Flow Behavior Of Concentrated Suspensions Research Articles

Operando observation of concentrated SiO2 suspensions by optical coherent tomography during flow curve measurements: The relationship between polymer dispersant structures and surface interactions

HypothesisFlow curve measurement is commonly used to characterize the flow behavior of concentrated suspensions. However, dynamic changes in the suspension inner microstructures under highly sheared conditions have not been correctly understood even though they strongly affect the measured shear stress. We hypothesize that the real particle dynamics during shearing could be effectively revealed by a systematic investigation that combines macroscopic flow curve measurements with operando microstructural observation employing an optical coherent tomography (OCT) apparatus and surface interaction measurements with the colloidal probe atomic force microscopy (AFM) method. ExperimentsThe model system was spherical SiO2/toluene suspensions stabilized by polyethyleneimine (PEI) partially complexed with different fatty acids. Inner structures of the suspensions during flow curve measurements were observed by the OCT technique. The surface-surface interactions in toluene were analyzed using the colloidal probe AFM method. FindingsOperando OCT observations revealed that during flow curve measurements, the suspensions can have completely different microscopic flow modes depending on the fatty acid species complexed to PEI and the solid concentrations. These microscopic flow modes could not be recognized using the typical flow curve measurements alone. The different flow modes can be explained by surface interactions measured by the colloidal probe AFM method.

Effects of Cell Density and Biomacromolecule Addition on the Flow Behavior of Concentrated Mesenchymal Cell Suspensions

With the rapidly growing interest in the use of mesenchymal stromal cells (MSCs) for cell therapy and regenerative medicine applications, either alone as an injected suspension, or dispersed within injectable hydrogel delivery systems, greater understanding of the structure-function-property characteristics of suspensions of adhesion-dependent mesenchymal cells is required. In this paper, we present the results of an experimental study into the flow behavior of concentrated suspensions of living cells of mesenchymal origin (fibroblasts) over a wide range of cell concentrations, with and without the addition of hyaluronic acid (HA), a commonly utilized biomolecule in injectable hydrogel formulations. We characterize the change in the shear viscosity as a function of shear stress and shear rate for cell volume fractions varying from 20 to 60%. We show that high volume fraction suspensions of living mesenchymal cells, known to be capable of homotypic interactions, exhibit highly complex but reproducible rheological footprints, including yield stress, shear thinning and shear-induced fracture behaviors. We show that with the addition of HA, we can significantly modify and tailor the rheology of these cell suspensions at all volume fractions. Using FACS and confocal imaging, we show that the observed effect of HA addition is due to a significantly modulation in the formation of cellular aggregates in these suspensions, and thus the resultant volume spanning network. Considering the aggregates as fractal structures, we show that by taking into account the changes in volume fractions with shear, we are able to plot a master curve for the range of conditions investigated and extract from it the average adhesion force between individual cells, across a population of millions of cells. The outcomes of this study not only provide new insight into the complexity of the flow behaviors of concentrated, adhesive mesenchymal cell suspensions, and their sensitivity to associative biomacromolecule addition, but also a novel, rapid method by which to measure the average adhesion force between individual cells, and the impacts of biomacromolecules on this important parameter.

Design of ceramic paste formulations for co-extrusion

The rheological and flow behaviour of ceramic pastes with varying solids loadings (solids volume fraction) has been studied. The pastes were shown to exhibit power law slip flow at both low and high solids loadings, with no slip yield stress. As would be expected, the extrusion rheometry data showed an increase in the die entry extrusion pressures with solids loading, in a trend similar to that of the Dougherty–Krieger equation. The die land flow however was shown to exhibit, only within a narrow range of solids loadings of approximately 53 to 56 v/v %, a trend in the die land extrusion pressure that was relatively independent of solids loading. Outside of this range the die land extrusion pressures increased significantly with solids loading in a trend similar to that of the die entry pressures. Using the Mooney analysis method, this was shown to be due to the development of slip flow with solids loading, as is consistent with other studies into the flow behaviour of concentrated suspensions. The slip velocity was also shown to be related to the wall shear stress by way of power law relationship also consistent with results from previous studies of concentrated suspensions using shear thinning liquid phases. The results presented in this report show that by using paste formulations that lie within said range of solids loadings, improved plug flow can be achieved in the die land without a significant change in the die land extrusion pressure at a given flow rate. This offers an advantage in the co-extrusion of ceramic products such as micro-tubular solid oxide fuel cells, as a uniform velocity profile results in improved control of the laminate structure.

Core–shell microgels as model colloids for rheological studies

We review recent work done on the rheology of thermosensitive suspensions. These systems consist of aqueous suspensions of core–shell particles having a solid polystyrene core and a shell of thermosensitive crosslinked poly(N-isopropylacrylamide) (PNIPA). In cold water the thermosensitive PNIPA-network is swollen leading to a high effective volume fraction of the particles in suspension. Approaching the volume transition at 32 °C the network shrinks by expelling water. Hence, the effective volume fraction can be adjusted by the temperature. We demonstrate that these suspensions are a well-characterized model system for the study of the flow behavior of concentrated suspensions. In particular, experimental work done on this system can be compared to the predictions of the mode-coupling theory (MCT) of the fluid-to-glass transition. Excellent agreement is found demonstrating that MCT captures the essential features of the dynamics of flowing suspensions. In particular, MCT predicts a melting of the glass by shear which is fully corroborated by the experimental data.

Processing of mullite–zirconia grains for slip cast ceramics

Dense composites were prepared by slip casting and sintering at 1600 °C from a commercially available mullite–zirconia powder which was finely ground in an attrition mill. Submicron alumina and a mixture of alumina and zircon were used as sintering aids. The effects of amount of dispersant and pH on colloid stability of diluted suspensions were determined to obtain the optimum conditions for suspension stabilization. The influence of powder composition and solid content on flow behavior of concentrated suspensions and on density of green cast were analyzed. Dried compacts achieved a relative density nearly to 60% theoretical density (TD) for fused zirconia mullite powder with and without 10 wt.% of sintering aids. Density, microstructure and crystalline phase content of sintered compacts such as were determined and analyzed in relation to the previous processing.

Squeeze flow of concentrated suspensions of spheres in Newtonian and shear-thinning fluids

The squeeze flow behavior of concentrated suspensions of spheres in both Newtonian and shear-thinning fluids is investigated experimentally. Analyzing the evolution of the squeeze force as a function of time for different controlled velocities, the suspension is found to present two main flow regimes. In the first regime the force decreases when the velocity decreases, which is expected and corresponds to a power-law fluid flow of the suspension. In the second regime the force increases when the velocity decreases which is an indication that the suspension is undergoing solid-fluid separation. It is found that the transition between the two regimes is controlled by a Peclet number defined as the ratio of the characteristic time of the fluid filtration through the porous media made up by the particles to the characteristic time of the suspension flow. A phase diagram delimiting the flowability domain under squeeze flow conditions for each suspension can be then determined. In the present study the influence of the rheological properties of the continuous phase is particularly investigated.

Industrial application of ultrasound based in-line rheometry: Visualization of steady shear pipe flow of chocolate suspension in pre-crystallization process

In the present work an in-line ultrasonic method for investigation of the rheological flow behavior of concentrated suspensions was created. It is based on a nondestructive rheological measuring technique for pilot plant and industrial scale applications. Elsewhere the author discusses a tremendous need for in-line rheological characterization of highly concentrated suspensions exposed to pressure driven shear flow conditions. Most existing on-line methods are based on destructive macro actuators, which are not suitable for materials with sensitive to applied deformation structure. Since the process of our basic interest influences the structure of suspension it would be difficult to separate the effects of rheometric measurement and weakly pronounced structural changes arising from a fine adjustment of the process parameters. The magnitude of these effects is usually associated with the complex flow dynamics of structured liquids and is sensitive to density or temperature fluctuations around the moving rheometric actuator. Interpretation of the results of such measurements can be hindered by process parameter influences on liquid product structure. Therefore, the author introduces an in-line noninvasive rheometric method, which is implemented in a pre-crystallization process of chocolate suspension. Use of ultrasound velocity profile pressure difference (UVP-PD) technique enabled process monitoring of the chocolate pre-crystallization process. Influence of seeded crystals on Rheology of chocolate suspension was recorded and monitored on line. It was shown that even slight velocity pulsations in chocolate mainstream can strongly influence rheological properties besides influencing flow velocity profiles. Based on calculations of power law fit in raw velocity profiles and calculation of wall shear stress from pressure difference measurement, a viscosity function was calculated and monitored on line. On-line results were found to be in a good agreement with off-line data. The results of the industrial test of the UVP-PD system brought practical knowledge and stipulated further development of a Smart UVP-PD noninventive on-line rheometer.

1409 二円板間の流れにおける濃厚サスペンションの塑性挙動

1409 二円板間の流れにおける濃厚サスペンションの塑性挙動

Effects of slip at the wall on the viscosity of suspensions with a polymeric matrix

The flow behavior of concentrated suspensions consisting of a Newtonian matrix (hydroxyl-terminated-polybutadiene, HTPB) mixed with two different sizes of aluminum powder (30% and above by volume) and two different sizes of glass beads (50% and above by volume) was investigated. The effects of the solid content, particle size, type of solid particle material, and the temperature on the shear stress and viscosity of the composite systems were studied. Slip at the wall was an important factor in suspensions with high filler content, large particle size, and measurements carried out at high temperatures requiring corrections for slip at the wall effects. Suspensions with filler contents of 50% and above by volume showed non-Newtonian behavior, whereas suspensions with smaller contents of filler were Newtonian. In most cases, the non-Newtonian suspensions were pseudoplastic at small shear rates, but dilatant at high shear rates. The dilatancy became evident only upon correction for slip at the wall. © 1999 John Wiley & Sons, Inc. Adv Polym Techn 18: 336–342, 1999

Flow behaviour of concentrated suspensions: predictions and measurements

In many applications it is advantageous to handle or process particles as suspensions in a fluid rather than as dry powder. In such cases one would like to manipulate and characterize their flow behaviour in order to ensure optimal performance. Several difficulties arise in the theoretical or experimental study of the systems under consideration, i.e. for concentrated suspensions. Here, the basic concepts and the major results of suspension rheology are critically reviewed. Both the continuum mechanics and the microstructural approaches are considered. The different parameters that affect the flow behaviour are systematically reviewed. Large particles as well as fine, colloidal particles are discussed. Whenever possible data reduction schemes and scaling laws are indicated, from which possible strategies for predicting flow behaviour are derived. Measurement difficulties are also included, fundamental limitations and requirements are pointed out.

Flow behavior of concentrated suspensions

Concentrated suspensions of noninteracting particles show dilatancy at high shear rates, whereas flocculated suspensions are generally pseudoplastic over a wide range of shear rates. The effects of colloidal attraction between particles on the dilatant flow are examined for aqueous suspensions of zinc sulfide (ZnS). The particle–particle interactions are controlled by surface treatment of particles and addition of surfactant. The steady‐flow properties were measured for suspensions at concentrations from 40% to 55% by volume and the degree of colloidal stability was evaluated by the particle concentration in the sediment. With decreasing particle concentration in the sediment, i.e., with increasing colloidal interactions, the dilatant behavior becomes less obvious. For the appearance of dilatant flow, the flocs must be broken down to primary particles in shear fields. The hydrodynamic forces required to break the flocs were calculated.

Rheological Characterization of Hydraulic Fracturing Slurries

Summary Few studies have dealt with the flow behavior of concentrated suspensions or slurries prepared with non-Newtonian carrier fluids. Therefore, the purpose of this investigation is to present experimental results obtained by pumping various hydraulic fracturing slurries into a fracture model and gathering data on differential pressure vs. flow rate. Several concentrations of hydroxypropyl guar (HPG), a wide range of proppant concentrations, and three test temperatures were studied. The effects of such variables as polymer gelling-agent concentration, proppant concentration, test temperature, and fracture-flow shear rate on the rheological properties of slurries were investigated. The correlations for predicting the relative slurry viscosity for these HPG fluids are presented. Substantial increases in viscosity of fracturing gels were observed, and results are discussed in light of several affecting variables. Results also are compared with those available for Newtonian and non-Newtonian concentrated suspensions. Application of these correlations to estimate the hindered particle-settling velocity in the fracture caused by the presence of surrounding particles also is discussed. The correlations presented can easily be included in any currently available 2D or 3D fracture-design simulators; thus, the information can be applied directly to predict fracture geometry and extension.

Interrelationships of Particle Structure and Flow in Concentrated Suspensions

Numerous commercial products either exist as concentrated suspensions of small particles or involve the processing of concentrated suspensions during some stage of their manufacture. Examples include foods, adhesives and glues, ceramic dispersions, paints, and polymer dispersions such as polyvinyl chloride plastisols. As a result, it is important for engineers to understand the flow behavior of these systems and how the flow behavior affects the way these materials can be processed.For mahy years, progress in understanding the flow behavior of concentrated suspensions was slow compared to progress on dilute systems, partly because of how the study of suspensions evolved. Building on Einstein's classical work for dilute suspensions of rigid spheres, many authors attempted to modify his equations to predict the flow behavior of more concentrated suspensions, but the extension of Einstein's work met with limited success, because nonhydrodynamic interactions cari be just as important as the hydrodynamic interactions considered by Einstein, and multiple particle interactions quickly complicate the problem as the particle concentration increases.

Rheology of Suspensions in Polymeric Liquids

This paper reviews the flow behavior of concentrated suspensions as may be of interest in the processing of composites and similar polymeric materials. Succinct synopses are arranged at the end of each section. In general, the state of the art is a good one: a small number of experiments on a new formulation frequently suffice for a general prediction of flow behavior when solids are suspended in viscous molten polymers, and in many instances, even a priori predictions of the viscosity are possible. Less viscous systems are much more difficult; several remaining problems and research areas are identified.