Rheological Properties Of Concentrated Suspensions Research Articles

Rheological properties of concentrated allophane, halloysite, and kaolinite suspensions

This study investigated the rheological properties of concentrated suspensions of allophane, halloysite, and kaolinite, which are representative of a series of weathering products of volcanic glass and feldspar minerals in volcanic rocks and deposits, to elucidate the mechanism of landslide generation involving these materials. All suspensions generally showed shear thinning during flow curve measurements, as represented by a steady decrease of viscosity with increasing shear strain rate from 0.1 to 1000 s−1. A difference among them was that the kaolinite suspension showed unstable flow behavior at high solid concentration, and the halloysite suspension had a similar behavior at low solid concentration. The shear stress at the beginning of flow—which represents the yield stress of the suspension—showed a power law correlation with the solid volumetric fraction, and that for kaolinite agreed well with previous static yield stress measurements. The stress of the halloysite suspension deviated from the power law curve at solid fraction < ~0.15. Dynamic viscoelasticity tests indicated that all suspensions showed a power law correlation between the modulus value and solid fraction. The crossover strain (when a solidlike-to-liquidlike transition occurs) of the kaolinite suspension was almost constant within the investigated solid fraction range, whereas that of the halloysite suspension decreased noticeably at solid fraction < ~0.15. Overall, the allophane suspension had the weakest strength and lowest resistance to oscillatory strain at a given solid fraction condition. The rheological properties of halloysite observed here may be related to the occurrence of flow-like, long-distance landslides in volcanic areas containing weathered tephra.

Cross-linked polymer microparticles with tunable surface properties by the combination of suspension free radical copolymerization and Click chemistry

We propose a general, versatile and broad in scope two-steps approach for the elaboration of cross-linked polymer microparticles (µPs) with tunable functionalities and surface properties. Surface-functionalized cross-linked polymer µPs with diameter in the 80 μm range are prepared by the combination of: 1) suspension free radical copolymerization of styrene, propargyl methacrylate and 1,6-hexanediol dimethacrylate, 2) subsequent covalent tethering of a variety of azide-functionalized moieties (i.e. rhodamine B fluorescent dye or poly(ethylene glycol) (PEG) brush precursor) by copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) and, 3) optional N-alkylation of the 1,2,3-triazole groups followed by anion exchange reaction to afford covalently-tethered 1,2,3-triazolium ionic liquids with iodide or cresol red counter-anions. The resulting µPs are characterized by laser diffraction, differential scanning calorimetry, as well as by optical, confocal fluorescence, scanning electron and atomic force microscopies. Finally, the rheological properties of concentrated suspensions (volume fractions of 0.40 and 0.44) of the different synthesized µPs dispersed in a 1:1 (vol/vol) mixture of polyalkylene glycol and water are studied. The modification of µPs surface properties contributes not only to change the stability of the suspensions against flocculation, but also to significantly modify their rheological behavior at high shear stresses. This represents a clear experimental evidence of the importance of non-hydrodynamic contact forces in the rheology of non-Brownian suspensions (NBSs).

Flow-Spurt Transition under Shear Deformation of Concentrated Suspensions

The rheological properties of concentrated suspensions containing 55 and 60 vol % aluminum particles are studied. This concentration range corresponds to the transition from the viscoelastic to the elastoplastic behavior of the suspensions. Moreover, this range is most favorable for powder injection molding of metals and ceramics. The measurements are performed in a rheometer at different shear stresses. Elastoplastic deformations take place at low stresses, while separation, i.e., the rupture of the adhesion contact between the bulk suspension and the boundary of a solid wall of the rheometer, is observed with an increase in the stress. The rupture of the adhesion contact occurs with time, and this process is characterized by a dependence of the lifetime (durability) of the contact on stress. This dependence is of an exponential character. The replacement of smooth working surfaces of the rheometer by rough ones leads to some increase in the stresses. This reflects the replacement of the adhesion separation by the formation of discontinuities in the bulk suspension at the asperities of the rough surface. The observed behavior of the concentrated suspensions in the aforementioned range of compositions is represented by a mechanical model, into which a new element has been introduced. It is represented by a friction pair with variable length (and, accordingly, variable friction force). The spurt arises when a threshold force that induces displacement over the friction surface is reached.

Rheometric flows of concentrated suspensions of solid particles

Publications on experimental and theoretical studies of the rheological properties of concentrated suspensions of solid particles have been analyzed. According to the currently accepted ideas, the rheology of suspensions is considered as a result of contact interaction of their constituent particles caused by the action of external forces, the formation and destruction of various types of conglomerate structures. A new rheological model of a highly concentrated suspension of solid particles in a Newtonian fluid is proposed, which describes both a continuous and discontinuous increase in the effective viscosity with a uniform increase in shear stresses. Accurate analytical formulas are obtained for the velocity profiles of suspensions in rotational viscometers “cone-plane” and “cylinder-cylinder”, as well as in a slit viscometer. The proposed model is modified to take into account the non-Newtonian properties of a dispersion medium, which exhibits pseudoplastic properties at low strain rates, and dilatant properties at large strain rates. The effective viscosity of such a suspension is presented as the sum of the contributions from the dispersion medium and the solid particles of the dispersed phase. To describe the rheology of the dispersion phase, the Ellis model was used. The velocity profiles in a pressure driven flat channel are obtained numerically by the finite element method. It is shown that, depending on the parameters of the model, the velocity profiles in a flat channel can take various complex forms.

Effects of dispersion state on rheological and electrical characteristics of concentrated multiwalled carbon nanotube suspensions

We report a dispersion state of concentrated multiwalled carbon nanotube (MWCNT) suspensions depending on their iterative milling process. The rheological properties of concentrated suspensions prepared with different milling times were measured using a Couette-typed rotation rheometer, and their electrical characteristics were investigated via an LCR meter. The relationships of the rheological and electrical properties with the dispersion state of the MWCNT were examined. The results of both the rheological and electrical measurements indicated that the de-bundling and dispersion of the MWCNT induced by the iterative milling yielded a percolation microstructure and an electrically conductive structure.

Influence of high mixing intensity on rheology, hydration, and microstructure of fresh state cement paste

From both the fundamental and applied points of view, there is a growing interest in characterizing the rheological properties of concentrated suspensions, such as cement paste. When a cement paste is sheared, its intrinsic network structure will respond to the shear induced stresses. The influence of mixing intensity on the rheological properties of fresh state cement paste is studied. The results showed that contrary to what is popularly believed, when subjected to a high mixing intensity, the rheological properties of cement paste can increase once a certain threshold mixing intensity is achieved. This increase in rheological properties was associated with changes in the chemical nature and physical nature of the cement pastes.

Rheological peculiarities of concentrated suspensions

The rheological properties of concentrated suspensions of metal oxides dispersed in transformer oil, which are used as electrorheological fluids, are systematically studied. Colloidal particles have intermediate sizes between nano- and microsized scales. Low-amplitude dynamic measurements show that the storage moduli of the examined suspensions are independent of frequency and these materials should be considered as solidlike elastic media. The storage modulus is proportional to the five-powdered particle volume concentration. At the same time, a transition through an apparent yield stress with a reduction in the viscosity by approximately six orders of magnitude is distinctly seen upon shear deformation. The character of the rheological behavior depends on the regime of suspension deformation. At very low shear rates, a steady flow is possible; however, upon an increase in the rate, an unsteady regime is realized with development of self-oscillations. When constant shear stresses are preset, in some range of stresses, thickening of the medium takes place, which can also be accompanied by self-oscillations. In order to gain insight into the nature of this effect, measurements are performed for samples with different volume/surface ratios, which show that, in some deformation regimes, suspension is separated into layers and slipping occurs along a low-viscosity layer with a thickness of several dozen microns. Direct observations show a distinct structural inhomogeneity of the flow. The separation and motion of layers with different compositions explain the transition to the flow with the lowest apparent Newtonian viscosity. Thus, the deformation of concentrated suspensions is associated with self-oscillations of stresses and slipping along a low-viscosity interlayer.

Short-ranged pair distribution function for concentrated suspensions of soft particles

Many rheological properties of concentrated suspensions of soft particles jammed together beyond the random close packing limit for hard-spheres are determined by the contacts between the particles and are computable from the short range value of the pair distribution function. It is a challenge to compute the structure of these amorphous suspensions because they are athermal, non-equilibrium materials. Here we develop a microscopic theory to predict the short ranged pair distribution function with a transport or conservation of mass equation for the stationary state that includes a concentration dependent mean force that captures the effect of the bulk suspension on the pair interaction. The resulting distribution function is singular as the concentration approaches random close packing as expected from the theory for hard spheres. The pair distribution function and elastic properties that can be calculated from it are validated by computer simulations and experiments with concentrated suspensions of particles with varying soft potentials.

Aggregation in colloidal suspensions and its influence on the suspension viscosity

Influence of aggregate formation on rheological properties of concentrated suspensions is discussed and reviewed. Methods of Stokesian and Brownian dynamic simulations of aggregation are discussed and the results of simulations for both Brownian and non-Brownian particles are presented.

A differential model for the rheological properties of concentrated suspensions with weakly viscoelastic matrices

A model for the rheological properties of a concentrated suspension in weakly viscoelastic fluid matrices is proposed. The model is derived according to the Roscoe differential procedure described in 1952. The analytical results produced recently by Greco et al. (J Non-Newton Fluid Mech 147:1–10, 2007) and Housiadas and Tanner (J Non-Newton Fluid Mech 162:88–92, 2009) for dilute suspensions of neutrally buoyant, non-Brownian rigid spheres in weakly viscoelastic matrix fluids are the key results which are used as a base to predict the properties of concentrated suspensions. The results are compared with the few available experimental data from the literature, showing promising trends for the viscometric properties of the suspensions. In particular, one sees the rapidly increasing value of −N2/N1 as concentration increases.

Yield Stress of Multimodal Powder Suspensions: An Extension of the YODEL (Yield Stress mODEL)

The prediction of the rheological properties of concentrated suspensions is of great importance both in industrial processes (ceramics, cements, and pharmaceutics) and natural phenomena (debris flow, soil erosion). In a previous paper, we presented a new model (YODEL) that can predict the yield stress of concentrated particulate suspensions. The model is based on first principles and takes into account particle size distribution, interparticle forces, and microstructural features. It was validated using data from the literature on four different alumina powder suspensions. The current paper extends the application field of the YODEL, successfully, to multimodal distributions of much interest in the cement and concrete field. The key parameter governing the predictive capacity of the YODEL for multimodal distributions was shown to be the maximum packing fraction of the powder mixtures. The de Larrad compressive packing model was used to provide a maximum packing fraction for mixtures from their particle size distributions. The YODEL can predict yield stresses of multimodal suspensions within 10% of the experimental results. Further improvement of the maximum packing fraction prediction should help in our goal of yield stress prediction from basic powder and suspension characteristics.

Microstructure development in concentrated suspensions in a spinning ball rheometer

The spinning ball rheometer has been proposed as a method to measure the microstructure effect on the bulk rheological properties of concentrated suspensions. Recent experiments have shown that the measured extra torque on the spinning ball decreases as the radius of the spinning ball becomes comparable to the size of the suspended particle. We have performed a series of three-dimensional boundary element calculations of the rheometer geometry to probe the microstructure effects that contribute to that apparent slip. We present a series of quasi-static results based on random initial configurations as well as fully three-dimensional transient calculations, both of which are compared to the available experimental data. For the two cases, the apparent viscosity decreased as the size of the spinning ball decreased relative to the suspended particle. Comparison of the quasi-static and transient simulations indicates that the microstructure development is critical even at short times. In the transient calculations, the viscosity was observed to increase substantially relative to the torque based on the random initial configuration. Published in 2005 by John Wiley & Sons, Ltd.

Deposition and reentrainment of Brownian particles in porous media under unfavorable chemical conditions: some concepts and applications.

The deposition and reentrainment of particles in porous media have been examined theoretically and experimentally. A Brownian Dynamics/Monte Carlo (MC/BD) model has been developed that simulates the movement of Brownian particles near a collector under "unfavorable" chemical conditions and allows deposition in primary and secondary minima. A simple Maxwell approach has been used to estimate particle attachment efficiency by assuming deposition in the secondary minimum and calculating the probability of reentrainment. The MC/BD simulations and the Maxwell calculations support an alternative view of the deposition and reentrainment of Brownian particles under unfavorable chemical conditions. These calculations indicate that deposition into and subsequent release from secondary minima can explain reported discrepancies between classic model predictions that assume irreversible deposition in a primary well and experimentally determined deposition efficiencies that are orders of magnitude larger than Interaction Force Boundary Layer (IFBL) predictions. The commonly used IFBL model, for example, is based on the notion of transport over an energy barrier into the primary well and does not address contributions of secondary minimum deposition. A simple Maxwell model based on deposition into and reentrainment from secondary minima is much more accurate in predicting deposition rates for column experiments at low ionic strengths. It also greatly reduces the substantial particle size effects inherent in IFBL models, wherein particle attachment rates are predicted to decrease significantly with increasing particle size. This view is consistent with recent work by others addressing the composition and structure of the first few nanometers at solid-water interfaces including research on modeling water at solid-liquid interfaces, surface speciation, interfacial force measurements, and the rheological properties of concentrated suspensions. It follows that deposition under these conditions will depend on the depth of the secondary minimum and that some transition between secondary and primary depositions should occur when the height of the energy barrier is on the order of several kT. When deposition in secondary minima predominates, observed deposition should increase with increasing ionic strength, particle size, and Hamaker constant. Since an equilibrium can develop between bound and bulk particles, the collision efficiency [alpha] can no longer be considered a constant for a given physical and chemical system. Rather, in many cases it can decrease over time until it eventually reaches zero as equilibrium is established.

Effect of blend ratio on rheological properties of aqueous SiC suspensions

The relationship between blend ratio and rheological properties of concentrated suspensions is of great importance since it is the key to get high solid suspensions. The rheological properties of bidisperse aqueous suspensions made of two SiC powders with different particle size [ d (0.5)=1.63 and 18.43 μm, respectively] has been studied as a function of blend ratio ξ (the volume fraction of larger particle size). The results showed that the value of critical blend ratio ξ, at which the viscosity is minimized, is in close relation to the shear rate applied. At shear rates below 10 S −1, the critical ξ was greater than 70%. But at shear rates from 10 to 500 S −1, ξ turned to be 50%. The change of shear region from shear-thinning behavior to shear-thickening behavior may be used to account for the variation of critical ξ. Dynamic oscillatory tests showed that the moduli and the linear viscoelastic region of suspensions with higher ξ are smaller than those with low ξ and the increment of blend ratio ξ leads to the change of suspension from nearly an elastic response to a liquid like response.

Study on gelcasting of silicon nitride-bonded silicon carbide refractories

Gelcasting is a very useful technique for producing ceramic green parts with near-net-shapes. Although it has been successfully applied to shape fine ceramic systems where micrometer- and submicrometer-size particles are used as starting powders, there is little report on gelcasting of refractories manufactured by coarse raw materials. In the present paper, dispersion and rheological properties of concentrated suspensions of SiC/Si mixtures with coarse particles (∼350 μm) and their gelcasting processes were discussed. A stable SiC/Si suspension with solids loading up to 70 vol.% was prepared, from which uniform green bodies with complex shapes were successfully gelcast. The performances of green bodies and sintered samples were measured and their microstructures were studied. After nitridation of the gelcast components at 1450 °C for 10 h in 0.1 MPa nitrogen atmosphere, the bulk density of 2.61 g cm −3 and flexural strength of 55.4 MPa were obtained.

Prediction of rheological properties in polymer colloids

AbstractThe rheological properties of concentrated suspensions, containing sterically stabilized polymer colloids, are reviewed. It is shown that the various rheological characteristics are strongly interrelated in such systems. They deviate from Brownian hard sphere behaviour, but these deviations are closely related to the interparticle potential and other colloidal characteristics like particle diffusivity. As a result the rheological properties can be predicted from the colloidal characteristics and vice versa. This is demonstrated by means of available data on well characterized model dispersions. In the experiments volume fraction, particle size, suspending medium and temperature have been changed. Data reduction schemes and scaling laws for these parameters are discussed.

The Effect of Alkali and Alkaline Earth Cations on the Rheology of Concentrated Quartz Suspensions

Rheological properties of concentrated suspensions (∼70 wt%) of relatively coarse particles of crushed quartz (∼11 μm) in solutions of various metal chlorides were studied. The apparent viscosity of these settling suspensions, as measured with a “settling viscometer,” was compared with the zeta potential of the particles, the degree of aggregation, the sediment volume, and the degree of hydration of the metal cation. As the salt concentration increased and the suspensions coagulated, their viscosity increased. However, suspensions that were qualitatively completely aggregated showed large variations in apparent viscosity at 100 s−1, depending on the particular metal cation of the salt solution. The viscosities increased in the order Li<Na<K<Cs and Mg<Ca<Ba. These trends were attributed to an increase in attractive forces between colliding particles resulting from a decrease in the degree of hydration of the particle surfaces as a result of adsorption of cations from solution.

Rheological Behavior of Concentrated Suspensions of Particulates in Unsaturated Polyester Resin

Measurements were taken of the bulk rheological properties of concentrated suspensions of particulates in unsaturated polyester resins using a cone‐and‐plate rheometer. Over the range of shear rates investigated, suspensions of calcium carbonate showed Newtonian behavior, whereas suspensions of clay showed both yield behavior at low shear rates and shear‐thinning behavior at high shear rates. Suspensions of both calcium carbonate and clay together exhibited no normal stress effects. However, suspensions of polyethylene powders were found to exhibit positive values of normal stress at low concentrations, but negative values of normal stress at and above a certain critical concentration. In view of the fact that negative values of normal stress were observed at low shear rates (1–10 s−1), the possibility of inertial effects is ruled out because we observed inertia‐induced negative values of normal stress with three different Newtonian liquids at very high shear rates, say above 250 s−1. Having checked the possibilities of other probable sources that might have caused the negative normal stresses observed, we have concluded that the negative values of normal stress observed with concentrated suspensions of polyethylene powders in unsaturated polyester resin are real, not an artifact. Interestingly enough, the normal stress effects observed with concentrated suspensions of polyethylene powders very much resemble the observation reported by Kiss and Porter, who used liquid crystalline polymer solutions.

Rheology of unsaturated polyester resins. I. Effects of filler and low‐profile additive on the rheological behavior of unsaturated polyester resin

AbstractMeasurements were taken of the bulk rheological properties of concentrated suspensions of particulates in unsaturated polyester resins, using a cone‐and‐plate rheometer. The particulates used were clay, calcium carbonate, and milled glass fiber. With clay and milled glass fibers, shear‐thinning behavior of suspensions was observed at low shear rates or low shear stresses as the concentration of particulates was increased, whereas concentrated suspensions of calcium carbonate exhibited Newtonian behavior over the range of shear stresses or shear rates investigated. The cone‐and‐plate rheometer was also used for measurements of the bulk rheological properties of various mixtures of polyester resin and low‐profile additives. For low‐profile additives, solutions, in styrene, of poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA) were used. It was found that the bulk viscosities of all mixtures of polyester resin and PVAc solution lie between those of the individual components, whereas the bulk viscosities of some mixtures of polyester resin and PMMA solution go through a minimum and a maximum, depending on the composition of the mixture. While all mixtures of polyester resin and PVAc solution exhibited negligible normal stress, some mixtures of polyester resin and PMMA solution exhibited noticeable normal stresses. It should be mentioned that polyester resin follows Newtonian behavior. It turned out that all mixtures of polyester resin and PVAc solution exhibited clear, homogeneous solutions, whereas mixtures of polyester resin and PMMA solution exhibited optical heterogeneity, i.e., turbidity. When polyethylene powders were used as low‐profile additives, suspensions of polyester resin and polyethylene powders exhibited negative values of normal stress as the concentrations of suspension reached a critical value. When both filler and low‐profile additive were put together in polyester resin, the rheological behavior became quite complex, indicating that some interactions exist between the filler and the low‐profile additive.

A plastic-viscoelastic constitutive equation to represent the rheological behavior of concentrated suspensions of small particles in polymer melts

The rheological properties of concentrated suspensions of small particulates in polymer melts are critically discussed. These systems give the appearance of a yield value in steady shear flow but also show many typical responses of a viscoelastic fluid. A tensor constitutive equation is developed to represent the behavior of such systems which combines a tensor yield criterion with a non-linear viscoelastic model. The response of this model is considered in both simple shear flow and uniaxial elongational flow.