Rheological Behavior Of Concentrated Suspensions Research Articles

Deformation Properties of Concentrated Metal-in-Polymer Suspensions under Superimposed Compression and Shear

Concentrated metal-in-polymer suspensions (55 vol.% and 60 vol.%) of aluminum powder dispersed in low molecular weight polyethylene glycol) demonstrate elastoplastic properties under compression and shear. The rheological behavior of concentrated suspensions was studied in a rotational rheometer with uniaxial compression (squeezing), as well as shearing superimposed on compression. At a high metal concentration, the elasticity of the material strongly increases under strain, compared with the plasticity. The elastic compression modulus increases with the growth of normal stress. Changes in the shear modulus depend on both normal and shear stresses. At a low compression force, the shear modulus is only slightly dependent on the shear stress. However, high compression stress leads to a decrease in the shear modulus by several orders with the growth of the shear stress. The decrease in the modulus seems to be rather unusual for compacted matter. This phenomenon could be explained by the rearrangement of the specific organization of the suspension under compression, leading to the creation of inhomogeneous structures and their displacement at flow, accompanied by wall slip. The obtained set of rheological characteristics of highly loaded metal-in-polymer suspensions is the basis for understanding the behavior of such systems in the powder injection molding process.

Utilizing the Discrete Element Method for the Modeling of Viscosity in Concentrated Suspensions.

The rheological behavior of concentrated suspensions is a complicated problem because it originates in the collective motion of particles and their interaction with the surrounding fluid. For this reason, it is difficult to accurately model the effect of various system parameters on the viscosity even for highly simplified systems. We model the viscosity of a hard-sphere suspension subjected to high shear rates using the dynamic discrete element method (DEM) in three spatial dimensions. The contact interaction between particles was described by the Hertz model of elastic spheres (soft-sphere model), and the interaction of particles with flow was accounted for by the two-way coupling approach. The hydrodynamic interaction between particles was described by the lubrication theory accounting for the slip on particle surfaces. The viscosity in a simple-shear model was evaluated from the force balance on the wall. The obtained results are in close agreement with literature data for systems with hard spheres. Namely, the viscosity is shown to be independent of shear rate and primary particle size for monodisperse suspensions. In accordance with theory and experimental data, the viscosity grows rapidly with particle volume fraction. We show that this rheological behavior is predominantly caused by the lubrication forces. A novel approach based on the slip of water on a particle surface was developed to overcome the divergent behavior of lubrication forces. This approach was qualitatively validated with literature data from AFM measurements using a colloidal probe. The model presented in this work represents a new, robust, and versatile approach to the modeling of viscosity in suspensions with the possibility to include various interaction models and study their effect on viscosity.

Tape casting of alumina/zirconia suspensions containing graphene oxide

Abstract The introduction of carbon derivatives (nanotubes, graphene, etc.) as a second phase in ceramic matrices has limitations arising from their difficult processing. This paper studies the colloidal stability and the rheological behaviour of concentrated suspensions of alumina with 5 vol.% Y-TZP (AZ) and the effect of the addition of 2 vol.% of graphene oxide (AZGO) on the suspension stability, rheological behaviour and tape casting performance. The colloidal stability was studied using zeta potential measurements in terms of concentration of deflocculants and pH and homogenisation was optimised adjusting the sonication mode and time. The best results were obtained for pulsed mode. The optimum rheological properties were obtained for solid loadings of 53 vol.% and 40 vol.% for AZ and AZGO. Homogeneous, flexible tapes with thickness of ∼120 μm were obtained reaching densities of >60% of theoretical density in which secondary phases are well dispersed.

Aqueous colloidal processing of SiC with Y3Al5O12 liquid-phase sintering additives

Abstract The aqueous colloidal processing of SiC with Y3Al5O12 liquid-phase sintering additives was investigated for two different additive systems, one the mixture of Y2O3 and Al2O3 in a 3:5 molar ratio and the other directly Y3Al5O12. The investigation involved the study of the colloidal stability of the different components, and the comparison of the rheological behaviour of concentrated suspensions of SiC, SiC + 3Y2O3:5Al2O3, and SiC + Y3Al5O12 as a function of the sonication condition, dispersant content, and solid loading. This allowed appropriate conditions for the preparation of well-dispersed, single-phase, and multi-component concentrated suspensions of SiC to be identified. It was found that the multi-component suspensions have better rheological behaviour than the single-phase ones, and that in terms of rheology and slip casting the Y3Al5O12 additives are more functional than the conventional 3Y2O3 + 5Al2O3 additives. It was also demonstrated that the Y3Al5O12 additive is as effective as the 3Y2O3 + 5Al2O3 additive in densifying SiC via liquid-phase sintering, with there existing no differences either in the microstructure or in room-temperature mechanical properties (hardness, toughness, and fracture mode). Implications of interest for the wet-shaping of complex SiC parts are discussed.

Local transient rheological behavior of concentrated suspensions

This paper reports experiments on the shear transient response of concentrated non-Brownian suspensions. The shear viscosity of the suspensions is measured using a wide-gap Couette rheometer equipped with a Particle Image Velocimetry (PIV) device that allows measuring the velocity field. The suspensions made of PMMA particles (31$\mu$m in diameter) suspended in a Newtonian index- and density-matched liquid are transparent enough to allow an accurate measurement of the local velocity for particle concentrations as high as 50%. In the wide-gap Couette cell, the shear induced particle migration is evidenced by the measurement of the time evolution of the flow profile. A peculiar radial zone in the gap is identified where the viscosity remains constant. At this special location, the local particle volume fraction is taken to be the mean particle concentration. The local shear transient response of the suspensions when the shear flow is reversed is measured at this point where the particle volume fraction is well defined. The local rheological measurements presented here confirm the macroscopic measurements of Gadala-Maria and Acrivos (1980). After shear reversal, the viscosity undergoes a step-like reduction, decreases slower and passes through a minimum before increasing again to reach a plateau. Upon varying the particle concentration, we have been able to show that the minimum and the plateau viscosities do not obey the same scaling law with respect to the particle volume fraction. These experimental results are consistent with the scaling predicted by Mills and Snabre (2009) and with the results of numerical simulation performed on random suspensions [Sierou and Brady (2001)]. The minimum seems to be associated with the viscosity of an isotropic suspension, or at least of a suspension whose particles do not interact through non-hydrodynamic forces, while the plateau value would correspond to the viscosity of a suspension structured by the shear where the non-hydrodynamic forces play a crucial role.

Tape casting of strontium and cobalt doped lanthanum chromite suspensions

Lanthanum chromite (LaCrO 3) is currently the most widely studied material as interconnector layers for solid oxide fuel cells (SOFC). The complexity of microstructures and geometries of SOFC devices, which are usually built-up by lamination of the different constitutive layers, make it necessary a precise control of processing parameters to achieve the desired combination of properties. Much effort has been devoted to the processing of electrodes and electrolytes but the other layers, such as that of interconnecting material, have received scarce attention. This work deals with the preparation and optimisation of the rheological behaviour of concentrated suspensions of Sr- and Co-doped LaCrO 3 and the subsequent tape casting to produce homogeneous thin sheets to be used in the SOFC stack. The starting powder was produced by combustion synthesis from the corresponding nitrates and urea as a fuel, and had a final composition of La 0.80Sr 0.20Cr 0.92Co 0.08O 3. These powders were dispersed in ethanol with commercial copolymers (Hypermer, KD6) to solids loading of up to 58 wt%. The binding system (BS) consisted of a mixture of a binder, polyvinyl butyral-co-vinyl alcohol-co-vinyl acetate (PVA-PVAc), and two plasticizers, polyethyleneglycol (PEG400) and benzylbutylphthalate (BBP). The effect of the binding system content and the binder-to-plasticizer ratio on the tape casting performance and the characteristics of the green and the sintered tapes, were studied, as well as the influence of the casting parameters (casting speed and blades height).

Elongational and shear rheology of carbon nanotube suspensions

Rheological behavior of concentrated suspensions of chemical vapor deposition carbon nanotubes in uniaxial elongation and simple shear is studied experimentally and theoretically. Nanotubes are suspended in viscous host liquids—castor oil or its blends with n-decane. The elongational measurements are performed by analyzing self-thinning (due to surface tension effect) liquid threads of nanotube suspensions. A quasi-one-dimensional model is used to describe the self-thinning process, whereas corrections accounting for thread nonuniformity and necking are introduced a posteriori. The effects of nanotube concentration and aspect ratio, viscosity of the suspending liquid, and initial diameter of the self-thinning thread in uniaxial elongation are elucidated. The results for uniaxial elongation are compared with those for simple shear. The correspondence in the results of the shear and elongational measurements is addressed and interpreted. The results conform to the Herschel–Bulkley rheological constitutive equation (i.e., power law fluids with yield stress). However, the yield stress in elongation is about 40% higher than in simple shear flow, which suggests that the original Herschel–Bulkley model need modification with the yield stress being a function of the second invariant of the deviatoric stress tensor. The present effort is the first to study capillary self-thinning of Herschel–Bulkley liquids, which are exemplified here by suspensions of carbon nanotubes.

Gelcasting of alumina suspensions containing nanoparticles with glycerol monoacrylate

An acrylic monomer of low toxicity containing two hydroxyl groups has been synthesized and used for gelcasting in water. The results have been compared to those achieved with the use of a commercially available monomer (2-hydroxyethyl acrylate). Due to the chemical structure of the synthesized monomer, no addition of the crosslinking agent was necessary for gelation and similar results in terms of rheology of suspensions, density and microstructure of the bodies were obtained with respect to those obtained with the commercial monomer. However, higher time for gelation was observed. Two alumina powders with very different particle sizes were used in this study: a commercial submicron-sized powder ( d 50 = 0.35 μm) and a nanometer-sized alumina obtained by freeze-drying from aluminum sulphate solutions. The rheological behavior of concentrated suspensions was studied in order to establish their stability and to analyse the effect of the different monomers used in the process. Once the suspensions were optimized, the influence of the size of the powder on the gelation process was studied. The sintered density of submicrometer-sized alumina was higher (99%) than that measured when the bimodal suspension was used due to the difficulty to obtain highly concentrated suspensions from nanometric powder.

Colloidal behaviour of mullite powders produced by combustion synthesis

The combustion synthesis method is very attractive to produce homogeneous powders of pure or mixed oxidic ceramics without further thermal treatment. Previous work reports the synthesis of monophasic mullite powders by a suspension combustion process using two combustions aids, ammonium nitrate and hydrogen peroxide, and an excess of colloidal silica suspension. The combustion synthesis leads to coarse, agglomerated powders that must be crushed. The effect of attrition milling on both particle size and morphology was studied. Aqueous suspensions of the milled powders were prepared using two dispersants: citric acid and a polyacrylic acid-based polyelectrolyte. The stability of the powders was studied by measuring the zeta potential. The effect of the dispersants on the rheological behaviour of concentrated suspensions was also studied. The sintered densities and the microstructures of specimens obtained by slip casting were studied, demonstrating that mullite powders obtained by a single step combustion process can be processed through conventional shaping routes.

Rheological behavior of concentrated suspensions as affected by the dynamics of the mixing process

The rheological characterizations of concentrated suspensions are generally carried out assuming “well-mixed” suspensions. However, the variation of the concentration distributions of the ingredients of the formulation, i.e., the “goodness of mixing”, the size and shape distributions of the particle clusters and the rheological behavior of the suspension all depend on the thermo-mechanical history that the suspension is exposed to during the mixing process. Here, various experimental tools are used for the characterization of the degree of mixedness (concentration distributions) of various ingredients along with the characterization of rheological material functions, wall slip behavior and the maximum packing fraction of a graphite/elastomer suspension. The degree of mixedness values of the ingredients of the suspensions processed using batch and continuous processes and under differing operating conditions were characterized quantitatively using wide-angle X-ray diffraction and thermo gravimetric analysis and were elucidated under the light of the electrical properties of the suspension as affected by the mixing process. Upon achieving better homogeneity of the graphite particles and the binder and decreases in the size and breadth of the size distributions of particle clusters (as inferred from electrical measurements and maximum packing fraction values), the elasticity (storage modulus) and the shear viscosity (magnitude of the complex viscosity from small-amplitude oscillatory shear and shear viscosity from steady torsional and capillary rheometry) of the suspension decreased significantly and the wall slip velocity values increased. These findings demonstrate the intimate relationships that exist between the rheological behavior of concentrated suspensions and the thermo-mechanical history that they are exposed to during the processing stage and suggest that the preparation conditions for suspensions should be carefully selected and well documented to achieve reproducible characterization of rheological material functions.

Determination of phenomenological constants of shear-induced particle migration model

Particles in initially well-mixed suspensions subject to inhomogeneous shear flows will migrate and establish a particle concentration gradient and a non-Newtonian velocity profile. In this study, a phenomenological diffusive flux model coupled with flow equations was employed to describe the shear-induced particle migration in a concentrated suspension. The focus of the paper is on the determination of the two phenomenological constants in the diffusive flux model k c and k η . They were determined inversely by employing a least square analysis on the experimental pressure data with different capillary die ratios of length to diameter of the die. The pressures and the flow patterns of the non-Newtonian concentrated suspension were predicted in terms of these two phenomenological constants. The results indicated that particle migration should be accounted for to properly characterize the rheological behaviour of concentrated suspensions.

Rheology of Mixed Palygorskite-Montmorillonite Suspensions

The rheological parameters of a clay suspension can be used to evaluate particle-particle interactions. Although there have been many studies of the rheological behavior of suspensions of reference clays, only few studies were performed with suspensions of mixed clays, and the rheological properties of these suspensions are not yet fully understood. The length-to-width ratio of some soil clays was evaluated by Egashira and Matsumoto (1981) based on viscosity measurements of very dilute suspensions of these clays. The influence of montmorillonite additions on the rheological behavior of kaolinite suspensions was determined by Keren (1989, 1991). The particle arrangement in three soil-clay suspensions was studied by Zhao et al. (1991) based on rheological measurements. The effects of mineralogical composition and particle size on the rheological behavior of concentrated suspensions of lateritic soil clays were investigated by Cerpa et al. (1999). The rheological properties of suspensions of reference palygorskite clays were investigated in a previous study (Neaman and Singer, 2000). To determine if the rheology of palygorskite suspension is affected by the addition of montmorillonite, we investigated the rheological properties of palygorskite-montmorillonite suspensions. Three palygorskite samples were used for this study: clays from Mt. Flinders (Queensland, Australia), Mt. Grainger (South Australia), and Yucatan (Sacalum, Mexico). Wyoming montmorillonite (SWy-1, Crook Country, USA) was used for preparation of palygorskite-montmorillonite mixtures. The clays were ultrasonically dispersed in water and the <2-μm size fraction was separated by sedimentation under gravity. The clays were then lightly treated by 0.1 N HCl (sample:solution ratio of 1:10) for the removal of carbonates. The samples were rapidly heated in the acid to boiling and the pH values of the suspensions were immediately adjusted to pH 7 by addition of 0.1 N NaOH. The sample from Mt. Grainger contains considerable amounts of free iron oxides which were extracted by …

Rheological aging and rejuvenation in microgel pastes

We have studied experimentally the rheological behavior of concentrated suspensions of soft deformable microgels below the yield point. We have found history-dependent effects which are interpreted in terms of aging and rejuvenation phenomena, analogous to those existing in glassy systems. The stress amplitude controls the long-time memory and determines the slow evolution of the suspensions.

The colloidal and rheological properties of bentonite suspensions

The objective of this paper is to give an overview of the colloidal properties of clays and to describe the work performed until the present moment in the area of adsorption of non-ionic polymers onto clay and the rheology of clay suspensions. The review is particularly focused on Na +-montmorillonite clay of the smectite group (Bentonite), due to its high swelling capacity, the ability to adsorb significant amounts of polymer on its surfaces, and the formation of a gel-like structure with yield characteristics and viscoelastic properties at relatively low clay concentrations. A significant amount of research has been performed in these areas with applicability to various fields, such as soil science and drilling fluids, where interaction of organic polymers with the clay surface and the rheological response in the presence of electrolytes under different temperature and pressure conditions are of considerable relevance. In order to understand the interaction of these compounds with the clay surface and the rheological response of the resulting suspensions, it becomes essential to understand the structure of clay minerals, the interaction of clay particles in aqueous medium, the swelling behaviour and electrokinetic properties of Na +-montmorillonite clay. For this reason, the above topics are addressed, along with the general features of stabilisation of colloidal particles and the adsorption mechanism of non-ionic compounds on hydrophilic surfaces. A comprehensive review of the most relevant studies of the adsorption of non-ionic surfactants and polymers onto both silica and clay is presented. Finally, an overview of the rheological behaviour of concentrated suspensions and models used to describe the flow behaviour of clay suspensions is given, along with the effect of electrolytes, polymers, high pressure and high temperature on the flow behaviour of Na +-montmorillonite suspensions.

The relationship between particle aggregation and rheology in mixed silica–titania suspensions

The rheological behaviour of concentrated suspensions of coarse SiO 2 particles containing various amounts of fine TiO 2 has been studied using a viscometer which prevents settling of the suspended material. At a volume fraction of 0.468 the mixed SiO 2/TiO 2 suspensions exhibit shear thinning behaviour, with the yield stress and viscosity being strongly dependent on pH and the TiO 2 content of the suspension. Four different TiO 2 samples are investigated, each having different inorganic surface coatings. The TiO 2 particles adsorb on the surface of the SiO 2 particles and coat them, resulting in suspensions with isoelectric points corresponding to those of suspensions containing TiO 2 alone. The changes in yield stress as a function of pH are influenced by the TiO 2 content of the suspension, and when the silica:TiO 2 volume ratio is greater than ∼0.03, the yield stress vs pH curve resembles that of a pure TiO 2 suspension. The maximum yield stress is observed at a pH corresponding to the isoelectric point of the TiO 2 component, and at this pH, networks of TiO 2 aggregates can form. The formation and breakdown of these aggregates has been monitored in concentrated suspensions using focused beam reflectance measurement (FBRM). FBRM results clearly demonstrate that increases in the yield stress of the suspensions are accompanied by an increase in aggregate size. At a given pH, changes in plastic viscosity with increasing TiO 2 content in the suspensions appear to result from an increase in the maximum packing density due to the corresponding change in the particle size distribution.

Rheological modeling of fractal and dense suspensions

The development of a rheological model for aggregated suspensions is necessarily based upon a suitable characterization of the structure of the disperse phase and of the structural modifications produced by a deformation or a velocity field. The disperse phases of real aggregate particle suspensions, both dilute and concentrated, may present a wide variety of structures, which can be conveniently characterized by using the concepts of fractal geometry. In the present paper we formulate a rheological model able to correlate the structural processes induced by shear sflow conditions and the consequent shear dependence of viscosity with the shear stress changes experienced by the suspension. The flow curves calculated from the model, both for dense and fractal aggregates, closely resemble those observed for real colloidal and non-colloidal suspensions. The model appears particularly advantageous in describing the transition from shear thinning to plastic behavior, which usually occurs with increasing volume fraction or aggregation of the disperse phase. The role played by the aggregation state of the disperse phase become predominant in the low shear stress range, where aggregates may be composed of many particles, and, consequently, where the fractal dimensionality D becomes an important parameter in determining the compactness of the aggregate structure and the rheological behavior of concentrated suspensions. The validity of the proposed model is checked further through an analysis of experimental viscosity data relative to two series of epoxy-acrylic systems, containing titanium dioxide and aluminum silicate at different disperse phase concentrations.

Rheological behaviour of concentrated suspensions of soft spheres

On etudie les proprietes rheologiques de suspensions concentrees de spheres molles dans un domaine de concentration ou tout l'espace est rempli de particules plus ou moins gonflees. Ces suspensions ressemblent a des gels fragiles et presentent un module dynamique pratiquement independant de la frequence. Sous un ecoulement de cisaillement, les spheres se deforment a 45° des lignes de courant. Puisqu'au dessus d'une concentration critique, le module elastique apparait dependant de la rigidite individuelle des particules, on propose une modelisation theorique de ces systemes a partir d'une structure coeur-peau de chaque particule ce qui permet un calcul simple du module elastique par minimisation de l'energie libre. L'accord avec les donnees experimentales est satisfaisant en utilisant des valeurs plausibles des parametres ajustables.

Polymer coatings with a destabilizing action on suspensions

The rheological behaviour of concentrated suspensions of spherical glass particles can be influenced drastically by surface pretreatment of the particles. This treatment consisted of silanization with mono- or difunctional organo-silicon compounds. Results from ESCA, IR and SEM analysis strongly suggest that silanization with the reagent dimethyldichlorosilane (DMDCS) results, depending on the processing, in a more or less homogeneous layer of oligomer and/or polymer. The particles were suspended in an apolar (DOP) and in a polar liquid (glycerol/water). All suspensions exhibited perfect or nearly Newtonian behaviour except DMDCS-coated glass in glycerol/water. With the latter, under oscillatory shear conditions, behaviour of a continuous coagulation network structure was found: dynamic moduli are hardly dependent on frequency while network structure breakdown occurred already at the lowest accessible deformation (10 −3). Under steady shear rate conditions pseudoplastic behaviour was found, with relative differential viscosities at high shear rates (10 2 s −1) approaching the value of equivalent suspensions of uncoated spheres. Results are interpreted with the giant floc model, according to which macroscopic shear is understood as microscopic slip along shear planes between coagulated domains. The origin of coagulation in these systems is not in van der Waals forces because bond strengths as judged from rheology are too high. Polymer bridging between coating polymers is suggested as flocculation mechanism.

Effect of surface treatment of a polymeric filler on its interaction with a plasticizer

AbstractThe interfacial behavior of a filler–plasticizer system was investigated using the pair poly(sodium methacrylate‐co‐divinylbenzene) beads–diisooctyl phthalate. The effect of various surface treatments of the filler was studied. These consist of a selective extraction of surfactant used in its preparation by emulsion copolymerization and in the adsorption of other types of surfactants. Changes in the compatibility between the two basic components were determined from measurements of the rate of penetration of the plasticizer through the filler bed, from the rheological behavior of concentrated suspensions and from the extractability of the filler from the plasticized PVC matrix.

The rheology of concentrated suspensions of spheres in simple shear flow by numerical simulation

The newly developed simulation method known as Stokesian dynamics is used to investigate the rheological behaviour of concentrated suspensions. Both the detailed microstructure (e.g. pair-distribution function) and the macroscopic properties are determined for a suspension of identical rigid spherical particles in a simple shear flow. The suspended particles interact through both hydrodynamic and non-hydrodynamic forces. For suspensions with purely hydrodynamic forces, the increase in the suspension viscosity with volume fraction ϕ is shown to be caused by particle clustering. The cluster formation results from the lubrication forces, and the simulations of a monolayer of spheres show a scaling law for the cluster size: lc ∼ [1 − (ϕ/ϕm)½]−1, where ϕm is the maximum volume fraction that can shear homogeneously. The simulation results suggest that the suspension viscosity becomes infinite at the percolation-like threshold ϕm owing to the formation of an infinite cluster. The predicted simulation viscosities are in very good agreement with experiment. A suspension with short-range repulsive interparticle forces is also studied, and is seen to have a non-Newtonian rheology. Normal-stress differences arise owing to the anisotropic local structure created by the interparticle forces. The repulsive forces also reduce particle clustering, and as a result the suspension is shear-thickening.