient; L_D (m³ N⁻¹ s⁻¹), reflection coefficient σ (dimensionless) at zero gradients of temperature and hydrostatic pressure. Considering intrinsic relationships between these constants where and L_PD = −σL_P, we have ascertained that the bentonitic fossiliferous Mancos shale had a lower L_P and a higher σ compared to the kaolinitic and siliceous shale from Abo Formation indicating a higher degree of compaction post-diagenesis (lower porosity) and higher filtration efficiency. Mechanistic processes involved in solute transport and matrix morphology indicate key multi-scale transformations, ionic- and atomic-exchange competitions on high energy sites like cation-exchange sites, isomorphic substitution at argillaceous mineral edges, atomic-clipping within basal spacing, preferential pathway migration, dead-end pores that give rise to localized solute exclusionary processes and solute attenuation giving rise to anomalous osmotic gradients.     

Determination of the Inductive Constant of the Succinimide Group from the Reaction Series of Thermolysis of Nitro Esters

The inductive constant σ* of the succinimide group was determined using the previously obtained equation correlating the rate constant k _l ofliquid-phase homolysis of the O-NO₂ bond in nitro esters with the σ* constants of electronegative substituents. The kinetics of thermolysis of N-nitroxymethyl- and N-(2-nitroxyethyl)succinimide in the melt and solutions was studied manometrically. Adequate data for k _l were obtained. The σ* constant of the succinimide group was determined to be 4.5. The σ* constants for the phthalimide group and some other imide substituents were determined by a similar procedure.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 6, 2005, pp. 954–957.Original Russian Text Copyright © 2005 by Goncharov, Dubikhin, Nazin, Fedorov, Shastin.     

Observations of the near-wall accumulation of suspended particles due to shear and electroosmotic flow in opposite directions

On the basis of previous studies, the particles in a dilute (volume fractions φ_∞ < 4 × 10^–3) suspension in combined Poiseuille and electroosmotic “counterflow” at flow Reynolds numbers Re ≤ 1 accumulate, then assemble into structures called “bands,” within ∼6 μm of the channel wall. The experimental studies presented here use a small fraction of tracer particles labeled with a different fluorophore from the majority “bulk” particles to visualize the dynamics of individual particles in a φ_∞ = 1.7 × 10^–3 suspension. The results at two different near-wall shear rates and three electric field magnitudes E show that the near-wall particles are concentrated about 150-fold when the bands start to form, and are then concentrated about 200-fold to a maximum near-wall volume fraction of ∼0.34. The growth in the near-wall particles during this accumulation stage appears to be exponential. This near-wall particle accumulation is presumably driven by a wall-normal “lift” force. The observations of how the particles accumulate near the wall are compared with recent analyses that predict that suspended particles subject to shear flow and a dc electric field at small particle Reynolds numbers experience such a lift force. A simple model that assumes that the particles are subject to this lift force and Stokes drag suggests that the force driving particles toward the wall, of O(10^–17 N), is consistent with the time scales for particle accumulation observed in the experiments.     

Phase Stability and Elastic Properties of Chromium Borides with Various Stoichiometries

Phase stability is important to the application of materials. By first‐principles calculations, we establish the phase stability of chromium borides with various stoichiometries. Moreover, the phases of CrB₃ and CrB₄ have been predicted by using a newly developed particle swarm optimization (PSO) algorithm. Formation enthalpy–pressure diagrams reveal that the MoB‐type structure is more energetically favorable than the TiI‐type structure for CrB. For CrB₂, the WB₂‐type structure is preferred at zero pressure. The predicted new phase of CrB₃ belongs to the hexagonal P‐6m2 space group and it transforms into an orthorhombic phase as the pressure exceeds 93 GPa. The predicted CrB₄ (space group: Pnnm) phase is more energetically favorable than the previously proposed Immm structure. The mechanical and thermodynamic stabilities of predicted CrB₃ and CrB₄ are verified by the calculated elastic constants and formation enthalpies. The full phonon dispersion calculations confirm the dynamic stability of WB₂‐type CrB₂ and predicted CrB₃. The large shear moduli, large Young’s moduli, low Poisson ratios, and low bulk and shear modulus ratios of CrB₄? PS_C and CrB₄? PS_D indicate that they are potential hard materials. Analyses of Debye temperature, electronic localization function, and electronic structure provide further understanding of the chemical and physical properties of these borides.     

Rheology of heterophase polymer systems

This lecture attempts to elucidate rheological behavior of multiphase polymer systems through a comparison with our studies on much simpler systems such as suspensions of (a) non-aggregating and (b) aggregating monodisperse spheres in viscoelastic media, (c) polymer latex in the same polymer liquids, and (d) emulsions or blends of two polymers with or without an emulsifying block copolymer. For the system (a) not only the viscosity η but also the modulus obey the known simple dependence on volume fraction ϕ of hard-sphere suspensions, while for the system (b) the flow induced-aggregation and dissociation of the particles govern the rheology. In the system (c), relaxations of entanglements of the adsorbed chains as well as the spatial distribution of the latexes are essential. For the emulsion (d) of a biased composition range (e.g., ϕ₁ > ϕ₂) the matrix phase 1 dominates, unless η₁ << η₂. When η₁ ≥ η₂, deformation and/or bursting of the dispersed phase 2 take place. For those of an even composition, the viscosity is additive of those of the components and is enhanced by adding the emulsifying block copolymer component.     

On the predictability of the high-frequency elastic modulus of semicrystalline polymers as function of crystallinity

The relation of the high-frequency elastic moduli of semicrystalline polymers to volume fraction crystallinity is correctly described by the Hashin-Shtrikman theory, without any disposable constants, as a function of the ratio of the modulus of the amorphous to that of the crystalline phase. Hence the (high-frequency) reduced modulus of semicrystalline polymers is largely a function of the temperature T/T_g. The importance of T/T_m for the modulus of the crystalline phase precludes the existence of a single universal reduced modulus versus temperature curve.     

Studies of particle-particle interactions using polystyrene latices and time-average light scattering

Dilute dispersions of monodisperse negatively-charged polystyrene latex particles, radii 161 Å, have been examined by time-average light scattering at various latex volume fractions and electrolyte concentrations. The latter were varied from the low value produced by maintaining mixed bed ion-exchange resin beads in the systems (ca. 10^–5 mol dm^–3) to the value of 5×10^–3 mol dm^–3 obtained by the addition of sodium chloride. From angular scattering measurements determinations of the structure factors were made; these were produced as a consequence of the particle-particle interactions in the system. By extrapolation of the structure factor to zero scattering angle, values were obtained for the osmotic compressibility and hence the osmotic pressure of the systems as a function of the latex volume fraction. It was found that the experimental data obtained could be interpreted in terms of a hard-sphere model for the particle-particle interaction. Good agreement was obtained provided that the particles were assigned a hard-sphere radius which was determined by the electrostatic repulsion between the particles.     

Characterization of silicone oil emulsions stabilized by TiO2 suspensions pre-adsorbed SDS

To study the effects of pre-adsorbed emulsifier on Pickering emulsion stability, the preparation of silicone oil emulsions by TiO₂ suspensions pre-adsorbed sodium dodecyl sulfate (SDS) at the fixed TiO₂ concentration of 0.15 g was carried out below a fiftieth of critical micelle concentration (cmc) of SDS, where all added amounts of SDS are adsorbed on the TiO₂ particles. The stability of the Pickering emulsions incorporating TiO₂ suspensions pre-adsorbed SDS was investigated by measuring the volume fraction of emulsified silicone oil, adsorbed amounts of TiO₂ suspensions pre-adsorbed SDS, oil droplet size, and some rheological responses such as the stress-strain sweep curve and strain and frequency dependences of dynamic viscoelastic moduli. The silicone oil was almost emulsified by TiO₂ suspensions pre-adsorbed SDS above cmc/10³. Increasing in the adsorbed amount of SDS on the TiO₂ particles leads to an increase in the adsorbed amounts of TiO₂ suspensions pre-adsorbed SDS. Such silicone oil emulsions for the first time showed two yield stresses in the stress-strain sweep curve as well as the oscillatory stress-strain curve. The respective yield stresses also increase with an increase in the adsorbed amounts of TiO₂ suspensions pre-adsorbed SDS. From such characteristic rheological properties and a partial sedimentation of some TiO₂ particles remained in the dispersion medium, we proposed the formation of a three dimensional network of the flocculated TiO₂ particles pre-adsorbed SDS on the silicone oil droplets.