Showing papers on "Rheometer published in 2002"

Filament-stretching rheometry of complex fluids

Abstract: ▪ Abstract Filament-stretching rheometers are devices for measuring the extensional viscosity of moderately viscous non-Newtonian fluids such as polymer solutions. In these devices, a cylindrical liquid bridge is initially formed between two circular end-plates. The plates are then moved apart in a prescribed manner such that the fluid sample is subjected to a strong extensional deformation. Asymptotic analysis and numerical computation show that the resulting kinematics closely approximate those of an ideal homogeneous uniaxial elongation. The evolution in the tensile stress (measured mechanically) and the molecular conformation (measured optically) can be followed as functions of the rate of stretching and the total strain imposed. The resulting rheological measurements are a sensitive discriminant of molecularly based constitutive equations proposed for complex fluids. The dynamical response of the elongating filament is also coupled to the extensional rheology of the polymeric test fluid, and this can...

Cheese Rheology and Texture

Abstract: Cheesemaking - An Overview Cheese Types Cheesemaking References Fundamental Rheological Methods Definition of Rheology Basic Concepts Fundamental Methods Uniaxial Compression Shear Rheometry Extensional Rheometry References Uniaxial Testing of Cheese Uniaxial Compression Measurements Structure and Composition Effects Stress-relaxation Measurements Torsion Measurements Tension Measurements Creep Measurements Bending Measurements Vane Measurements Shear Measurements Lubricated Squeezing Flow Measurements References Fracture Properties of Cheese Fracture Mechanics Brittle Fracture Griffith Criterion Determination of KI Fracture Tests on Cheese Cutting, Slicing, and Shredding Cutting with Wire and Blade Eye/slit Formation and Growth References Linear Viscoelasticity of Cheese Mathematical Relations in Linear Viscoelasticity Types of SAOS Measurements Time-Temperature Superposition Application of SAOS in Cheese Rheology Linear Viscoelastic Region of Cheeses Mozzarella: Time-Temperature Superposition Example Cox-Merz Rule References Nonlinear Viscoelasticity of Cheese Pipkin Diagram Sliding Plate Rheometer Large Amplitude Oscillatory Shear Flow Spectral Analysis Discrete Fourier Transform Determining Material Properties Amplitude Spectrum Stress-Shear Rate Loops Effect of Wall Slip Constitutive Model for Cheese Relaxation Modulus Obtained from SAOS Relaxation Modulus Conforming to LAOS References Cheese Texture Texture Development in Cheese Cheese Manufacturing Factors that Affect Texture Measurement of Texture Uniaxial Tests for Cheese Texture Measurement Compression Test Torsion Test and Vane Rheometry Texture Map Dynamic Tests Empirical Tests Crumbliness References Measuring Cheese Melt and Flow Properties Meltability Empirical Tests Objective Tests Steady Shear Viscometry Capillary Rheometry Squeeze-Flow Rheometry UW Meltmeter Viscoelasticity Index for Cheese Meltability Dynamic Shear Rheometry Helical Viscometry Cheese Melt Profile Measurement UW Melt Profiler Determination of Melt Profile Parameters Graphical Method Modeling Melt Profile Constant Temperature Test Conduction Heating References Measuring Cheese Stretchability Empirical Methods Instrumented Methods Vertical Elongation Horizontal Elongation Compression Tests Helical Viscometry Fiber-spinning Technique The Weissenberg Effect References Factors Affecting Functional Properties of Cheese Properties of Milk Cheesemaking Procedures Addition of Starter Culture and Coagulants Cheese Composition Post-Manufacturing Processes Aging/Ripening References

Synthetic Biodegradable Aliphatic Polyester/Montmorillonite Nanocomposites

Abstract: Biodegradable aliphatic polyesters (BAPs), synthesized from diols and dicarboxylic acids, and organophilic montmorillonite (OMMT) were intercalated by a solvent-casting method using chloroform as a cosolvent to produce nanocomposite (BAP/OMMT). The d spacings of both BAP and BAP/OMMT were examined by X-ray diffraction analysis, and the microstructure of BAP/OMMT was examined by transmission electron microscopy. Melting temperature changes and residuals were measured by thermal gravimetric analysis. Tensile strength and elongation were also examined with a universal testing machine. Increases in both the thermal stability and the mechanical strength of BAP/OMMT were observed for several different OMMT loadings. The rheological properties of the BAP/OMMTs were also examined with a rotational rheometer having a parallel-plate geometry. The shear viscosity at low shear rate exhibited a Newtonian plateau even at high loading and showed a higher degree of shear thinning at higher shear rate. Both the Newtonian ...

Slip at polymer–polymer interfaces: Rheological measurements on coextruded multilayers

Abstract: Polypropylene (PP) and polystyrene (PS), with closely matched viscosity, were coextruded into 8, 32, and 64 alternating layers. The apparent steady shear viscosity of these multilayer samples was measured with an in-line slit rheometer and with a parallel-plate rheometer. In both cases the apparent viscosity decreased with the number of layers providing strong evidence for interfacial slip. The velocity difference across the interface (interfacial slip velocity) versus shear stress, ΔVI(τ) was calculated from the apparent viscosity measurements. ΔVI(τ) showed sigmoidal behavior: a region of very low slip ( 103 Pa followed by a linear region ΔVI=τ/β∞. These data could be fit with a modified Ellis model. The same function fit the different number of layers and both slit and parallel-plate data indicating ΔVI(τ) is a material property of the PP/PS pair. Slip was also observed in PS/PMMA (polymethyl methacrylate) and PP/aPA (amorphous nylon) p...

Effects of bubble deformation on the viscosity of dilute suspensions

Abstract: The relative viscosity (µrel = suspension viscosity/suspending fluid viscosity) of low Reynolds number, dilute and surfactant-free bubble suspensions in simple shear is studied with a rotating cylinder, Couette rheometer. The conditions of the experiments correspond to capillary numbers (Ca) of order 1 and bridge previous experimental, theoretical and numerical results that focused on either Ca � 1o rCa � 1. The suspensions are shear thinning with µrel > 1 for small Ca. At large Ca, µrel approaches a constant that is less than 1. These results are explained by a scaling analysis that considers how regions of viscous dissipation in and around bubbles change as bubbles are deformed by the flow. © 2002 Elsevier Science B.V. All rights reserved.

Effect of Fiber Concentration and Axial Ratio on the Rheological Properties of Cellulose Fiber Suspensions

Abstract: The steady flow and the dynamic viscoelastic properties of cellulose fiber suspensions were investigated as functions of the suspension concentration and the fiber shape using a parallel-plate type rheometer. Various concentrations of the suspensions were made from various types of cellulose fibers, i.e., microcrystalline cellulose, bacterial cellulose, and fibrillated cellulose fibers. All the suspensions showed non-Newtonian flow even at very low concentrations. The flow property of each suspension showed a plateau of the shear stress, i.e., the yield stress, over a critical concentration. The critical concentration obtained from the experiment agreed well with the value theoretically calculated from the axial ratio of the fibers. The dynamic moduli of the suspensions were almost independent of the angular frequency, and they increased with the fiber concentration. The dynamic storage moduli increased in proportion to the 9/4th power of the fiber concentration. This power of 9/4 is coincident with that theoretically required for polymer gels. This fact suggests that the rigidity of the suspensions has appeared by the same mechanism from the order of cellulose fibers to microcrystalline cellulose fibers, and even to polymer molecules.

Revisiting the 1954 suspension experiments of R. A. Bagnold

Abstract: In 1954 R. A. Bagnold published his seminal findings on the rheological properties of a liquid-solid suspension. Although this work has been cited extensively over the last fifty years, there has not been a critical review of the experiments. The purpose of this study is to examine the work and to suggest an alternative reason for the experimental findings. The concentric cylinder rheometer was designed to measure simultaneously the shear and normal forces for a wide range of solid concentrations, fluid viscosities and shear rates. As presented by Bagnold, the analysis and experiments demonstrated that the shear and normal forces depended linearly on the shear rate in the 'macroviscous' regime; as the grain-to-grain interactions increased in the 'grain-inertia' regime, the stresses depended on the square of the shear rate and were independent of the fluid viscosity. These results, however, appear to be dictated by the design of the experimental facility. In Bagnold's experiments, the height (h) of the rheometer was relatively short compared to the spacing (t) between the rotating outer and stationary inner cylinder (h/t=4.6). Since the top and bottom end plates rotated with the outer cylinder, the flow contained two axisymmetric counter-rotating cells in which flow moved outward along the end plates and inward through the central region of the annulus. At higher Reynolds numbers, these cells contributed significantly to the measured torque, as demonstrated by comparing Bagnold's pure-fluid measurements with studies on laminar-to-turbulent transitions that pre-date the 1954 study. By accounting for the torque along the end walls, Bagnold's shear stress measurements can be estimated by modelling the liquid-solid mixture as a Newtonian fluid with a corrected viscosity that depends on the solids concentration. An analysis of the normal stress measurements was problematic because the gross measurements were not reported and could not be obtained.

Extensional Rheometry of Entangled Solutions

Abstract: The seminal ideas of de Gennes and Doi and Edwards have provided the theoretical framework for much of the recent effort to model the rheological behavior of entangled polymer melts and solutions. Recent theoretical work has incorporated a number of important additions to the basic Doi-Edwards theory, including an explicit description of chain stretch and additional relaxation mechanisms such as contour length fluctuations (CLF) and convective constraint release (CCR). However, very little quantitative data has been published on the rheological behavior of entangled systems in strong flows. Hence, a comprehensive examination of the theoretical developments has not been possible. The experiments described in this paper use the filament stretching rheometer to obtain transient extensional stress growth data and steady state uniaxial extensional viscosity data for a number of entangled, narrow molecular weight distribution polystyrene solutions in the strain-rate regime characterized by a significant degree of both chain alignment and stretch. These results are then compared with theoretical predictions for a number of the current generation of reptation-based models, including mechanisms for chain stretching, contour length fluctuations, and convective constraint release. These comparisons demonstrate that when the model parameters are properly obtained from linear viscoelastic measurements, the recent model due to Mead, Larson, and Doi (Macromolecules 1998, 31, 7895) provides quantitative predictions for this class of flows for solutions spanning the complete range from very lightly to highly entangled solutions.

Viscoelastic properties of pig kidney in shear, experimental results and modelling

Abstract: In this paper, the results from a series of rheological tests of fresh pig kidney have been reported. Using a standard strain-controlled rheometer, the oscillation strain sweep experiment showed a linear viscoelastic strain limit of the order of 0.2% strain. To determine the components of dynamic moduli in terms of frequency, shear oscillation tests were done at strain 0.2% using a stress-controlled rheometer. Shear stress relaxation tests were carried out with a fixed strain of 0.2% and 0.02 s rise time. The model we have developed uses a multi-mode upper convected Maxwell (UCM) model with variable viscosities and time constants, to which we have added a Mooney hyper-elastic response, both multiplied by a damping function. Different forms of damping functions that control the non-linearity of strain-stress profile have been tested. The model was fitted to our experimental data, and matched the entire test data reasonably well with a single set of parameters.

Investigation of the elongational behavior of polymer solutions by means of an elongational rheometer

Abstract: An elongational rheometer is used to measure the relaxation times λ and the steady terminal elongational viscosities ηE,t of aqueous solutions of ionic and nonionic polymers at various concentrations in Θ and in good solvents. In the corresponding ηE,t/λ nomogram the results yield two typical limiting curves—one for solutions of nonionic polymers and another for solutions of ionic polymers. The curves characterize the elongational behavior of the polymer solutions. The elongational flow results in data that are intermediate to the two limiting curves for polymers in aqueous salt solutions and for aqueous solutions of mixtures of ionic and nonionic polymers. By means of an empirical correlation, the relaxation time and the steady terminal elongational viscosity are normalized such that the nomogram can be applied to polymers in various solvents. Thus a simple and reliable method for determining the behavior of polymer solutions in elongational flows is achieved.

On the Effect of Coarse Aggregate Fraction and Shape on the Rheological Properties of Self-Compacting Concrete

Abstract: Rheological measurements have been undertaken to illustrate the applicability on fresh self-compacting concrete of a model for the relative viscosity and yield stress of suspensions. The model is based on linear viscoelasticity and classical theory for composite materials and it takes into account the amount, shape (aspect ratio of ellipsoids), and maximum packing density of the particles (here aggregates), as well as the rheological properties of the matrix (here mortar). Self-compacting concretes made from different types of coarse aggregates: spheres (glass beads), sea dredged, crushed, and a mix of sea dredged and crushed aggregates, have been tested in a coaxial cylinder concrete rheometer (BML). The investigations indicate that the aspect ratio, angularity, and surface texture of aggregates affect the viscosity and yield stress. The magnitude of these effects depends on the property in question, viscosity and yield stress.

Shear-banding structure orientated in the vorticity direction observed for equimolar micellar solution

Abstract: The non-monotonic shear flow of a viscoelastic equimolar aqueous surfactant solution (cetylpyridinium chloride-sodium salicylate) is investigated rheologically and optically in a transparent strain-controlled Taylor Couette flow cell. As reported before, this particular wormlike micellar solution exhibits first a shear thinning and then a pronounced shear-thickening behavior. Once this shear-thickening regime is reached, a transient phase separation/shear banding of the solution into turbid and clear ring-like patterns orientated perpendicular to the vorticity axis, i.e., stacked like pancakes, is observed (Wheeler et al. 1998; Fischer 2000). The solution exhibit several unique features as no induction period of the shear induced phase, no structural build-up at the inner rotating cylinder, jumping pancake structure of clear and turbid ringlike phases, and oscillating shear stresses appear once the pancake structure is present. According to our analysis this flow phenomenon is not purely a mechanical or rheological driven hydrodynamic instability but one has to take into account structural changes of the oriented micellar aggregates (flow induced non-equilibrium phase transition) as proposed by several authors. Although this particular flow behavior and the underlying mixture of shear induced phases and mechanical instabilities is not fully understood yet, some classification characteristics based on a recent theoretical approach by Schmitt et al. (1995) and Porte et al. (1997) where a strong coupling between the flow instability (non-homogeneous flow profile due to the bands) and the structural changes causes the observed transient phenomena can be derived. In reference to the presented model the observed orientation of the rings is typical for complex fluids that undergo a spinodal phase separation coupled with a thermodynamic flow instability. In contrast to other shear banding phenomena, this one is observed in parallel plate, cone-plate, and Couette flow cell as well as under controlled stress and controlled rate conditions. Therefore, it adds an additional aspect to the present discussion on shear banding phenomena, i.e., the coupling of hydrodynamics and phase transition of rheological complex fluids.

Rheology and flow-birefringence from viscoelastic polymer-clay solutions

Abstract: The shear orientation of viscoelastic clay-polymer solutions was investigated by means of rheology and flow birefringence (Δn). The polymer chains are in dynamic adsorption/desorption equilibrium with the clay particles to form a “network”. The elastic behavior of the network was characterized by constant stress, oscillatory shear, and stress relaxation experiments. Constant stress experiments indicated a yield stress upon which shear flow started and no strain recovery could be observed. Oscillatory shear experiments showed a broad elastic region followed by flow when a critical strain was reached. Stress relaxation experiments showed several relaxation times when the same critical strain was reached. Experiments under steady flow characterized the transient behavior of the network. With increasing steady shear rate a pronounced minimum in birefringence was observed at a critical shear rate. The shear rate dependent viscosity showed near power law behavior and no corresponding critical feature. While birefringence detects orientational effects on a microscopic length scale, rheology averages over macroscopic changes in the sample. The same degree of orientation could be achieved under constant shear rate or constant stress conditions.

Transient normal stress response in a concentrated suspension of spherical particles

Abstract: The transient normal force response in a concentrated suspension of spherical particles upon startup of shear, following a period of rest, was found to depend on the direction in which shear was restarted. When shear was restarted in the same direction, the normal force signal rapidly grew to its positive steady-state value. However, when shear was restarted in the opposite direction, the normal force signal was initially negative and decreased, within the response time of the instrument, to a negative minimum, from which it gradually increased, passing through zero, to its positive steady-state value. This is believed to be the first experimental confirmation of this phenomenon, which had been suggested by the numerical simulations of previous investigators.

Behavior of magnetorheological fluids utilizing nanopowder iron

Abstract: Iron nanopowders for use in magnetorheological (MR) fluids were synthesized using a Microwave Plasma Synthesis technique developed at Materials Modification Inc (Fairfax VA). Transmission electron microscopy and surface area analysis measured iron particle size at 15–25 nm. The nanopowders were mixed into hydraulic oil to create nano-scale MR fluid. A micro-scale fluid was created using 45 μm iron particles as well as a hybrid fluid using a 50/50 mix of micro- and nanoparticles. All three fluids had a solids loading of 60% (w/w or weight by weight fraction). The fluids were tested in a flow mode rheometer fabricated from a modified damper using a sinusoidal input dynamometer over a speed range of 12.7 to 177.8 mm/s (0.5 to 6 in/s) and an input current range of 0 to 2 A. The yield stress and plastic viscosity of the MR fluid were characterized using a Bingham plastic model.

Electronic packages having good reliability comprising low modulus thermal interface materials

Abstract: An electronic package includes a heat-generating electronic component such as an integrated circuit chip, a thermally conductive member, which may be an integrated heat spreader, and a low modulus thermal interface material in heat conducting relation between the electronic component and the thermally conductive member. Increased thermal performance requirements at the electronic component level are met by the thermal interface material, which includes a polymer matrix and thermally conductive filler, which has a storage shear modulus (G′) at 125° C. of less than about 100 kPa, and which has a gel point, as indicated by a value of G′/G″ of ≧1, where G″ is the loss shear modulus of the thermal interface material. The values for G′ and G″ are measured by a strain-controlled rheometer.

Experimental investigation of creep and recovery behaviors of magnetorheological fluids

Abstract: Creep and recovery behaviors of a magnetorheological (MR) fluid under constant stress shear were investigated. This experiment was accomplished by using a rheometer with parallel-plate geometry. The applied constant stress ranges from a small value to a big one which approaches to the yield stress. The effects of magnetic field strength and the temperature on the creep behavior were addressed. The experimental results indicate that MR fluids behave as linear viscoelastic bodies at small stresses; with increasing constant stresses, nonlinear viscoelastic, viscoplastic, or purely plastic properties will dominate. Hence, MR fluid behavior ranges from predominantly elastic at small stresses to plastic at high stresses. Moreover, the creep and recovery behaviors are explained by a typically thick column structure.

Investigation of the solid–liquid transition of highly concentrated suspensions in oscillatory amplitude sweeps

Abstract: Suspensions of quasimonodisperse polymethylmethacrylate spheres with mean diameters of 4.7 and 3.1 μm, dispersed in a low-molecular-weight polydimethylsiloxane, were characterized using oscillatory shear amplitude sweeps. Thereby, the solid volume concentration was varied. The influence of the sample preparation, the mode of the experiment (controlled shear rate, controlled shear stress), and the parameters of the amplitude sweep such as logarithmic ramp time, measurement time, and frequency were investigated. The logarithmic ramp and the measurement time were found to be the substantial factors which especially influence the experimental results at low shear stress amplitudes. The frequency has an effect only at higher shear stress amplitudes where the material behaves linearly again. All suspensions showed a Hookean solid behavior at low shear stress amplitudes and a Newtonian fluid behavior at high shear stress amplitudes. In the transition range between the Hookean and the Newtonian behavior the analysis of the primary shear strain-time signal showed that the nonlinear effects were coupled with the occurrence of higher odd harmonics.

Electrorheological fluid under elongation, compression, and shearing.

Abstract: Electrorheological (ER) fluid based on zeolite and silicone oil under elongation, compression, and shearing was investigated at room temperature. Dc electric fields were applied on the ER fluid when elongation and compression were carried out on a self-constructed test system. The shear yield stress, presenting the macroscopic interactions of particles in the ER fluid along the direction of shearing and perpendicular to the direction of the electric field, was also obtained by a HAAKE RV20 rheometer. The tensile yield stress, presenting the macroscopic interactions of particles in the ER fluid along the direction of the electric field, was achieved as the peak value in the elongating curve with an elongating yield strain of 0.15-0.20. A shear yield angle of about 15 degrees -18.5 degrees reasonably connected tensile yield stress with shear yield stress, agreeing with the shear yield angle tested well by other researchers. The compressing tests showed that the ER fluid has a high compressive modulus under a small compressive strain lower than 0.1. The compressive stress has an exponential relationship with the compressive strain when it is higher than 0.1, and it is much higher than shear yield stress.

Squeezing flow viscometry for nonelastic semiliquid foods--theory and applications.

Abstract: In most conventional rheometers, notably the coaxial cylinders and capillary viscometers, the food specimen is pressed into a narrow gap and its structure is altered by uncontrolled shear. Also, most semiliquid foods exhibit slip, and consequently the measurements do not always reflect their true rheological properties. A feasible solution to these two problems is squeezing flow viscometry where the specimen, practically intact and with or without suspended particles, is squeezed between parallel plates. The outward flow pattern mainly depends on the friction between the fluid and plates or its absence ("lubricated squeezing flow"). Among the possible test geometries, the one of constant area and changing volume is the most practical for foods. The test can be performed at a constant displacement rate using common Universal Testing Machines or under constant loads (creep array). The tests output is in the form of a force-height, force-time, or height-time relationship, from which several rheological parameters can be derived. With the current state of the art, the method can only be applied at small displacement rates. Despite the method's crudeness, its results are remarkably reproducible and sensitive to textural differences among semiliquid food products. The flow patterns observed in foods do not always follow the predictions of rheological models originally developed for polymer melts because of the foods' unique microstructures. The implications of these discrepancies and the role that artifacts may play are evaluated in light of theoretical and practical considerations. The use of squeezing flow viscometry to quantify rheological changes that occur during a product's handling and to determine whether they are perceived sensorily is suggested.

From two-dimensional model networks to microcapsules

Abstract: The synthesis of microcapsules for technical, cosmetic, and pharmaceutical purposes has attracted much interest in recent years. The design of new capsules requires profound knowledge of their mechanical properties. Rheological studies provide interesting information on intrinsic membrane features and they can also be used to obtain information on bursting processes and shear-induced release of encapsulated compounds. In this article we shall discuss the basic rheological properties of different types of ultra-thin membranes, which can be used to form stable capsules walls. We have also analyzed the typical structures of these cross-linked films using Brewster-angle microscopy. Tiny oil or water droplets, which are surrounded by ultra-thin membranes, form simple types of microcapsules. In addition to the interface shear rheology, we have measured the Young's modulus (elongational modulus) and the Poisson ratio using a modified spinning drop apparatus. The shear-induced deformation and orientation of microcapsules was investigated in optical rheometers (rheoscopes). In the regime of small deformations the results were in fairly good agreement with a theoretical model recently proposed by Barthes-Biesel. Due to the simple synthesis and well-defined structure, microcapsules can also serve as model systems to understand the complicated flow properties of red blood cells (erythrocytes).