Thixotropic Suspensions Research Articles

Some applications of the AMPC model of the shear yield stress of particulate fluids

The AMPC structural model of shear yield stress of single component suspensions is extended to three important rheological phenomena encountered in particulate fluids. These are shear yield stress of multi-component systems, time dependent shear yield stress of thixotropic suspensions, and compressive yield stress of particulate fluids. The shear yield stress of multi-component suspensions is modeled by summing the interparticle interactions due to all possible combinations of particle sizes and solid constituents. The time dependent shear yield stress of thixotropic muds is simulated by invoking the proportionality between yield stress and interparticle bond density across the shear plane. The compressive yield stress of particulate fluids is calculated by assuming that consolidation occurs by shear but at a narrower gap between the particles. The ability of the model to describe these diverse phenomena reasonably well seemingly validates its basic premise, namely, a 3-dimensional space-filling network of particles whose mechanical properties can be estimated from the theory of strength of particulate assemblage.

Thixotropic casting of ceramic-metal functionally gradient materials

Functionally gradient materials (FGM), a recent development in composite materials, consist of a continuously graded interface between two component phases. Previously reported FGM fabrication methods are reviewed, including vapour deposition, plasma spraying, electrophoretic deposition, controlled powder mixing, slipcasting, sedimentation forming, centrifugal forming, laser cladding, metal infiltration, controlled volatilization, and self propagating high-temperature synthesis. A novel approach to FGM fabrication is presented involving thixotropic casting — vibratory casting of highly concentrated highly thixotropic suspensions castable only under vibration. The hydroxyapatite/316L stainless steel system (a biocompatible ceramic/metal system) was used due to appropriate matches in true densities, thermal expansion, and sintering temperatures. Solids loadings from 76.08–82.67 wt% were trialled. Solids loading was found to be critical. At 78.17 wt%, an ideal continuous FGM formed, showing a gradual transition from pure ceramic to pure metal across 60 mm. At the nearest increments trialled (±0.3 wt%) gradation was far from optimal: 77.87 wt% gave a sharp ceramic/metal interface with negligible grading; 78.47 wt% gave a relatively homogeneous sample with only a small degree of grading, from a slightly metal-rich end to a slightly metal-poor end.

Dielectric behaviour of flowing thixotropic suspensions

The dielectric properties of sheared suspensions of carbon black in mineral oil are analyzed. These systems are known to be thixotropic, i.e. their vis

The torsion behaviour of stir-cast Cu-11wt.%Sn alloy at room temperature

It is well known that, in the traditional casting technique, metal exhibits a shrinkage, leading to porosity, hot tearing and cracks when the liquid transforms into a solid. One of the recent techniques which has been developed to avoid these defects consists of obtaining by a stir-casting process (rheocasting) a thixotropic suspension of degenerate spheroidal dendrites. Torsion test bars obtained by a new stir-casting process (registered trademark, GIRCAST) illustrate the improvement in mechanical properties at room temperature.

The pharmacokinetics of subcutaneous bolus cytosine arabinoside in an arachis oil plus aluminium distearate suspension.

An attempt was made to create a delayed release preparation of cytosine arabinoside (araC) which could be administered subcutaneously, and would produce plasma levels similar to steady state infusion concentrations. A thixotropic suspension of araC in arachis oil and aluminium distearate was formulated. This preparation was similar to that previously used with bleomycin oil suspension and procaine penicillin. Two hundred mg/ml of araC in arachis oil containing varying amounts of aluminium distearate were administered firstly to New Zealand White rabbits and then to patients with acute myelogenous leukaemia. This preparation was well tolerated by both rabbits and patients but did not delay the release of araC from the subcutaneous tissues.

Mixing of Thixotropic Fluids

Rheometric data of a thixotropic bentonite suspension were used for the identification of selected rheological models of thixotropic flow behavior. Rheometric data, obtained for different deformation histories, were compared with theoretical predictions derived from the rheological model for the same deformation histories. At low shear rates, superanomalous behavior of the bentonite suspension has been observed. The capability of the rheological models to reproduce this unusual phenomenon is discussed. Torques necessary to agitate a thixotropic suspension by means of a small‐scale anchor agitator were calculated on the basis of the rheological equation and compared with torques obtained, for the same deformation histories, experimentally on the same agitator. Good agreement between theory and experiment has been found.

Partial melting of feldspathic quartzite around late Caledonian minor intrusions in Appin, Scotland

SummaryGranophyre patches on the sites of former sedimentary feldspars in the feldspathic Appin Quartzite show that partial melting took place in the inner parts of the aureoles of late Caledonian minor intrusions. Mobilization of the melt and refractory quartz residue as a thixotropic suspension is shown to result in the formation of veins and breccias. Textures are related to the wetting of quartz/quartz grain bounderies by the eutectic melt and the later growth of crystals during cooling. From the presence of high (β) quartz coexisting with melt, minimum contact temperatures are estimated as 780°C, showing that some convective circulation of magma took place.

Reactions and Equi ibria in Magnesium Oxych oride Cements

Phase equi ibria in the system MgO‐MgC 2‐H2O at 2°±3°C were determined and the reactions by which the equi ibrium phases, 5Mg(OH)2 MgC 2 8H2O and 3Mg(OH)2 MgC 28H2O, deve op were studied by X‐ray diffractometry. As reactive MgO is disso ved by magnesium ch oride so utions, a thixotropic suspension is converted to a ge, which then crysta izes to form the ternary oxych oride phases. Insufficient y active MgO disso ves more s ow y so that, in an open system, bu k composition can shift by evaporation of water, resu ting in crysta ization of a nonequi ibrium phase assemb age, with residua magnesium ch oride so ution and unreacted MgO. Imp ications of these nonequi ibrium reactions for the performance of magnesium oxych oride cements are discussed.

Structure of Thixotropic Suspensions in Shear Flow. III. Time-Dependent Behavior

The time-dependent viscosity of bentonite-water suspensions was studied experimentally in Couette flow for various transitions between constant shear rates. It was found that after a sudden change in the shear rate the time dependence of the viscosity can be described by η(t)=A exp[−(t/τ1)4.0]+B exp(−t/τ2)+C, with τ2>τ1. In general, the characteristic times: 1) become longer as the differences in shear rates become smaller; 2) decrease with increasing concentration; 3) show little temperature effect for a 20°C temperature change. A theory for the time-dependent behavior is given which can reproduce the essential features of the experimental results.

Time-dependent behavior of thixotropic suspensions

The time-dependent behavior of the viscosity of bentonite-water suspensions was studied experimentally inCouette flow for various transitions between constant shear rates. It was found that, in general, the characteristic times become longer as the final shear rate becomes smaller and, for the same final shear rate, as the differences in shear rates become smaller. The characteristic time increases with increasing temperature and decreasing concentration. A discussion of the kinetics of the time dependence is given.

Structure of thixotropic suspensions in shear flow: II. Optical properties

A theory of the opacity of bentonite-water suspensions as a function of the shear rate is given. The experimental opacity behavior confirms previous theoretical predictions of the aggregate size distribution. The extinction coefficient of bentonite was determined at two wavelengths.

Structure of thixotropic suspensions in shear flow: I. Mechanical properties

A theory for the steady state viscosity η of thixotropic clay-water suspensions is given. The numerical results of this analysis can be approximated by u + u2/10 = K/γ̇ where u = (ηr − 1)/C, γ̇ is the shear rate, η∞ is the viscosity for γ̇ → ∞, ηr =η/η∞, and K and C are parameters depending on concentration and temperature. For large shear rates this expression reduces to the Bingham behavior ηr = 1 + τ0/η∞γ̇, where τ0 is the Bingham yield stress. Experiments on a Wyoming bentonite at various concentrations and temperatures give good agreement with the theoretically predicted behavior. The comparison of experimental and analytical results yields the cohesive force per particle and the ratio of the major and minor dimensions of the bentonite particles.

A rotational double coaxial cylinder viscometer

Design principles for a rotational coaxial cylinder viscometer which directly measures viscosity and yield stress without need for any corrections have been developed. A viscometer with a rotating external cylinder is combined with one with a rotating inner cylinder. The torque is measured on a common tube which makes the inner cylindrical surface in the first viscometer and the outer one in the second one. Two rotational double cylinder viscometers are described in detail. The determinations are illustrated by measurements onNewtonian fluids, time-dependent shear thinning fluids, and a thixotropic suspension and by determination of zero shear viscosity of polymer solutions.

Examples of construction of infiltration curtains in the Polish people's republic using thixotropic suspensions

Examples of construction of infiltration curtains in the Polish people's republic using thixotropic suspensions

Rheology of Thixotropic Suspensions of Magnesium Montmorillonite

Rheology of Thixotropic Suspensions of Magnesium Montmorillonite

The Rheological Properties of Concentrated Suspensions. IV. The Viscoelastic Properties of Concentrated Dispersions in Molten Polymers

Abstract The dynamic viscoelastic properties of molten polyethylene with various fillers have been measured in nitrogen gas at frequencies from 0.01 to 0.75 c. p. s. The dynamic viscosity and dynamic modulus have been measured at the temperatures of 150∼230 °C. Glass spheres, calcium carbonate powder and barium sulfate powder have been used as fillers. The average diameter of the particles covers the range of 0.04∼91 μ. The polyethylene used was one of a low density, “ Sumikathene F-70-6,” whose number average molecular weight is 28000. The dynamic viscosity and modulus of molten polyethylene are increased by the addition of fillers. The viscoelastic properties at low frequencies depend differently on the concentration in each suspension. In suspensions of glass spheres of the larger particle sizes (particle diameter : 24.0, 32.3, 49.5 and 91.1 μ), the concentration dependence of the dynamic viscosity, η′, follows a generalized Robinson’s formula : ln(η′⁄η′0)=Kφ⁄(1−S′φ) where η′0 is the dynamic viscosity of polyethylene ; φ, the volume fraction of the suspended particles, and K and S′, 2.5 and 1/0.74 respectively, regardless of the particle size. In suspensions of calcium carbonate of the intermediate particle size (particle diameter : 2.4 μ), the dependence of the dynamic viscosity on the concentration and on the amplitude of oscillation is observed more remarkably than that in suspensions of glass spheres. In suspensions of calcium carbonate and barium sulfate of the smaller particle sizes (particle diameter: 0.04, 0.08 and 0.27 μ), the dynamic viscosity and modulus increase appreciably above a critical concentration of the filler, a point which depends on the particle size. Beyond this critical concentration, the particles seemed to be linked with each other by single polymer molecules. The dynamic viscosity and modulus at lower frequencies increase remarkably as the mean distance between the surfaces of particles becomes less than the molecular dimension of the polymer. In suspensions of the larger particles of glass spheres, this critical concentration can not be observed up to the highest concentration. The temperature dependence of the dynamic viscosity at lower frequencies approximately agrees with that of unfilled polymers. The dependence of dynamic properties on the time and on the amplitude of oscillations is observed in suspensions of calcium carbonate and in those of barium sulfate of comparatively small particles. In the barium sulfate suspensions of the finest particles, the dynamic viscosity and modulus increase with time under periodic strains and decrease with the increase in the amplitude of oscillations. The amplitude dependence of the dynamic modulus is more remarkable than that of the dynamic viscosity generally. In such thixotropic suspensions, the dynamic properties were measured with as small a strain as possible.

A study in thixotropy of concentrated pigment suspensions

The thixotropic recovery times of concentrated pigment suspension inNewtonian liquids, such as toluene, paraffin oil, glycerol, etc., were studied between various “levels” of rates of shear. The thixotropic recovery time was related to the pigment concentration, pigment's degree of flocculation, sedimentation volume, viscosity of the liquid phase, and temperature. Theminima of the investigated functions corresponded to the sedimentation volumes equal unity; a transition from theexpanded to thecompressed type of suspension caused a profound change in the above relationships. It was found that the thixotropic recovery time decreases with increasing pigment concentration and increasing pigment's degree of flocculation as long as the suspension remains in the region ofexpanded systems. The recovery time is always proportional to the viscosity of the liquid phase. The temperature effect is two-fold: the thixotropic recovery time is decreased by a decrease in the viscosity of the liquid phase and by an increase in the degree of flocculation of pigment particles (the latter being subject to the restrictions of expanded systems). The observed minima of the recovery time—pigment concentration and recovery time—degree of flocculation functions may explain the existing confusion of thoughts and may illustrate complexity of the subject. The concept of equivalent states was introduced in order to apply the formerly discovered relationships of equilibria states to the non-equilibria states. Some problems of elasticity and rigidity of thixotropic suspensions are reviewed. The general equation of thixotropic recovery was proposed. $$t_{D'}^{D''} = k \cdot \eta _v \cdot \frac{1}{{F_\infty }} \cdot \frac{1}{{\beta ^3 (\beta - 1)}}\left[ {\left( {\log \frac{{D_c }}{{D'}}} \right)^3 - \left( {\log \frac{{D_c }}{{D''}}} \right)^3 } \right]$$ In this equation the coefficientk remains still a function of the particles size and shape and density, while the termsη v, F∞, β, Dc, D′ andD″, correspond respectively to the viscosity of the liquid phase, hydrodynamic volume of the solid phase, degree of flocculation of pigment particles, the critical rate of shear of the rested system, and the two levels of rates of shear employed during experiment; the recovery time,t D′ D″ , is the time required for thixotropic build-up between these two levels of rates of shear,D′ andD″.

Effects of Processing on the Rheology of Thixotropic Suspensions

A study was undertaken to evaluate the effects of processing by various types of commercially available dispersion equipment under different conditions of operation on certain rheological parameters of thixotropic suspensions. Two suspending agents, bentonite U. S. P. and Veegum HV were used in the evaluation procedure and four types of dispersion equipment were used, a marine-type impeller mixer, a homogenizer, a colloid mill, and a high shear mixer. The rheological parameters were obtained from flow curves that were automatically plotted by means of a completely instrumented Ferranti-Shirley cone-plate viscometer. The type of dispersion equipment employed and the operating conditions used were related to the rheological parameters of the suspension systems. The rheological parameters evaluated were thixotropic index, yield value, spur value, and plastic viscosity.

Thermodynamic Properties of Water in Suspensions of Montmorillonite

AbstractThe partial molar free energies, entropies and heat contents of the water in suspensions of Li-, Na-, and K-clay were measured relative to pure water by means of a tensiometer. This method of obtaining these thermodynamic properties is far more sensitive than the more common method utilizing adsorption isotherms and a form of the Clausius-Clapeyron equation. The partial molar free energy of the water decreased with time after stirring and increased rapidly upon stirring. This was taken as evidence that a gradual increase in the degree of orientation of water molecules associated with the clay particles occurred as the particles fell into the more random edge-to-surface thixotropic structure. The partial molar entropies and heat contents of the water in the suspensions were found to be less than those for pure water. Hence, this evidence also supported the postulate that an ordered water structure exists in the thixotropic suspensions.

Yield Value of Bentonite Suspensions

I HAVE recently been measuring the yield values of thixotropic aqueous suspensions of bentonite by determining the force required to move a thin flat plate in its own plane, through the suspension1–3. The results indicate that the yield value, as measured, cannot be regarded, as an intrinsic property of the suspension but depends to a very marked extent on the material of the plate. The plate (3 cm. × 2·5 cm. × 0·008 cm.), immersed in the suspension, is suspended from a horizontal arm attached rigidly to a torsion wire. The vessel containing the suspension is slowly lowered (0·18 cm./min.) and the stress on the plate measured by the deflexion, recorded photographically as a function of time, of the horizontal arm. The suspensions (pH c. 8·5) are aged for at least a month after mixing and are thoroughly dispersed by a high-speed stirrer immediately before each experiment. The time of repose, until the yield point is reached, is approximately constant (c. 4 min.).