Effect of a magnetic field on the rheological properties of cellulose ether solutions

Application of a magnetic field is shown to be accompanied by an increase in the viscosity of hydroxyethyl cellulose and ethyl cellulose solutions and an additional assembly of macromolecules, as is evident from a gain in the radii of light-scattering particles. The concentration dependences of supramolecular-particle radius and solution viscosity in the presence of a magnetic field are described by curves with maxima.     

The effect of particle shape on the structure and rheological properties of carbon-based particle suspensions

The structure and rheological properties of carbon-based particle suspensions, i.e., carbon black(CB), multi-wall carbon nanotube(MWNT), graphene and hollow carbon sphere(HCS) suspended in polydimethylsiloxane(PDMS), are investigated. In order to study the effect of particle shape on the structure and rheological properties of suspensions, the content of surface oxygen-containing functional groups of carbon-based particles is controlled to be similar. Original spherical-like CB(fractal filler), rod-like MWNT and sheet-like graphene form large agglomerates in PDMS, while spherical HCS particles disperse relatively well in PDMS. The dispersion state of carbon-based particles affects the critical concentration of forming a rheological percolation network. Under weak shear, negative normal stress differences(ΔN) are observed in CB, MWNT and graphene suspensions, while ΔN is nearly zero for HCS suspensions. It is concluded that the vorticity alignment of CB, MWNT and graphene agglomerates under shear results in the negative ΔN. However, no obvious structural change is observed in HCS suspension under weak shear, and accordingly, the ΔN is almost zero.     

The effect of oligomeric electrolytes on the aggregative stability and rheological properties of aqueous mineral suspensions

Oligomeric electrolytes of benzene and naphthalene series are synthesized, and the effects of chain length and hydrophilic group position in their molecules on not only the aggregative stability but also rheological and electrokinetic properties of concentrated mineral suspensions are studied. On the basis of the data of adsorption experiments, a parallel orientation of oligomer molecules on the surface of dispersed phase particles is revealed. By the sedimentation and electron microscopy techniques, it is established that, at the optimal concentration of oligomers, systems display aggregative stability. It is shown that the plasticizing effect of oligo-electrolytes is due to the combined action of electrostatic and adsorption-solvation factors.     

The effect of water-soluble polymers on the stability and rheological properties of suspensions of fibrous activated charcoal

The stability and rheological characteristics of polymer-containing aqueous suspensions of fibrous activated charcoal are determined. It is shown that the improved stability of suspensions is observed in the presence of carboxymethyl cellulose, oxypropyl methyl cellulose, polyvinylpyrrolidone, and a new water-soluble cellulose derivative, whereas the presence of polyacrylamide, poly(acrylic acid), poly(vinyl alcohol), and starch leads to the formation of unstable suspensions having a broad particle size distribution. A correlation between the concentration of dispersed-phase particles corresponding to the transition to non-Newtonian flow and the characteristics of suspension stability (such as the rate of sedimentation, the volume of the sediment, and the particle size distribution) is established. It is suggested that the high degree of charcoal particle surface hydrophilization by polyvinylpyrrolidone and by the water-soluble cellulose derivative is mainly responsible for the stability of the modified suspensions.     

Study of the rheological properties of dispersed lignite suspensions

The aim of this study was to determine the basic rheological parameters of dispersed highly concentrated lignite aqueous suspensions. Typical dilatant behavior was observed, where critical parameters of dilatancy were determined for different size fractions and concentrations. To modify the non-Newtonian behavior of the lignite highly concentrated suspensions, the effect of surfactant addition on the flow parameters of the suspensions was determined. Received: 5 March 1999/Accepted in revised form: 1 September 1999     

Measurement of rheological properties of corn stover suspensions

Corn stover is currently being evaluated as a feedstock for ethanol production. The corn stover suspensions fed to reactors typically range between 10 and 40% solids. To simulate and design bioreactors for processing highly loaded corn stover suspensions, the rheologic properties of the suspension must be measured. In systems with suspended solids, rheologic measurements are difficult to perform owing to settling in the measurement devices. In this study, viscosities of corn stover suspensions were measured using a helical ribbon impeller viscometer. A calibration procedure is required for the impeller method in order to obtain the shear rate constant, k, which is dependent on the geometry of the measurement system. The corn stover suspensions are described using a power law flow model.     

Dynamical and rheological properties of soft colloid suspensions

Soft colloids comprise a wide class of materials, ranging from linear polymers over polymeric assemblies, such as star polymers and dendrimers, to vesicles, capsules, and even cells. Suspensions of such colloids exhibit remarkable responses to imposed flow fields. This is related to their ability to undergo conformational changes and elastic deformations, and the adaptation of their dynamical behavior. The rational design of soft particles for targeted applications or the unraveling of their biological function requires an understanding of the relation between their microscopic properties and their macroscopic response. Here, mesoscale computer simulations provide an invaluable tool to tackle the broad range of length and time scales. In this article, we discuss recent theoretical and simulation results on the rheological behavior of ultrasoft polymeric colloids, vesicles, capsules, and cells. The properties of both, individual particles and semi-dilute suspensions, are addressed.     

Synthesis and rheological properties of cation-exchanged Laponite suspensions

In this paper we report our new approach to synthesize cation-exchanged Laponite suspensions. General observations of the prepared samples indicated that an aqueous suspension of 1 wt.% Laponite retained its free flowing liquid phase characteristics even after aging for several weeks. When bivalent cationic metals (Cu, Co, Ni) were ablated into the suspension, the strong charge of the crystal face was reduced and, on standing, the suspension gelled becoming highly viscous. This sol–gel transition was induced by the formation of a space-filled structure due to both van der Waals and electrostatic bonds between the positively charged rims and negatively charged faces. Rheological properties of such prepared suspensions were measured using a Brookfield DV-II Pro Viscometer with a small sample adapter (SSA18/13RPY). The yield strengths of 2.2 N/m², 3.2 N/m², and 1.7 N/m² were measured for Ni-, Co-, and Cu-modified Laponite suspensions, respectively. These yield strengths are sufficiently high for suspending weighting materials such as barite which requires the gel strength of about 0.5 N/m².     

Influence of cross-link density on rheological properties of temperature-sensitive microgel suspensions

The influence of the cross-link density on rheological properties of thermosensitive microgels was investigated. The temperature-sensitive hydrogel particles consisted of poly (N-isopropylacrylamide) (PNiPAM) chemically cross-linked with several different molar ratios of N,N′-methylenebisacrylamide. The variation of cross-link density leads to soft spheres that possess a different particle interaction potential and a different swelling ratio. With increasing temperature the microgel particles decrease in size and with it the effective volume fraction, which leads to strong changes in rheological properties. The relative zero-shear viscosity and the plateau modulus at different temperatures superpose to mastercurves when plotted versus the effective volume fraction. Up to an effective volume fraction of 0.5 the microgels behaved like hard spheres and the maximum volume fraction, as determined from the divergence of the zero-shear viscosity, was mainly dominated by the polydispersity of the spheres and not by the cross-link density. The plateau modulus, on the other hand, revealed soft-sphere behavior and the interaction potential became softer with decreasing cross-linker content. Received: 15 December 1999 Accepted: 15 February 2000     

Dispersion stabilities and rheological properties of fumed silica suspensions

Herein, we have reviewed fumed silica suspensions in dispersing fluids, polymer melts, and polymer solutions, focusing on their dispersion stability and rheological properties as a function of the surface character of fumed silica powders and the silica volume fraction, ?. Hydrophilic fumed silica powders are well dispersed at ? < 0.01 in polar dispersing fluids or polar polymer melts, and their phase states change from sol to gel with increasing ?. Such changes should also be strongly related to the rheological responses of the hydrophilic fumed silica suspensions, which change from Newtonian flow behavior to gel-like elasticity with increasing ?. On the other hand, hydrophobic fumed silica powders are stabilized in both polar and nonpolar dispersing fluids, depending on the interactions between the surface hydrophobic moieties and the dispersing fluids, in addition to those between the residual surface silanol groups and dispersing fluid, except for the particle–particle interactions. Moreover, the effects of the adsorption and desorption of polymers, as well as of non-adsorbing polymers on the dispersion stability and rheological behavior of fumed silica suspensions are discussed, by taking account of their optical microscopic observation and SANS curves.     

Influence of the exchangeable cations on stability and rheological properties of montmorillonite suspensions

The effects of NaCl and CaCl₂ on the colloid stability and rheological properties of Na- and Ca-montmorillonite dispersions were studied. The distribution of cations between the surface and the bulk phase was determined. For both of monocationic montmorillonite, the critical coagulation concentration were 250 mmol NaCl/dm³ and 2 mmol CaCl₂/dm³. The changes in the Bingham yield stresses of Na- and Ca-montmorillonite dispersions as functions of the NaCl and CaCl₂ concentration could be explained in terms of the surface excess amount and equilibrium concentration of the cations, the second electroviscous effect and the formation of a gel structure.     

The effect of polymeric surfactants on the rheological properties of nanoemulsions

We have investigated the rheological properties of submicron emulsions and how they are affected by the structure of polymeric surfactants. We have prepared oil-in-water emulsions stabilized with five steric surfactants, two of them belonging to the Myrj family and three belonging to the Pluronic family, with key differences on their structures. Droplet size and volume fraction have been kept constant to analyze only the influence of the surfactant. The viscoelasticity has been characterized by dynamic oscillatory shear experiments, while the shear viscosity was measured during steady shear flow tests. The results show a qualitatively similar gel-like behavior for all the emulsions, but with remarkable quantitative differences. Surfactants with longer hydrophilic tails produced emulsions with higher viscoelasticity. Pluronics, having a central hydrophobic part between two hydrophilic tails, produced emulsions with notably higher viscoelasticity and yield stress than Myrjs with comparable hydrophilic tails. The reason for this seems to be a more efficient steric barrier at the interface, induced by this central hydrophobic part.     

Viscoelasticity of suspensions of magnetic particles in a polymer: effect of confinement and external field

We study the suspensions of magnetic particles, the precursor state of magnetic gels and elastomers. We use magnetic particles with a permanent magnetization which is high enough to overcome thermal energy and low enough to guarantee a long live time of the sample. These particles form a space-filling structure at very low volume fractions (approximately 0.5 vol %), which modifies the viscoelastic response of the matrix significantly. In confined geometry the particles form clusters of a size that depends on the sample thickness. Even small external fields induce a strong anisotropy in the mechanical and optical properties of the suspension. The action of the applied magnetic field induces a gel-like response in one direction but leaves the other directions liquidlike. The viscosity is a very sensitive mechanical test for the anisotropy of the material. Light scattering data confirm our mechanical results.     

Size effect on the rheological behavior of nanoparticle suspensions in associating polymer solutions

Associating polymers are hydrophilic long-chain molecules containing a small amount of hydrophobic groups and tend to create bonds between chains by reversible associating interactions. The effects of associating polymer on the steady-shear viscosity and dynamic viscoelasticity are studied for suspensions of silica nanoparticles with diameters of 8, 18, and 25 nm. The silica particles of 8 nm are entrapped in the transient network of associating polymer by reversible adsorption. The enhancement of network results in the high viscosity with a Newtonian flow profile in the limit of zero shear rate. In suspensions of 25 nm silica, the hydrophobes extending from the chains adsorbed onto different particles can form a micelle by association interactions. The multichain bridging gave rise to the shear-thinning flow and high storage modulus at low frequencies. The suspensions of 25 nm silica are characterized as flocculated systems. Because of intermediate curvature, the flexible bridges are formed between silica particles of 18 nm. When the flexible bridges are highly extended within the lifetime in shear fields, the suspensions show shear-thickening flow. The shear-thickening flow can be attributed to the elastic effect of flexible bridges.     

Interfacial and rheological properties of humic acid/hematite suspensions

This work deals with the effect of humic acid (HA) adsorption on the interfacial properties, the stability, and the rheology of aqueous iron oxide (hematite) suspensions. It is first of all demonstrated that HA effectively adsorbs onto hematite, mainly at acid pH. Since the charge of the HA chains is negative, it will be electrostatically attracted to the hematite surface below the point of zero charge of the particles, when they are positively charged. Electrophoresis measurements of hematite suspensions as a function of pH in the presence and absence of HA clearly demonstrate the adsorption of negatively charged entities on the oxide. Since the HA-covered particles can be thought of as "soft" colloids, Ohshima's theory was used to gain information on the surface potential and the charge density of the HA layer (H. Ohshima, in: A.V. Delgado (Ed.), Interfacial Electrokinetics and Electrophoresis, Dekker, New York, 2002, p. 123). A different procedure was also used to ascertain the degree of modification experienced by the hematite surface when placed in contact with HA solutions. The contact angles of selected liquids on pretreated hematite layers lead to the conclusion that the humic acid molecules impart to the particles a significant electron-donor character, in turn increasing their hydrophilicity. All this amount of information is used in the work for the interpretation of the rheological properties of hematite suspensions; the results are consistent with a stabilizing effect of HA adsorption on the suspensions, mainly as a consequence of the increased electrostatic repulsion between particles.     

Effects of steric and hydrophobic forces on the rheological properties of ZrO2 suspensions

 The effect of pH on the flow behavior of ZrO₂ suspensions containing polyacrylic and octanoic acids was evaluated. In the flocculated pH regime, the flow behavior is highly shearthinning and can be described by a power-law model. The shear-thinning behavior increases with increasing degree of flocculation. Maximum shearthinning was observed at the zero zeta potential condition. Hydrophobic interaction arising from adsorbed octanoic acid was found to enhance the shear-thinning behavior. No such enhancement was observed for adsorbed polyacrylic acid. It was also illustrated that the viscosity–pH behavior is a mirror image of the yield stress–pH behavior. A quantitative particle-pair interactions model incorporating steric and hydrophobic interactions was proposed to explain the effects of polyacrylic and octanoic acids on the maximum yield stress. Received: 23 May 1997 Accepted: 4 June 1997     

The effect of added liposomes on the rheological properties of a hydrogel: a systematic study

Rheological characteristics of liposome-containing-hydrogels were studied. Sonicated unilamellar vesicles (SUV), prepared by probe sonication and multilamellar vesicles (MLV) prepared by thin film hydration were loaded in a hydrogel containing carbopol 974 NF and hydroxyethylcellulose (Natrosol 250 HX). Phosphatidylcholine (PC) or hydrogenated-PC (HPC) liposomes, plain or mixed with cholesterol (chol) were used. Static (steady-stress sweep-tests) and dynamic (frequency sweep-tests) rheological measurements were carried out. All gels had a shear thinning behaviour (fitted well by Cross model). Zero-rate shear viscosity and power law index values, revealed that PC liposome addition in the hydrogel had minimum effect on its rheological properties even at the highest lipid concentration used (20 mg/ml). Oppositely, HPC (or HPC/chol) liposome addition resulted in significant modulations of the same rheological characteristics (which increased with increasing lipid concentration). HPC liposomes also caused a significant increase in gel relaxation time, which indicates that the elastic character of the gel strengthens as HPC liposome concentration increases. Concluding, liposome composition (membrane rigidity) and lipid concentration, but not liposome size, seem to be very important factors that determine the rheological modulations caused by liposome addition in gels.     

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.     

The effect of magnetic field on conductive films

Conductive paints consisting of nickel powder (conductive material), solvent, and binder polymer were treated in a magnetic field. The efficiency of magnetic treatments on conductivity of coating films was evaluated. The effect of the following factors on conductivity of composite films by magnetic treatment was studied: metal powder concentration, magnetic flow density, time difference between film preparation and magnetic treatment, drying time of paint films, and effect of distance between terminals. Results showed that the volume resistivity of paint films treated magnetically was lower than that for untreated films at each nickel content. Magnetic treatment provided high conductivity even at low magnetic flow density, and conductivity increased with magnetic flow density.     

The effect of a magnetic field on the thermal destruction of cobalt formate

The thermal decomposition of cobalt formate in a flow of an inert gas led to the formation of cobalt nanoparticles in pores of various substrates (silica gel, alumina, activated carbon, and montmorillonite). Electron microscopic studies showed that the particle-size distribution of cobalt depended on the external magnetic field strength; the average particle size and distribution variance decreased as the field strength increased. It was assumed that the external magnetic field affected the nucleation constant of cobalt nanoparticles. Original Russian Text ? P.A. Chernavskii, V.I. Zaikovskii, G.V. Pankina, N.S. Perov, A.O. Turakulova, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 3, pp. 586–589.