Showing papers on "Rheometer published in 1995"

Foam Mechanics at the Bubble Scale

Abstract: By focusing on entire bubbles rather than films or vertices, a simple model is proposed for the deformation and flow of foam in which dimensionality, polydispersity, and liquid content can easily be varied. Simulation results are presented for the linear elastic properties as a function of bubble volume fraction, showing a melting transition where the static shear modulus vanishes and the relaxation time scale peaks. Results are also presented for shear stress versus strain rate, showing intermittent flow via avalanchelike topological rearrangements and Bingham-plastic behavior.

The Fibroblast-Populated Collagen Microsphere Assay of Cell Traction Force—Part 2: Measurement of the Cell Traction Parameter

Abstract: In Part 1 of this work, we formulated and analyzed a mathematical model for our fibroblast-populated collagen microsphere (FPCM) assay of cell traction forces (Moon and Tranquillo, 1993). In this assay, the FPCM diameter decreases with time as the cells compact the gel by exerting traction on collagen fibrils. In Part I we demonstrated that the diameter reduction profiles for varied initial cell concentration and varied initial FPCM diameter are qualitatively consistent with the model predictions. We show here in Part 2 how predictions of a model similar to that of Part 1, along with the determination of the growth parameters of the cells and the viscoelastic parameters of the gel, allow us to estimate the magnitude of a cell traction parameter, the desired objective index of cell traction forces. The model is based on a monophasic continuum-mechanical theory of cell-extracellular matrix (ECM) mechanical interactions, with a species conservation equation for cells (1), a mass conservation equation for ECM (2), and a mechanical force balance for the cell/ECM composite (3). Using a constant-stress rheometer and a fluids spectrometer in creep and oscillatory shear modes, respectively, we establish and characterize the linear viscoelastic regime for the reconstituted type I collagen gel used in our FPCM traction assay and in other assays of cell-collagen mechanical interactions. Creep tests are performed on collagen gel specimens in a state resembling that in our FPCM traction assay (initially uncompacted, and therefore nearly isotropic and at a relatively low collagen concentration of 2.1 mg/ml), yielding measurements of the zero shear viscosity, μ 0 (7.4 x 10 6 Poise), and the steady-state creep compliance, J c 0 . The shear modulus, G (155 dynes/cm 2 ), is then determined from the inverse of J e 0 in the linear viscoelastic regime. Oscillatory shear tests are performed in strain sweep mode, indicating linear viscoelastic behavior up to shear strains of approximately 10 percent. We discuss the estimation of Poisson's ratio, v, which along with G and μ 0 specifies the assumed isotropic, linear viscoelastic stress tensor for the cell/collagen gel composite which appears in (3). The proliferation rate of fibroblasts in free floating collagen gel (appearing in (1)) is characterized by direct cell counting, yielding an estimate of the first-order growth rate constant, k (5.3 X 10 -6 s -1 ). These independently measured and estimated parameter values allow us to estimate that the cell traction parameter, τ 0 , defined in the active stress tensor which also appears in (3), is in the range of 0.00007-0.0002 dyne cm 4 /mg collagen cell. This value is in agreement with a reported measure of traction obtained directly via isometric force measurement across a slab of fibroblast-containing collagen gel.

The multipass rheometer

Abstract: This article describes the technical performance and initial results for a new, two‐piston, ‘‘multipass rheometer.’’ Fluid is contained within a capillary by two servo‐hydraulically controlled pistons. Both steady flow and oscillatory data can be obtained from the device with mean pressure as an independent adjustable variable. Steady and oscillatory data are presented for a silicone oil and a viscoelastic solution and certain data are compared with results obtained from a Rheometrics RDS‐II mechanical spectrometer or with literature data. The results show that the multipass rheometer is capable of extending the experimental ranges of many conventional rheometers, particularly in terms of pressure control, high shear rates, and multiple, successive flow measurements.

Nonequilibrium behavior of dense suspensions of uniform particles: Volume fraction and size dependence of rheology and microstructure

Abstract: The rheological and microstructural properties of dense suspensions of uniform, charge stabilized colloidal spheres with diameters greater than 200 nm are investigated at volume fractions just below the ordering transition up to 0.6. Shear stresses marking static and dynamic yielding, discontinuous shear thinning, and shear thickening are weakly dependent on volume fraction and particle size when scaled on the crystal’s elastic modulus G0. As shear stress is increased microstructures evolve through similar states independent of volume fraction. As rest, presheared suspensions exhibit long‐range orientational order. Above the dynamic yield stress, the suspensions deform with a polycrystalline microstructure which, at intermediate shear rates, evolves to hexagonally close‐packed planes lying parallel to the rheometer walls. At higher shear rates the suspensions melt. Thickening is only observed above a volume fraction of 0.4–0.5, depending on particle size and at shear rates above that where the hexagonal c...

Determination of yield stress fluid behaviour from inclined plane test

Abstract: The aim of this paper is to determine precisely under which conditions an inclined plane can be used as a rheometer, which could represent a practical and rapid technique for various types of industrial or natural viscoplastic coarse suspensions. We first examine its efficiency and relevancy for determining fluid yield stress in a straight way by measuring the deepest fluid layer able to stay on the inclined plane. We have made experiments with different materials (clay-water suspensions) whose yield stress ranged from 35 to 90 Pa, using 1 m long open rectangular channels with a slope ranging from 10 to 30° and a width ranging from 5 to 25 cm. Our procedure involved measuring the final fluid depth far from edges a long time after the end of the slow gravity-induced emptying of a dam placed upstream. The fluid yield stress was also estimated independently by fitting a Herschel-Bulkley model to simple shear rheometry data obtained within a relatively wide shear rate range. A good agreement between inclined rectangular channel tests and independent usual rheometrical tests is obtained even for aspect ratios (flow depth to channel width ratio) as large as 1 when one assumes that, when the fluid has stopped, the side and bottom wall shear stresses are equal to the fluid yield stress. These results prove the efficiency of the inclined plane test for determining yield stress when appropriate experimental precautions are taken for both tests. In addition we examine the possibility of determining the simple shear flow curve of a mud suspension from fluid depth, velocity and discharge measurements of different steady flows in a wide open channel (8 m long; 60 cm wide) equipped with a recirculating system. The results obtained from inclined plane tests are in good agreement with independent rheometrical data (with torsional geometries). However it is technically difficult to cover a wide shear rate range from the inclined plane technique since this requires a rather wide channel flow rate range.

A large‐scale field coaxial cylinder rheometer for the study of the rheology of natural coarse suspensions

Abstract: When one wants to study the behavior of coarse suspensions directly, one needs a special large‐scale rheometer with a wide gap. As a first step towards the understanding of coarse suspension behavior a wide‐gap coaxial cylinder rheometer (1.2 m diameter, 0.2 m gap, sample volume: 0.5 m3) was built. This apparatus, which should be cheap and strong, was designed to be towed and used in the field to test fluids including particles whose diameters do not exceed 1 cm. The validity of hypotheses and the various theoretical problems of the rheometrical calculations in the case of a wide‐gap rheometer when used to test a coarse suspension are then examined. The main advantage of this geometry appears to be that edge effects should not be too important. Its main disadvantage, especially for coarse suspensions with a complex behavior (yield stress, thixotropy, presence of coarse particles), is the heterogeneity of shear rate from one tool to the other. Experiments with various natural materials (water–clay mixtures...

Gap size and shear history dependencies in shear thickening of a suspension ordered at rest

Abstract: An investigation is made of the flow properties of a suspension of uniform 197 nm diameter spheres at a volume fraction of 0.50 dialyzed to equilibrium against 10−3 M KCl which displayed long‐range orientational order at rest. In wide gap rheometers (i.e., tool gaps ≥0.5 mm), microstructures and flow properties are similar to those seen in many suspensions which are ordered at rest. Low shear rate response is found to be independent of rheometer tool gap down to 4.5 μm. However, the shear rate marking the onset of thickening is a decreasing function of rheometer tool gap for gaps smaller than 15 μm. These results are consistent with abrupt shear thickening seen in dense suspensions containing uniform particles arising from the formation of percolating clusters produced during shear melting.

Effect of gap on the viscosity of monodisperse polystyrene melts: Slip effects

Abstract: The low shear rate slip behavior of monodisperse polystyrene melts was characterized with the parallel plate geometry in a torsional rheometer. It was found that all melts studied exhibited some slip behavior. Tool material of construction showed no appreciable effect. Results were analyzed according to deGennes’ theory of slip, and values of the characteristic slip dimension, b, were calculated for each molecular weight. The slip dimension was found essentially proportional to the molecular weight to the first power. A slider block and spring model is proposed in an attempt to explain slip phenomena in the low shear rate limit.

Mudbanks of the southwest coast of India. IV: Mud viscoelastic properties

Abstract: A small-strain rheological model has been developed to characterize the complex equivalent viscosity in the equation of mud motion due to water waves The dynamic viscosity and the sheaf moduli of elasticity in this model are determined from creep and dynamic shear tests in a controlled-stress rheometer, and their dependence on the forcing stress amplitude frequency and the solids volume fraction is examined For a comparative evaluation of mud rheology, the model has been applied to mud samples obtained from the southwest coast of India, Lake Okeechobee in Florida and Mobile Bay in Alabama, and to an aqueous clay mixture In each case within the measured stress amplitude range, the viscoelastic coefficients are found to depend significantly on the forcing frequency for a given solids volume fraction The India mud is found to be the most viscous of those tested This high viscosity results in very high rates of wave energy dissipation off the coast of Kerala during the monsoon season

Injection molding of semicrystalline polymers. I. Material characterization

Abstract: Various material data for an isotactic polypropylene were acquired for the simulation of the injection molding of this material. Viscosity as a function of shear rate and temperature was measured using a capillary rheometer at high shear rates and a cone-and-plate rheometer at low shear rates. Heat-flow properties, characterizing kinetics and induction time of quiescent crystallization, were obtained from DSC measurements. Material data characterizing shear-induced crystallization were obtained from extrusion experiments through a slit die with subsequent quenching of the material in the die after various rest times. The thickness of the shear-induced crystallization layer was measured along with the birefringence in this layer. A model of shear-induced crystallization developed by Janeschitz-Kriegl and co-workers was used to fit the kinetic data. Thus, kinetic parameters such as the limiting shear rate below which no shear-induced crystallization can occur and the characteristic time for the relaxation of birefringence were obtained. © 1995 John Wiley & Sons, Inc.

A rheometer for the measurement of a high shear modulus covering more than seven decades of frequency below 50 kHz

Abstract: A new rheometer is described. It consists of a transducer unit supplied with an electric impedance analyzing unit. The transducer unit converts a mechanical impedance into an electrical impedance by the piezoelectric effect. A detailed quantitative analysis of the interaction between the sample and the transducer is given. The real and imaginary parts of the shear modulus of a viscoelastic sample can be found in the frequency range of 1 mHz–50 kHz, modulus range of 0.1 MPa–10 GPa, and the temperature range of 150–300 K. The sample volume is 0.3 cm3 and the strain amplitude is exceedingly small. The small size of the transducer allows for good temperature control and equilibration. It has a simple construction based on inexpensive components. Results on the supercooled liquid 2‐metyl‐2,4‐pentandiol at the glass transition obtained by the method are included.

Polymerization and Mechanical Degradation Kinetics of Gluten and Glutenin at Extruder Melt‐Section Temperatures and Shear Rates

Abstract: Thermal polymerization and mechanical degradation rates were measured for wheat gluten and glutenin at 25-30% moisture. Changes in soluble protein and disulfide bonds were measured at 85°C to 180°C, residence times of 2-60 sec and shear rates of 10-275 sec -1 . Polymerization rates without shear were evaluated using isothermal analysis. An extrusion rheometer was used to simulate extruder conditions to determine the combined polymerization-degradation rates. Molecular weight distributions of soluble reaction products were determined using liquid size exclusion chromatography. Activation energies were 0.60-2.1 kcal/mol, 2.0-8.2 kcal/mol, and 0.50-0.90 kcal/mol and reaction orders were 0.5-2.0, 0.8-1.5, and 0.6-0.7, respectively for polymerization without shear, polymerization with shear, and degradation.

Extrude distortion in the capillary/slit extrusion of a molten polypropylene

Abstract: Experiments were carried out in both sliding plate and capillary rheometers with a polypropylene resin to determine the conditions for the onset of slip, surface, and gross melt fracture. It was found that there was no distinction between surface and gross melt fracture, which is commonly observed in the case of polyethylenes. Furthermore, the flow curves determined by using capillaries having various diameters are diameter independent implying the absence of slip. However, experiments with slit dies having rough surfaces suggest wall slip. Further analysis has shown that the effect of viscous heating masks the detection of slip from the diameter-dependency of the flow curves. The effect of a thin layer of fluoropolymer (Teflon PA, DuPont) on the critical shear stress for the onset of wall slip and melt fracture, as well as on the relationship between the wall shlip and the shear stress, were also examined. It was found that the presence of such layers increases the slip velocity, while it decreases the critical shear stress for the onset of slip.

Rheological and dielectric changes during isothermal epoxy-amine cure

Abstract: Dynamic viscosity and ionic conductivity were measured simultaneously during the cure of a digylcidyl ether of bisphenol-A (DGEBA) epoxy resin with diamino-diphenyl sulfone (DDS) by mounting a microdielectric sensor into the plates of a rheometer. Two different cure temperatures were examined. Periodically, throughout the cure, samples were removed from the plates of the rheometer, quenched, and analyzed for the glass transition temperature and epoxide conversion. The relationship between conductivity and viscosity appeared to be independent of cure temperature. A linear relation with a slope of −1 was observed between the natural logarithms of conductivity and viscosity during the cure up to approximately 85% cure conversion. It was hypothesized that the reaction rate was hindered by diffusion at this stage in the polymerization. A free volume relationship was used to successfully correlate conductivity with viscosity up to the diffusion limited region. ©1995 John Wiley & Sons, Inc.

The development of the bending beam rheometer; basics and critical evaluation of the rheometer

Abstract: The bending beam rheometer is proposed by the Strategic Highway Research Program (SHRP) for direct measurements of the rheological properties of asphalts at the lowest pavement temperatures. The purpose of this paper is to present the basic concepts upon which the rheometer was developed, to describe the testing procedure and the important procedural factors that affect test results, and to discuss the methodology of data interpretation. Typical results from the research program using a large number of asphalts to evaluate the rheometer are presented to illustrate the differences in behavior of asphalts from different sources and of different grades. The paper highlights the features of the testing system and the specimen preparation procedure. The results of the rheometer evaluation indicate that the rheometer is a repeatable test that can be used effectively to characterize the visco-elastic nature of asphalts at the lowest pavement temperatures. The paper includes discussion of the rationale behind selecting the specification parameters and the specification criteria to enhance binder contribution to resistance to thermal cracking.

Oscillatory, creep and steady flow behavior of Xanthan‐thickened oil‐in‐water emulsions

Abstract: In the handling, mixing, storage, and pipeline transportation of emulsions, knowledge of rheological properties is required for the design, selection, and operation of the equipment involved. The rheological behavior of xanthan gum-thickened oil-in-water emulsions is studied with a cone-and-plate system using a constant-stress rheometer. Xanthan gum solutions and xanthan-thickened oil-in-water emulsions are strongly shear-thinning and viscoelastic in nature. The effects of polymer and oil concentrations on the rheological behavior of emulsions are investigated. The relative viscosity for the thickened emulsions, at any given oil concentration, increases with an increase in the shear rate, whereas the unthickened emulsions show the opposite trend. The theoretical models give reasonable predictions for the relative viscosity, storage modulus, and loss modulus of xanthan-thickened emulsions. The ratio of storage to loss moduli increases considerably with the increase in polymer and oil concentrations. The creep/recovery experiments confirm that the xanthan-thickened emulsions are highly viscoelastic in nature and that the degree of elasticity increases with the increase in polymer and oil concentrations.

Nonlinear Rheology of Polymer Melts under Shear Flow

Abstract: The nonlinear rheology of an unentangled polymer melt under shear flow is considered theoretically. The finite chain extensibility is taken into account explicitly. The tangential stress and the first and the second normal-stress differences are calculated as a function of shear rate 9. It is shown that in the shear thinning regime, the viscosity r,~ decreases as p-2/3.

Dynamics of Compressed Polymer Layers Adsorbed on Solid Surfaces

Abstract: The viscoelastic properties of thin polymer films confined between a sphere and a plane are investigated by use of a surface force apparatus designed as a rheometer operating at the molecular scale. Axial oscillatory motions yield a shear field from which a complex elastic modulus of the confined fluid can be deduced. As the sphere-plane distance is decreased, the fluid made of two adsorbed polymer layers and the free solution exhibits a liquid-solid like transition at distance h c . The measured complex modulus can be split into an elastic modulus G c accounting for the compression of the adsorbed chains and a complex shear modulus G H * (ω) expressing the draining process at the interface. The elastic modulus G c is directly connected to the gradient of static repulsive forces, and a compression modulus E N of the adsorbed overlapping layers can be deduced from G c . The flow term G H * (ω) is discussed in terms of viscoelastic parameters (zero-shear viscosity ν 0 , average relaxation time τ 0 ) as a function of sphereplane distance h. For moderately compressed layers, a correlation length ξ can be defined either from the compression modulus E N or from the zero-shear viscosity η 0 . Both lengths scale as (h c -h) -4/9

Shear deformation of high solid fraction (>0.60) semi-solid SnPb under various structures

Abstract: The behavior of high solid fraction (>0.60) semi-solid SnPb alloys was studied in pure shear deformation using a Couette geometry rheometer. Three structures were investigated: a classical coarse dendritic structure, a globular structure obtained by reheating a fine dendritic structure (R.G.) and a globular structure generated by vigorous stirring during solidification (S.G.). The slurries were subjected to shear rates ranging from 0.01 to 50 s −1 . Experimental data for dendritic structure and for R.G. are compared to the model proposed by Nguyen et al. ( Proc. 2nd Int. Conf. on Processing of Semi-Solid Alloys and Composites , Boston, USA, 1992, p.296; and Int. J. Plast. 10 (1994) 693). The S.G. structure exhibited a very different behavior compared with globular structures obtained from a fine dendritic precursor (R.G.). This behavior is interpreted in terms of an agglomeration/disagglomeration model.

Biopolyester melt behavior by torque rheometry

Abstract: A torque rheometer method has been developed to evaluate the melt behavior of biopolyesters. The main features of this technique include : (i) a modified equipment configuration, so as to maintain the melt temperature within ± 1°C of the set temperature after the first few minutes of polymer melting and thermal equilibration, and (ii) a kinetic equation that can be used to determine the melt behavior from the torque-time data. Applying this methodology to a poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer shows that the degradation kinetics follow Arrhenius behavior with temperature, that the degradation rate increases with increasing shear, and that the activation energy for random chain scission is independent of shear rate. This effect of shear on the degradation process may be a result of viscous heating. The degradation rate is increased because of the increased internal energy of the polymer chains as a result of the mechanical deformation. There is no evidence of direct mechanical degradation. Last, the viscosity values obtained with the torque rheometer are consistent with those obtained by capillary rheometry.

Comparison of the extensional and shear viscosity characteristics of aqueous hydroxyethyl cellulose solutions

Abstract: The extensional viscosity characteristics of aqueous solutions of varying concentrations of three samples of hydroxyethylcellulose (HEC) of differing molecular masses have been determined using a commercially available opposed jet extensional rheometer. The results have been compared to the shear flow characteristics of the solutions and the Trouton ratio, T R , of the various systems calculated. For all polymer solutions studied, T R was found to approach the predicted Newtonian value of 3 at low extensional strain rates. This observation was taken as a strong indication that the opposed jet rheometer was producing reliable extensional viscosity data. At higher strain rates, T R generally increased with increasing strain rate and, for any given sample, with increasing polymer concentration thereafter. All HEC solutions studied were found to exhibit strain-thinning behavior in extensional flow and shear-thinning behavior in shear flow, with the most pronounced effects being observed with the highest molecular mass sample (M v ∼4.5×10 5 g/mol). The relative degrees of strain and shear thinning exhibited by the various HEC solutions have been quantified using the power law analysis. For the two samples of lowest molecular mass (M v ∼6.5×10 4 and ∼1.9×10 5 g/mol) the value of a power law index, n, in both extensional and shear flow, was close to unity. For all solutions of the sample of highest molecular mass, the value of n in extensional flow was significantly higher than that observed in shear flow. Regions of apparent strain thickening were observed for a few of the lowest viscosity solutions, but these observations were attributed to the onset of significant inertial effects rather than any coil-stretch transition phenomena