Showing papers on "Rheology published in 1996"

Effect of droplet size on the rheology of emulsions

Abstract: The effect of droplet size on the rheological behavior of water-in-oil and oil-in-water emulsions was investigated using a controlled-stress rheometer. Results indicate that the droplet size has a dramatic influence on emulsion rheology. Fine emulsions (water-in-oil or oil-in-water) have much higher viscosities and storage moduli than the corresponding coarse emulsions. The shear-thinning effect is much stronger in the case of fine emulsions. When coarse droplets are replaced by fine droplets (keeping total volume fraction of the dispersed phase constant), the resulting emulsion exhibits a minimum in rheological properties (viscosity, storage and loss moduli, time constant) at a certain proportion of fine droplets. However, the minimum in viscosity occurs only at low shear stresses. At high stresses, the viscosity of the mixed emulsion increases as the proportion of fine droplets increases. The study of the aging effect on the rheological behavior shows that water-in-oil emulsions age much more rapidly than the oil-in-water emulsions.

Numerical simulation of a concentrated emulsion in shear flow

Abstract: A three-dimensional computer simulation of a concentrated emulsion in shear flow has been developed for low-Reynolds-number finite-capillary-number conditions. Numerical results have been obtained using an efficient boundary integral formulation with periodic boundary conditions and up to twelve drops in each periodically replicated unit cell. Calculations have been performed over a range of capillary numbers where drop deformation is significant up to the value where drop breakup is imminent. Results have been obtained for dispersed-phase volume fractions up to 30% and dispersed- to continuous-phase viscosity ratios in the range of 0 to 5. The results reveal a complex rheology with pronounced shear thinning and large normal stresses that is associated with an anisotropic microstructure that results from the alignment of deformed drops in the flow direction. The viscosity of an emulsion is only a moderately increasing function of the dispersed-phase volume fraction, in contrast to suspensions of rigid particles or undeformed drops. Unlike rigid particles, deformable drops do not form large clusters.

Stokesian Dynamics simulation of Brownian suspensions

Abstract: The non-equilibrium behaviour of concentrated colloidal dispersions is studied by Stokesian Dynamics, a general molecular-dynamics-like technique for simulating particles suspended in a viscous fluid. The simulations are of a suspension of monodisperse Brownian hard spheres in simple shear flow as a function of the Peclet number, Pe, which measures the relative importance of shear and Brownian forces. Three clearly defined regions of behaviour are revealed. There is first a Brownian-motion-dominated regime (Pe ≤ 1) where departures from equilibrium in structure and diffusion are small, but the suspension viscosity shear thins dramatically. When the Brownian and hydrodynamic forces balance (Pe ≈ 10), the dispersion forms a new ‘phase’ with the particles aligned in ‘strings’ along the flow direction and the strings are arranged hexagonally. This flow-induced ordering persists over a range of Pe and, while the structure and diffusivity now vary considerably, the rheology remains unchanged. Finally, there is a hydrodynamically dominated regime (Pe > 200) with a dramatic change in the long-time self-diffusivity and the rheology. Here, as the Peclet number increases the suspension shear thickens owing to the formation of large clusters. The simulation results are shown to agree well with experiment.

Relationship between rheology and morphology of model blends in steady shear flow

Abstract: Immiscible polymer blends display a complex flow behavior caused by the coupling between morphology and rheology. The flow induced microstructure has been studied on model systems of nearly inelastic polymers. For these systems, the elastic properties of the blend are mainly governed by the interface. Measurements of the storage modulus and of the first normal stress difference, both reflecting this enhanced elasticity, have been used to probe the blend morphology. From oscillatory measurements after cessation of flow the mean diameter of the disperse phase, as generated by the previous flow, has been calculated using the model of Palierne [Rheol. Acta 29, 204 (1990)]. A procedure based on a direct fitting of the dynamic moduli with the model is compared with one that uses a weighted relaxation spectrum. The steady state normal stress data, on the other hand, have been related to the morphology of the blend by means of the model of Doi and Ohta [J. Chem. Phys. 95, 1242 (1991)]. Since this model predicts a direct proportionality between the contribution of the interface to the normal stress and the specific interfacial area, the size of the droplets can be calculated once the proportionality constant is known. The conditions for which the model is valid have been determined and the required constant is obtained by comparing the results with those from the dynamic moduli. The resulting droplet sizes have been used to develop a data reduction scheme: The specific interfacial area is found to be inversely proportional to the ratio of interfacial tension over shear stress for several blends with various concentrations and viscosity ratios.

Rheology of a Lower Critical Solution Temperature Binary Polymer Blend in the Homogeneous, Phase-Separated, and Transitional Regimes

Abstract: Small amplitude oscillatory shear rheology is employed in order to investigate the linear viscoelastic behavior of the lower critical solution temperature blend polystyrene/poly(vinyl methyl ether), PS/PVME, as a function of temperature and composition. At low temperatures, where the mixture is homogeneous, the dependence of the zero shear viscosity (η0) on concentration is measured and is well-described by means of a new mixing rule, based on surface fractions instead of volume fractions. Shift factors from time-temperature superposition (TTS) exhibit a Williams−Landel−Ferry (WLF) behavior. As the macrophase separation temperature is approached (the phase diagram being established by turbidity measurements), the blend exhibits a thermorheologically complex behavior. A failure of TTS is observed at low frequencies, both in the homogeneous pretransitional and in the two-phase regimes. Its origin is attributed to the enhanced concentration fluctuations, which exhibit a critical slowing down near the phase b...

Shear thickening in model suspensions of sterically stabilized particles

Abstract: The onset of shear thickening (dilatancy) has been studied in submicron model suspensions of sterically stabilized spherical particles. Stress controlled rheometers have been used for this purpose, so that measurements can be performed beyond the onset of sudden shear thickening. Systematic data are presented for the effect of particle size, particle concentration, and the nature of the suspending medium on the onset of shear thickening. As a first approximation, the critical shear rate changes inversely proportional with the medium viscosity. The changing solvency of the medium for the stabilizer polymer introduces additional changes through effects on the thickness and stiffness of the steric barrier. Thinner, stiffer barriers cause lower critical shear rates. In the softer systems the critical shear stress becomes independent of particle concentration in dense suspensions. This does not seem to be the case for the harder systems. The effect of particle radius a could shed some light on the underlying m...

Influence of amylose content on the viscous behavior of low hydrated molten starches

Abstract: Starches with various amylose contents (0%–70%) were processed on a twin‐screw extruder equipped with a special slit die rheometer. Relationships between thermomechanical treatment and starch macromolecular degradation are defined, and flow curves are discussed in order to determine the role of moisture content, product temperature, and mechanical energy on melt viscosity. The viscous behavior is described by a power‐law expression. Viscosity is more sensitive to moisture content and macromolecular degradation at lower amylose contents. Using multiple regression analysis, expressions for the different starches are proposed to describe the influence on the viscosity of amylopectin, which is the macromolecular component with short chain branching. The main differences observed when decreasing the amylose content are a lower viscosity and less pronounced shear thinning. These effects are interpreted in terms of entanglements.

Molecular dynamics simulations of the rheology of normal decane, hexadecane, and tetracosane

Abstract: Extensive nonequilibrium molecular dynamics simulations have been carried out for liquid decane, hexadecane, and tetracosane at densities corresponding to atmospheric pressure and near ambient temperatures. The strain‐rate‐dependent viscosity has been obtained for strain rates ranging over several orders of magnitude. At high strain rate, the viscosities for all alkanes studied here have similar values and exhibit similar power‐law shear‐thinning behavior with a slope between about −0.40 and −0.33. Accompanying this shear thinning is the onset of orientational order and the alignment of the alkane molecules with the flow direction. The alignment angle tends to 45° at very low strain rate and is significantly smaller at high strain rate. This suggests that the chains substantially align in the flow direction and that the dominant motion at high strain rate is the sliding of the chains parallel to the flow. At low strain rate, the shear viscosity shows a transition to Newtonian behavior. The Newtonian viscosity can be obtained from the plateau value of the shear viscosity at the lowest strain rates calculated from the nonequilibrium molecular dynamics simulation (NEMD). This is demonstrated by comparing the viscosity of decane obtained by extrapolating the NEMD simulation with an independent calculation using the standard Green–Kubo method. The transition from the non‐Newtonian regime to the Newtonian regime is also correlated with the disappearance of orientational order and with the longest relaxation time of the liquid alkanes simulated.

Interfacial Rheology and the Dynamic Properties of Adsorbed Films of Food Proteins and Surfactants

Abstract: Factors affecting the interfacial rheology of films of food proteins and surfactants at air-water and oil-water interfaces are reviewed. A distinction is made between interfacial shear rheology and interfacial dilatational rheology, and between soluble and insoluble films. A comparison is made between properties of adsorbed layers of small-molecule surfactants, proteins, and mixed systems of protein+surfactant. The relationship between interfacial rheology and the stability of emulsions and foams is critically discussed.

Rheological interpretation of deposits of yield stress fluids

Abstract: Various industrial or natural slurries are coarse, concentrated suspensions with a yield stress. Relevant practical methods are needed to determine the behaviour of such fluids. Here we provide a simple, theoretical approach to describe form of deposits remaining after free surface flow stoppage as a function of fluid characteristics. Thixotropy, inertial effects and sedimentation are neglected. It is demonstrated that the flow depth, as a function of the distance from the edge, can be predicted in any direction. Further analysis shows that there is a clear difference in the form of deposit edge between materials in which there is a grain-to-grain network of interaction and materials in which there is a network of colloidal interaction. These results provide a first order, practical, rheological and structural interpretation of current deposits of pastes, muds, slurries, fresh concrete or magmas.

Micellar Transition State in Casein Between pH 5.5 and 5.0

Abstract: pH-induced changes in casein micelles during direct acidification and bacterial fermentation of reconstituted skim milk at 20°C were monitored by scanning electron microscopy (SEM) in combination with biochemical and rheological methods. For SEM casein micelle observations, an original method of milk sample preparation with porous inorganic membranes was developed. Micrographs suggested that different stages of micellar association were related to pH and that between pH 5.5 and 5.0 casein micelles coalesced. Correlations between microstructural and biochemical changes in casein micelles, and rheological behavior of milk or gel, help to explain the different steps leading to the final protein network of the acid milk gel.

A numerical study of the rheological properties of suspensions of rigid, non-Brownian fibres

Abstract: Using techniques developed in our previous publication (Mackaplow et al. 1994), we complete a comprehensive set of numerical simulations of the volume-averaged stress tensor in a suspension of rigid, non-Brownian slender fibres at zero Reynolds number. In our problem formulation, we use slender-body theory to develop a set of integral equations to describe the interfibre hydrodynamic interactions at all orders. These integral equations are solved for a large number of interacting fibres in a periodically extended box. The simulations thus developed are an accurate representation of the suspensions at concentrations up to and including the semidilute regime. Thus, large changes in the suspensions properties can be obtained. The Theological properties of suspensions with concentrations ranging from the dilute regime, through the dilute/semi-dilute transition, and into the semi-dilute regime, are surprisingly well predicted by a dilute theory that takes into account two-body interactions. The accuracy of our simulations is verified by their ability to reproduce published suspension extensional and shear viscosity data for a variety of fibre aspect ratios and orientation distributions, as well as a wide range of suspension concentrations.

Rheology of Confined Polymer Melts

Abstract: The behavior of confined polymer melts in a shear flow is investigated using molecular dynamics simulations. Polymer molecules are modeled as bead−spring chains that interact via repulsive site−site potentials. The fluid is contained between atomistic walls and shear is imparted by moving the walls in opposite directions to simulate planar Couette flow. Experimental conditions are simulated by maintaining the walls at a constant temperature. The density, velocity, and temperature profiles during shear flow are monitored and compared to those of simple liquids under similar conditions. For the shear rates investigated, polymeric fluids exhibit a much stronger tendency for slip at the wall−fluid interface than simple fluids. The magnitude of slip increases with increasing shear rate. For short chains the magnitude of slip increases with increasing chain length but appears to reach an asymptotic value at approximately the entanglement length. The viscosity increases as the film thickness is decreased, in qua...

Suspensions : fundamentals and applications in the petroleum industry

Abstract: Suspensions: Basic Principles Particle and Suspension Characterization Rheology of Suspensions Flow of Suspensions in Pipelines Principles of Single-Phase Flow Through Porous Media Permeability Decline Due to Flow of Dilute Suspensions Through Porous Media Fines Migration in Petroleum Reservoirs Asphaltenes in Crude Oil and Bitumen: Structure and Dispersion Solids Production and Control in Petroleum Recovery Drilling Fluid Suspensions Suspensions in Hydraulic Fracturing Principles and Applications of Cement Slurries Suspensions in the Hot Water Flotation Process for Canadian Oil Sands Nature and Fate of Oil Sands Fine Tailings Glossary: Suspension Terminology

Rheology of a Magnetorheological Fluid

Abstract: We report on experimental investigations of the rheological behavior of aqueous magnetic suspensions. The suspended particles are monodisperse colloidal polystyrene spheres which contain magnetic Fe203-grains. In the absence of a magnetic field these suspensions behave as Newtonian fluids, whereas under the influence of a magnetic field due to the formation of an ordered structure the apparent viscosity of the suspension increases up to three orders of magnitude and they clearly exhibit non-Newtonian properties, such as shear thinning and yield stress. The apparent viscosity depends on the magnetic field according to aq - H2". Increasing the volume fraction of the particles in the range of 0.014 < 0 < 0.12 results in a linear increase in apparent viscosity and yield stress. Both apparent viscosity and yield stress depend also on the particle size as first measurements on particles with diameters ranging from 0.5 Am to 1.0,Am clearly show. All measurements were carried out with a rotation viscometer using ...

Relating the shear-thinning curve to the molecular weight distribution in linear polymer melts

Abstract: The double‐reptation ansatz for stress relaxation in broadly polydisperse melts of linear chains relates the molecular weight distribution φ(n) and the dynamic viscosity η(ω), and has been used to infer φ(n) from rheological data. I show that the microscopic theory of constraint release implies the double‐reptation model is valid for practical distributions φ(n). I also provide a heuristic theoretical argument for the Cox–Merz empirical rule that the shapes of the functions η*(ω) and η(γ) are similar, which allows the shear‐thinning curve to be predicted.

Rheology and processing of liquid crystal polymers

Abstract: Introduction to liquid crystal polymers. Theoretical aspects of the flow of liquid crystal polymers. Hamiltonian modelling of liquid crystal polymers and blends. Rheology and processing of liquid crystal polymer melts. Rheological and relaxation behaviour of filled LC-thermoplastics and their blends. The morphology and rheology of liquid crystal polymer blends. Processing of liquid crystal polymers and blends. Time-dependent effects in lyotropic systems. Processing and properties of rigid rod polymers and their molecular composites.

Magnetorheological fluid composites

Abstract: An interesting extension in the use of magnetic fluids has resulted from the development of magnetic fluid composites obtained by dispersing micrometre-sized non-magnetic particles in a magnetic fluid. The composites possess a yield stress in a magnetic field which can be described at sufficiently high strain rates by the Bingham relation , where is the shear stress perpendicular to the applied field, the extrapolated yield stress, the strain rate and the plastic viscosity. Thus, a composite, particle concentration , in a field 0.036 T with has a yield stress of 26 Pa. The yield stresses obtained experimentally for different and correspond well to values predicted theoretically by Rosensweig using a determination of based on a continuum concept of unsymmetric stress that develops in the deformed but unyielded anisotropic medium.

Computer simulation of rheological phenomena in dense colloidal suspensions with dissipative particle dynamics

Abstract: The rheological properties of colloidal suspensions of spheres and rods have been studied using dissipative particle dynamics (DPD). We have measured the viscosity as a function of shear rate and volume fraction of the suspended particles. The viscosity of a 30 vol% suspension of spheres displays characteristic shear-thinning behaviour as a function of shear rate. The values for the low- and high-shear viscosity are in good agreement with experimental data. For higher particulate densities, good results are obtained for the high-shear viscosity, although the viscosity at low shear rates shows a dependence on the size of the suspended spheres. Dilute suspensions of rods show an intrinsic viscosity which is in excellent agreement with theoretical results. For concentrated rod suspensions, the viscosity increases with the third power of the volume fraction. We find the same scaling behaviour as Doi and Edwards for the semidilute regime, although the explanation is unclear. The DPD simulation technique therefore emerges as a useful tool for studying the rheology of particulate suspensions.

Rheological Properties of Concentrated, Nonaqueous Silicon Nitride Suspensions

Abstract: The rheological properties of nonaqueous silicon nitride powder suspensions have been investigated using steady shear and viscoelastic measurements. The polymeric dispersant, Hypermer KD-3, adsorbed strongly on the powder surfaces, and colloidally stable, fluid suspensions up to a volume fraction of {Phi} = 0.50 could be prepared. The concentrated suspensions all displayed a shear thinning behavior which could be modeled using the high shear form of the Cross equation. The viscoelastic response at high concentrations was dominated by particle interactions, probably due to interpenetration of the adsorbed polymer layers, and a thickness of the adsorbed Hypermer KD-3 layer, {Delta} {approx} 10 nm, was estimated. The volume fraction dependences of the high shear viscosity of three different silicon nitride powders were compared and the differences, analyzed by using a modified Krieger-Dougherty model, were related to effective volume effects and the physical characteristics of the powders. The significantly lower maximum volume fraction, {Phi}{sub m} = 0.47, of the SN E-10 powder was referred to the narrow particle size distribution and the possibility of an unfavorable particle morphology.

Effect of Hydrophobic Modification on Phase Behavior and Rheology in Mixtures of Oppositely Charged Polyelectrolytes

Abstract: Phase behavior and rheology of polymer mixtures comprising aqueous solutions of oppositely charged polyelectrolytes are investigated. Emphasis is put on the effects of hydrophobic modification of the polymers and addition of salt. The associative phase separation usually observed when mixing oppositely charged polyelectrolytes is effectively prevented over a large miscibility region for the hydrophobically modified polymers. Also, in the extended one-phase region, the viscosity is 3-4 orders of magnitude higher for mixed polyelectrolyte systems compared to that observed for either one of the polymers. Addition of ordinary electrolytes to the mixture decreases the viscosity strongly, and at higher electrolyte contents a phase separation is induced. A mechanism explaining the observations is proposed.