David V. Boger

Yield Stress Measurement for Concentrated Suspensions

The measurement and use of the flow properties of highly concentrated solid‐liquid suspensions is a topic of considerable practical interest in a broad spectrum of industries. The yield stress is a rheological property that all highly concentrated suspensions may have in common. In this work four established methods for determining the yield stress are compared with a fifth and new method based on a vane test developed in soil mechanics. It is clearly shown that a single‐point measurement with the vane device is sufficient to determine accurately the yield stress in a region of high concentration where the four conventional methods are extremely tedious or not applicable. Furthermore, the vane method does not rely on any previous shearing of a suspension and hence is applicable for study of the kinetics of thixotropic systems. The work has been motivated by the need to know the yield stress of highly concentrated bauxite residue suspensions in order to establish a waste disposal strategy for the residue w...

Direct Yield Stress Measurement with the Vane Method

In the vane method for measuring the yield stress, the conventional analysis assumes that the stress is uniformly distributed on a cylindrical sheared surface to calculate the yield stress from the maximum torque and vane dimensions. By using two simple procedures, the present work shows that this assumption is justified at the moment of yielding. The yield stress calculated using the proposed methods compares favorably with that obtained with the conventional procedure. A comparison with the yield stress independently determined by other methods again confirms the usefulness of the vane technique as a simple but accurate method for direct yield stress measurement.

The flow of fiber suspensions in complex geometries

Abstract A continuum theory for dilute suspensions of large-aspect-ratio particles is applied to the flow of fiber suspensions through contractions. The theory, which incorporates the statistical orientation distribution function into the stress equation, predicts that the flow of dilute suspensions will differ qualitatively from the flow of the suspending fluid. The theory is in excellent agreement with experiments on the flow of suspensions of chopped-glass fibers through axisymmetric contractions, where substantial enlargement of the recirculating corner vortex is observed at volume fractions of 0.1% and less.

Surface chemistry-rheology relationships in concentrated mineral suspensions

The rheological behaviour of concentrated mineral suspensions can be controlled by understanding and regulating the net particle-particle interaction force. This paper summarizes the experimental results from studies showing how the yield stress of concentrated aqueous suspensions of alumina, zirconia and kaolin can be controlled through regulation of pH and the addition of inorganic electrolytes, ionic surfactants and polyelectrolytes (polymers). The various kinds of inter-particle forces acting in aqueous suspensions of fine particles are reviewed and used to interpret the experimental results.

Chemical and physical control of the rheology of concentrated metal oxide suspensions

Abstract Experimental and theoretical work defining the effect of the surface chemistry and particle physics on the rheology of metal oxide suspensions is reviewed. The influence of variables, including solids concentration, particle size and size distributions are examined at various inter-particle interaction conditions controlled by pH, electrolyte concentration, and/or addition of various additives for both flocculated and well-dispersed suspensions. The rheological properties examined include shear viscosity, shear and compressive yield stress, shear thinning and shear thickening. Data from various studies on metal oxide suspension systems at well-defined surface chemistry conditions are interpreted through analyses, primarily of pair interactions and suspension microstructure. Better insight has been gained concerning the governing rules of the surface chemistry and various physical factors on the rheological properties of metal oxide suspensions.

Yield stress measurements with the vane

Abstract Yield stress measurements were performed on a TiO 2 pigment suspension with the vane in both a rate controlled and a stress controlled mode. In the rate controlled mode, a constant rotational speed is applied to the vane immersed in the suspension, and the resulting stress is measured as a function of time. In the stress controlled mode, a constant stress or a constant stress-rate is applied to the vane immersed in the suspension, and the resulting creep angle is measured as a function of time. In both modes the yield stress is determined as the minimum stress required for continuous rotation of the vane. A Haake Rheometer and a modified Weissenberg Rheogoniometer facilitated the controlled rate measurements. The significant difference between these instruments is that the Weissenberg torsion bar is 550 times stiffer than the Haake spring. To ensure a comparable time frame of measurement with the Haake, it was necessary to use lower rotational speeds in the operation of the Weissenberg. Although agreement in the measured yield stress within 5% was established between the Haake at a rotational speed of 0.021 rad s −1 and the Weissenberg at a rotational speed of 0.0063 rad s −1 , the shapes of the respective stress-time profiles were strikingly different. The Haake exhibited a largely elastic response, whereas the Weissenberg exhibited a largely viscoelastic response. The yield stress measured by both the Haake and the Weissenberg corresponds to the transition stress between viscoelastic and fully viscous flow. A Bohlin Rheometer facilitated the stress controlled measurements. The yield stress measured by the Bohlin was up to 13% lower than that measured by the Haake and the Weissenberg, and seemed to correspond more closely to the transition stress between fully elastic and viscoelastic flow.

A fifty cent rheometer for yield stress measurement

The slump test, initially developed to determine the flow properties of fresh concrete, has been adopted as a means of accurately measuring the yield stress of strongly flocculated suspensions. The slump test offers a quick and easy way of measuring yield stress without the need for sophisticated electronic equipment, thereby giving plant operators an effective tool for determining yield stress. The model used to predict the yield stress from the conical slump test was devised by Murata (1984) and corrected by Christensen (1991). In the present case the theory has been adapted for a cylindrical geometry. Yield stress measurements obtained with the vane for numerous mineral suspensions under known surface chemistry conditions are compared to the slump measurements and theoretical prediction. Good agreement is obtained.

Rheological evidence of adsorbate-mediated short-range steric forces in concentrated dispersions

In order to examine quantitatively the effect of short-range forces on the state of dispersion in dense suspensions, the effect of a series of anionic adsorbates on the rheology of a model zirconia concentrated suspension has been studied in detail. The anionic additives include sulfate, phosphate, pyrophosphate and polyphosphates as well as simple organic acid anions such as lactate, malate and citrate. The adsorbates shifted the pH of maximum static yield stress and, equivalently, the pH of zero zeta potential (ζ), to lower pH values. The additives also lowered the magnitude of the maximum yield stress at ζ= 0. The results are all consistent with the adsorbates producing a steric barrier equivalent to the size of the adsorbed molecule along the attractive van der Waals interaction. Separately the polyphosphate adsorbates are shown to assume a flat orientation and do not appear to produce a thick electro-steric barrier to coagulation.

The rheology of a concentrated colloidal suspension of hard spheres

Abstract The rheology of a model hard-sphere suspension has been studied at high volume fraction. Particular emphasis was placed on observing the transition between liquid-like and solid-like behavior at the maximum packing volume fraction. Capillary viscometry has shown that the suspension viscosity at low concentration agrees well with theory and other experimental work on hard-sphere systems. At higher concentrations the rheological properties, measured using steady shear, oscillatory shear, and creep techniques, change rapidly from viscous Newtonian to shear-thinning viscoelastic. When the volume fraction is greater than the maximum packing volume fraction the behavior is like that of an elastic solid, and a yield stress can be measured using cone and plate instruments and the vane method. At high volume fractions the product of a characteristic shear rate (or Peclet number) and the low shear limiting viscosity is found to be almost independent of concentration. It is possible to superimpose all the steady shear data using a scaling based on the Cross equation.

The yield stress of concentrated flocculated suspensions of size distributed particles

An investigation of shear yield stress is made on well-characterized alumina suspensions of different distributed particle sizes at the vicinity of the particle isoelectric point (IEP) across a wide range of volume fractions. Experimental results are compared with recently developed models [; ] and structural effects on the yield stress are examined. The models predict the magnitude order of the yield stress below a volume fraction of approximately 0.42, suggesting that interparticle forces play a dominant role in determining the network strength in this concentration region. Deviations between experimental results and theoretical predictions are explained in terms of structural effects being controlled by a competition between weak particle–particle linkages and geometric resistance on the network strength. At higher volume fraction, the effect of geometric resistance on the deformation of suspensions becomes more pronounced. A number of models for the yield stress of size distributed suspensions are then proposed. Results suggest that the effect of polydispersity of particles on the yield stress of suspensions can be well characterized by a surface area average diameter and the broad size distributed suspension exhibits a higher yield stress than the narrow size distributed suspension of the same volume average diameter.