Submicron diameter polymethylmethacrylate (PMMA) particles suspended in a solvent mixture of tetralin and decalin have been observed to undergo a crystal to liquid phase transformation as a function of particle volume fraction[1]. This thermodynamic phase transformation and other measurements of the crystal structure [2], sedimentation rates [1], and response of particle microstructure to shear flow [3] suggest hard sphere behavior for the interparticle interactions.

The viscoelastic properties of thin films of graft copolymers of poly (hydroxy stearic acid)/poly (methyl methacrylate) adsorbed on mica surfaces have been investigated by oscillating one surface sinusoidally using a piezoelectric crystal and monitoring the response of the other on a piezo bimorph. Two distinct regions were found. At low separations the viscous response was negligible, although equilibrium force / distance profiles clearly showed that fluid could be squeezed from the intersurface gap. This was ascribed to hydrodynamic effects, the time scale of the dynamic experiments being much shorter than the equilibrium experiments. Several ways of investigating these effects further have been proposed. At larger separations the viscosity of the medium achieved its bulk value, as expected.

Poly-N-isopropylacrylamide (poly-NIPAM) is a water soluble nonionic polymer that exhibits a lower consolute solution temperature (LCST) of about 31°C. This paper describes the preparation and characterization of a monodisperse, hydrophilic polymer latex of poly-NIPAM, crosslinked by N,N'-methylenebisacrylamide. The presence of these crosslinking moieties will prevent dissolution of the particles at low temperatures; rather the particles will be swollen with solvent such that a microgel is formed. The swelling and rheological behaviour of the latex has been studied. The results show that at high temperatures the particles are ˜ 30 nm whilst at 25°C, the particles were ˜ 130 nm in diameter. This four fold increase in the particle diameter correspond to a ˜ sixty fold increase in volume. The rheological properties, both continuous and oscillatory shear, of these systems have also been studied and is strongly temperature dependant, reflecting the changes in phase volume of the NIPAM latex.

The forces between mica surfaces bearing adsorbed layers of graft copolymers of either polyethyleneoxide/ polymethylmethacrylate in water or polyhydroxystearic acid / polymethylmethacrylate in decalin have been measured. The forces are repulsive in nature and in both cases commence at a surface separation of ˜ 25 nm. They may be described by scaling theory. The results are compared to the rheology of particles stabilised by the same copolymers, in particular to the elastic modulus. It is found that these two parameters scale in the same way, but quantitative comparison reveals that the modulus predicted from the particle forces is some 1 – 2 orders of magnitude greater than that measured experimentally.

When an electric field is applied to a colloidal suspension of micronsize particles, the particles are attracted to each other in the direction of the external field. They line up to form chains and columns across the gap of the electrodes and therefore drastically change the rheological properties of the suspension. These electrorheological phenomena have been studied extensively by recent computer simulation work [1,2], and it is generally accepted that the interactions between the induced electric dipole moments of the particles are responsible for their alignment in the external fields. To avoid net migrations of particles in electric fields of small inhomogeneity, alternating fields are generally used in experiments.

An overview is given of theories of aggregates under flow. These generally assume some sort of structural breakdown as the shear rate is increased. Models vary with both the rigidity of the bonding and the level of treatment of hydrodynamics. Results are presented for simulations of a Rouse model of non-rigid, (i.e. central force) weakly bonded aggregates. In large scale simulations different structures are observed at low and high shear rates. The change from one structure to another is associated with a change in the rate of shear thinning. The model captures low shear rate features of real systems absent in previous models: this feature is ascribed to agglomerate deformations. Quantitatively, the model is two orders of magnitude out from experiment but some scaling is possible.

Solid state 13C NMR techniques have been used to study the mobility of both toluene and ethanol in slurries containing 70% alumina by weight. A comparison of relative signal intensities from cross polarization experiments, and from single excitation experiments under static conditions (no sample spinning) shows that a portion of the solvent molecules are immobilized with “solid” characteristics, while a majority of the molecules exist in mobile or “liquid” like environments. The immobilized solvent is rigid enough to cross polarize with protons, and hence it is characterized by long correlation times, and by a high viscosity. The remainder of the solvent is too mobile to cross polarize, and hence it is characterized by a lower viscosity. The results suggest that solvent is partitioned with the most viscous components being bound near the particle surfaces. These experiments and calculations will be discussed along with the implied effects of solvent partitioning on bulk rheological behavior.

We report a real-time, two-dimensional light scattering study of the evolution of structure in a concentrated electrorheological fluid during the ‘liquid-solid’ phase transition. We find that after particle chaining along the electric field lines, strong light scattering lobes appear at a finite scattering wavevector q orthogonal to the field lines, and then brighten as they move to q=0. This indicates the existence of an unstable concentration fluctuation that signifies the segregation of chains into columns. In fact, the observed growth kinetics of the characteristic length, as well as the form of the structure factor, are qualitatively similar to two-dimensional spinodal decomposition in a system with a conserved order parameter.

The coarsening observed orthogonal to the field lines in light scattering studies of a quiescent electrorhcological fluid during the “liquid-solid” phase transition can be qualitatively understood using the thermal mechanism of chain interaction proposed by Halsey and Toor. By analyzing fluctuations in the interaction between chains, we predict a power-law increase of the column width with time, with a time constant that decreases as a power of the electric field. Lubrication forces between the particles may control the time scale of these fluctuations. These predictions are in good accord with light scattering measurements.

We present the results of experimental and theoretical study of the orientational ordering of rigid rod like polymers in the presence of shear flow. Shear induced birefringence was measured to determine the dependence of orientational order parameter S=<3cos2θ−l>/2 on the shear rate at different values of the volume fraction. Experimental results are discussed in terms of an approximate solution of the rotational diffusion equation in the presence of shear flow based on the construction of the effective interaction energy.

A structural network model for investigating dense suspension is introduced. The suspension may be considered as particle dispersion in liquid as a continuous phase and it is the combination of the three-dimensional arrangement of the fine particles and the forces acting between them that determines the physico-chemical behavior at all stages of the processing. The model assumes that microstructural features of dense suspension can be represented by simple linear network with a breakdown threshold. The conditions under which such a structural network model is valid for studying flow are discussed. The cement paste, asphalt, sand-water and clay-water system were investigated respectively using this model. An oscillating rheometer and a conventional rotational viscometer were used to carry out this study. The dynamical and viscoelastic properties of dense suspension were observed. All the qualitative features of the network results can be described theoretically by a statistical analysis of this problem. Soil mechanics principles which are focused on critical-state theory were applied to describe the stress-strain behavior of dense suspension.

Shear-induced structural changes were observed in suspensions of binary blends of fluorescent particles of differing size and color. Median particle size ranged from 8.3 to 13.7 microns for the nearly spherical fluorescent particles. Concentrated aqueous suspensions were studied under shear flow in a Couette fixture modified to accommodate a fused silica cup. The color of the fluorescent particle blends was observed through the cup wall using photometric methods. Suspension color near the cup wall depended upon shear rate. A color shift occurred near the shear thinning to shear thickening transition. With increasing shear rate, the color contribution from the larger particles increased as the thickening transition occurred. Draining films of these suspensions on a flat fused silica plate were also studied using photometric readings from both the free surface and through the silica plate. Color measurements indicated highly non-uniform particle size distributions within the draining films, which varied with drain time.

An expression relating dichroism to the non-equilibrium structure was derived through Rayleigh-Gans light scattering theory. Comparison of this expression to a micromechanical expression for the thermodynamic contribution to the total stress demonstrates a stress-optical relationship. Optical dichroism and viscosity measurements on monodisperse, near hard-sphere suspensions as a function of shear rate, volume fraction, and particle size demonstrated the relationship experimentally. This work provides a rheo-optical method for determining the various contributions of the colloidal forces to the overall stress.

The rheological properties and microstructure of concentrated suspensions of silica particles in water have been studied. Two typical pH conditions, the effect of the addition of CaCl2 and different mixing procedures have been investigated. When CaCl2 is added, the suspensions become very thick only if the pH is far from the point of zero charge.

The application of nuclear magnetic resonance flow imaging (NMRI) to the study of Couette and falling-ball viscometry of solid/liquid suspensions is described. The suspension consisted of non-Brownian, monodisperse, neutrally-buoyant spheres of 50 vol%. NMRI data demonstrated flow-induced changes in the particle distribution in the suspension that strongly influence the accuracy in viscosity measurements using these flow geometries. In Couette flow, direct correlation between stress measurements and particle concentrations at various locations in the flow cell as a function of shear strain can be made. Our data directly confitrm the shearinduced particle migration theory proposed by Leighton and Acrivos.[1] In the falling-ball experiments, increased particle concentration at the leading edge and decreased concentration at the trailing edge of the ball was observed when the falling ball is big compared to the cylinder containing the suspension. This change in particle distribution can be directly related to the changes in fall velocity of the ball as a function of position in the cylinder.

Procedures for the manufacture of thin wall ceramic components from particulate suspensions using plastic forming methods which employ extensional flows are described. These include vacuum forming, blow moulding and film blowing. In order to understand how to select materials and to adjust the composition of such suspensions, the factors which control suspension rheology are identified. The measurement of extensional viscosity of ceramic suspensions is reported and compared with shear flow measurements.

The relationships among colloidal stability, suspension rheology, and solids loading are elucidated for zirconia-alumina mixtures containing 20 vol% ZrO2, based on solids. The lower colloidal stability of ZrO2 limits the rheological properties of this system. If the zeta potential of ZrO2 is less than 49 mV, high degrees of pseudoplasticity, high yield stress, high viscosity, and long relaxation times characterize the binary suspensions. These effects occur, despite the fact that ZrO2 is the minor ceramic constituent. If the zeta potential of ZrO2 is maintained above 49 mV, suspensions with high solids loading (55 vol%) can be prepared which behave as Newtonian fluids over the 0-to-200 s−1 shear rate range and as an elastic solid at higher rates. As the solids loading of a highly stable binary suspension is increased, the rheological properties change. They evolve from those of a near Newtonian-like fluid with nearly independent particles (40 vol%) to those of a pseudoplastic fluid with a weakly interacting particle network (50 vol%) to those of an elastic-like solid composed of crowded, strongly repulsive particles (55 vol%). Low-shear conditions for suspension-transport and mold-filling operations that ensure a homogeneous arrangement of ZrO2 and A12O3 particles in a gelcast ceramic seem promising.

Ceramic suspensions were formulated based on an alumina/plasticized-polymer system. The total polymer volume in suspension was held constant, while the relative amount of high-to-low molecular weight polyvinyl butyral (PVB) in suspension was varied. Experiments were performed to elucidate the effects of polymer molecular weight and distribution on the rheological properties of these casting suspensions as well as on the green microstructure of tape-cast components. The polymer properties affected not only the suspension viscosity at a given shear rate as expected, but also the shear thinning behavior of each suspension. Tapes (thickness ≈ 250 μm) were cast from these suspensions and their properties were evaluated. Pore volume, a measure of the packing efficiency, was found to depend strongly on the polymer molecular weight and distribution. In addition, preliminary lamination studies revealed that dimensional stability and anisotropy were also affected by the relative amount of high-to-low molecular weight PVB. A direct correlation was shown to exist between the shear thinning behavior of these suspensions and the resulting dimensional anisotropy exhibited by the tapes cast from each of them. These results demonstrate that polymeric aids influence not only the suspension rheology, but the green component microstructure as well, and, hence, are an integral aspect of ceramic processing.

Rheological testing has been developed to predict the extrudability of ceramic pastes. Inorganic batches containing cellulosic binders were mixed in a twin screw extruder prior to characterization in a capillary rheometer. Various die lengths were used to separate viscosity from wall slip parameters. A power law model was fit to the paste viscosity. The wall shear stress, assuming 100% slip, was a linear function of the wall velocity. The resulting rheological models were used in fluid flow finite element analysis for determination of extrusion pressure, and the evaluation of binders.

We have previously shown that suspension dry spinning of green ceramic fibers can produce continuous fibers of alumina in an ethyl methacrylate binder. The spinning behavior of the dope and the particle packing in the dried fiber are highly dependent on the interparticle interactions in the dope prior to spinning. In particular, the interactions between the ceramic powder and both the dispersant and the binder are critical in determining the processability and spinnability of the dope. We have used rheological techniques to probe these interactions in alumina dispersions in methyl ethyl ketone employing ethyl methacrylate as the binder and a phosphate ester as the dispersant. Measurements of the viscosity of the dispersion as a function of dispersant concentration demonstrate that there is an optimum dispersant concentration to achieve a minimum viscosity at a given binder concentration. In addition, the relative viscosity (viscosity of the dispersion relative to that of the dispersing medium) is found to decrease significantly with increasing binder concentration. These results highlight the importance of characterizing the power-binder-dispersant interactions to fully understand the stability and ultimate spinnability of a ceramic dope. In addition, this work illustrates the general applicability of rheological characterization in ceramic fiber spinning.