Showing papers on "Rheometer 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.

Yield stress measurements with the vane

Abstract: Yield stress measurements were performed on a TiO 2 pigment suspension with the vane in both a rate controlled and a stress controlled mode. In the rate controlled mode, a constant rotational speed is applied to the vane immersed in the suspension, and the resulting stress is measured as a function of time. In the stress controlled mode, a constant stress or a constant stress-rate is applied to the vane immersed in the suspension, and the resulting creep angle is measured as a function of time. In both modes the yield stress is determined as the minimum stress required for continuous rotation of the vane. A Haake Rheometer and a modified Weissenberg Rheogoniometer facilitated the controlled rate measurements. The significant difference between these instruments is that the Weissenberg torsion bar is 550 times stiffer than the Haake spring. To ensure a comparable time frame of measurement with the Haake, it was necessary to use lower rotational speeds in the operation of the Weissenberg. Although agreement in the measured yield stress within 5% was established between the Haake at a rotational speed of 0.021 rad s −1 and the Weissenberg at a rotational speed of 0.0063 rad s −1 , the shapes of the respective stress-time profiles were strikingly different. The Haake exhibited a largely elastic response, whereas the Weissenberg exhibited a largely viscoelastic response. The yield stress measured by both the Haake and the Weissenberg corresponds to the transition stress between viscoelastic and fully viscous flow. A Bohlin Rheometer facilitated the stress controlled measurements. The yield stress measured by the Bohlin was up to 13% lower than that measured by the Haake and the Weissenberg, and seemed to correspond more closely to the transition stress between fully elastic and viscoelastic flow.

A fifty cent rheometer for yield stress measurement

Abstract: The slump test, initially developed to determine the flow properties of fresh concrete, has been adopted as a means of accurately measuring the yield stress of strongly flocculated suspensions. The slump test offers a quick and easy way of measuring yield stress without the need for sophisticated electronic equipment, thereby giving plant operators an effective tool for determining yield stress. The model used to predict the yield stress from the conical slump test was devised by Murata (1984) and corrected by Christensen (1991). In the present case the theory has been adapted for a cylindrical geometry. Yield stress measurements obtained with the vane for numerous mineral suspensions under known surface chemistry conditions are compared to the slump measurements and theoretical prediction. Good agreement is obtained.

A rheological study of shear induced crystallization

Abstract: The isothermal crystallization of three isotactic polypropylene (iPP) types, with different molar mass (distributions), was studied after a well defined shear treatment of the melt at an elevated temperature and a subsequent quench to the crystallization temperature. For these experiments a standard rheometer of the cone and plate configuration was used. The development of the crystallization was monitored by dynamic oscillatory measurements. Shearing in the melt was shown to enhance subsequent crystallization at lower temperatures. Not only the total shear at constant rate is of importance, but also the chosen combination of rate and shearing time. Moreover, a pronounced influence of molar mass was detected. The exploration of the melting temperatures and times which are necessary for an erasion of the memory effects showed that the effect of shearing could not completely be erased, possibly as a consequence of mechanical degradation.

Analysis and testing of Bingham plastic behavior in semi-active electrorheological fluid dampers

Abstract: Electrorheological- (ER-) fluid-based dashpot dampers have smart capabilities because ER fluids undergo large changes in yield stress as electric field is applied. Our objective is the development and experimental validation of quasi-steady dashpot damper models, based on an idealized nonlinear Bingham plastic shear flow mechanism, for purposes of preliminary design and performance predictions. The data required for the Bingham plastic model is normally supplied by ER fluid suppliers, that is, plastic viscosity and dynamic yield stress as a function of applied field, as determined from a shear stress versus shear strain rate diagram. As force is applied to the dashpot damper, the ER fluid flows through an annulus between the concentric inner and outer electrodes. The idealized Bingham plastic shear flow mechanism predicts that three annular flow regions develop as a function of the local shear stress. In the central pre-yield or plug region, the local shear stress is less than the dynamic yield stress, so that the plug behaves like a rigid solid. The remaining two annular regions, adjacent to the electrodes, are in the post-yield condition and correspond to the shear stress exceeding the dynamic yield stress, so that the material flows. Equivalent viscous damping performance of an ER fluid dashpot damper is strongly coupled with the plug behavior. For a constant force, as the applied field increases, so does the plug thickness and equivalent viscous damping. For a constant applied field, as the force increases, the plug thickness and equivalent viscous damping both decrease. The passive and active or field-dependent damping behavior of an ER-fluid-based dashpot damper can be designed for a specific application using these quasi-steady Bingham plastic models.

The role of end-effects on measurements of extensional viscosity in filament stretching rheometers

Abstract: The transient extensional viscosity function of two semi-dilute polyisobutylene polymer solutions is investigated in a filament stretching rheometer of the type developed by Tirtaatmadja and Sridhar (J. Rheol., 37 (1993) 1081). A velocity compensation algorithm which yields a constant deformation rate at a single point in both Newtonian and non-Newtonian fluid samples is detailed. Good experimental reproducibility is obtained in the device and measurements with a viscous Newtonian oil yield steady-state Trouton ratios of η μ = 3 ± 0.5 . Both viscoelastic fluids show the onset of significant strain-hardening for Hencky strains greater than two, and transient extensional viscosities that increase by three orders of magnitude. Good agreement between the results for the two different fluids is obtained when tests are performed at identical values of the Deborah number. The maximum Hencky strain achievable in the device is ϵ = 5 and steady-state values of the extensional viscosity are never achieved over the range of strain rates attainable. Measurements show that the evolution of the tensile force exerted by the deforming filament is a strong function of the initial aspect ratio of the cylindrical test sample. A lubrication analysis for small sample aspect ratios demonstrates that this variation results from large radial pressure gradients arising from the non-homogeneous shear flow near the rigid disks. This simplified analysis provides a good description of the experimental observations, and tests conducted with the Newtonian oil suggest that only at Hencky strains ϵ > 2 does the extensional deformation of the filament dominate the shearing flow. In the case of viscoelastic fluid filaments, the initial shearing motion near the fixed endplates significantly affects the measured tensile stress in the filament at all later times in the extension. Careful attention is focused on the non-homogeneous deformation induced at the endplates that constrain the fluid sample at each end of the test apparatus. Measurements of the filament profile and surface curvature with a video-imaging system show distinct differences in the evolution of Newtonian and non-Newtonian samples. In both cases, different strain histories are experienced by fluid elements at different axial positions in the filament, and a spatially homogenous deformation is not achieved in the viscoelastic filaments until Hencky strains ϵ > 4 are attained. Observations at larger strains and high Deborah numbers, De > 3, indicate the onset of an elastic instability near the stationary endplate which results in the deforming fluid column partially decohering from the endplate.

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...

Stress relaxation and elastic decohesion of viscoelastic polymer solutions in extensional flow

Abstract: The evolution of the transient extensional stresses in dilute and semi-dilute viscoelastic polymer solutions was measured with a filament stretching rheometer of a design similar to that first introduced by Sridhar et al. The solutions were polystyrene-based Boger fluids which were stretched at constant strain rates in the range 0.6 ⩽ ϵ 0 ⩽ 4 s−1 and to Hencky strains of ϵ > 4. The test fluids all strain-hardened and Trouton ratios exceeding 1000 were obtained at high strains. In addition to measuring the transient tensile stress growth, the decay of the tensile viscoelastic stress in the fluid column following cessation of uniaxial elongation was also monitored as a function of the total imposed Hencky strain and the strain rate. The measured relaxation functions were found to be significantly different from those observed following cessation of steady shear flow. The extensional stresses initially decayed very rapidly upon cessation of uniaxial elongation followed by a slower viscoelastic relaxation. For the most elastic fluids, partial decohesion of the fluid filament from the endplates of the rheometer was observed in tests conducted at high strain rates. This elastic instability is initiated near the rigid endplate fixtures of the device and it results in the progressive breakup of the fluid column into individual threads or ‘fibrils’ with a regular azimuthal spacing. These fibrils elongate and bifurcate as the fluid sample is elongated further. In tests conducted at the highest Deborah numbers, complete sample decohesion from the endplates and rapid elastic recoil were sometimes observed. The critical stress and strain at the onset of the instability were determined by monitoring the tensile force exerted by the filament, the sample radius, and were used to construct an approximate stability diagram. Flow visualization experiments using a modified stretching device showed that the instability develops as a consequence of an axisymmetry-breaking meniscus instability in the non-homogeneous region of highly deformed fluid near the rigid endplate.

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.

Sliding plate rheometer studies of concentrated polystyrene solutions: Large amplitude oscillatory shear of a very high molecular weight polymer in diethyl phthalate

Abstract: A very high molecular weight, narrow molecular weight distribution polystyrene, having a molecular weight of 8.42×106, was investigated using a sliding plate rheometer. The solvent was diethyl phthalate. The solution exhibited wall slip during steady shear, even at very low shear rates and exhibited a marked normal‐stress‐driven secondary flow at high shear rates. However, there was no evidence of slip during the oscillatory shear tests. Using fast Fourier transform analysis, the results of the oscillatory shear tests on this solution are presented in terms of response surfaces in a space based on a Pipkin diagram (strain‐rate amplitude versus frequency). These reveal the linear and nonlinear regimes and the approach to a purely elastic regime at high frequencies. Wagner’s constitutive equation predicts the major trends in the experimental data but does not provide quantitative predictions over the entire range of experimental parameters.

Validation of BTRHEOM, the new rheometer for soft-to-fluid concrete

Abstract: An investigation was performed on the objectivity of a new rheometer for soft-to-fluid concrete, called BTRHEOM. Firstly, the influence of concrete wall friction on the measurement of the Bingham constants was studied by finite-element simulations. It has been shown that this influence is limited, in particular on the measurement of shear yield stress. However, the quality of the estimate of plastic viscosity can be improved by applying a reduction of 10% to the bulk value. Secondly, a series of comparative tests was conducted with two other rheometers with different operating principles. The comparison with the intrinsic values as measured by a big coaxial-cylinder viscometer, the CEMAGREF-IMG rheometer, showed a satisfactory result for yield stress, but not for plastic viscosity, probably due to the segregation of the coarse aggregate in the CEMAGREF-IMG rheometer. For two concretes without segregation, the two rheometers yielded the same values. Another comparison with the LAFARGE rheometer (a modified version of Tattersall's two-point test) showed a good correlation between the measurements with the two instruments. Finally, the slump test was modelled by a simple numerical approach, from which a linear relation between slump and yield stress was derived. This relationship agrees with the experimental results.

Rheology of suspensions

Abstract: This chapter is an in-depth review on rheology of suspensions. The area covered includes steady shear viscosity, apparent yield stress, viscoelastic behavior, and compression yield stress. The suspensions have been classified by groups: hard sphere, soft sphere, monodisperse, polydisperse, flocculated, and stable systems. The particle shape effects are also discussed. The steady shear rheological behaviors discussed include low- and high-shear limit viscosity, shear thinning, shear thickening, and discontinuity. The steady shear rheology of ternary systems (i.e., oil-water-solid) is also discussed. 228 refs., 45 figs., 1 tab.

A calibration technique to evaluate the power‐law parameters of polymer melts using a torque‐rheometer

Abstract: In this work we develop an empirical method to calibrate the measuring head of a torque-rheometer to obtain the power-law parameters of polymer melts. This method is based on a similar analysis developed by Lee and Purdon. However, in this work, only one geometrical parameter, with well-defined physical limits, is needed for calibration, instead of the two arbitrary parameters proposed by the previous authors. Moreover, this parameter is closely related to the ordinate of the logarithmic relationship between the torque (M T ) and the angular velocity (S 1 ), obtained from the torque-rheometer data. This allows us to define a calibration function for this parameter, which is used to evaluate the consistency index (m) of the melt. On the other hand, experimental results confirm the equivalence between the slope of the logarithmic values of torque and angular velocity obtained from the torque-rheometer data and those of shear stress vs. shear rate obtained from capillary rheometry. This further highlights the feasibility of evaluating the melt flow index (n) and the consistency index (m) from the torque-rheometer data.

Rheometry on magnetorheological (MR) fluids I. steady shear flow in stationary magnetic fields

Abstract: Test fixtures of a commercial concentric cylinder rheometer (Physica Rheolab MC 20) were modified to enable measurements under magnetic inductions up to 0.5 Tesla in a shear rate range of 0.1 up to 1000 s −1 and temperatures 0° to 150°C. In the 2 x90°-cups only two 90° sectors of the stationary part of the double concentric cylinder arrangement are submitted to the magnetic field which is created outside the test tools by an electromagnet. A prototype of a 360°-cup contains the electromagnet within the cup and avoids the correction necessary for the sector geometry. Measurements are shown for a carbonyl iron MR fluid and two nano MR fluids. An encouraging comparison of the viscosity function and MR effect (shear stress changes due to the field) measured by using the various cups is presented. The detailed investigation of the magnetic field distribution in the tools yields a distinct radial field gradient and also stray fields that make the quantification of the effective field in the gap difficult. The change of the field when the gap is filled with MR fluid is addressed. MR effects up to 13 000 Pa have been found, the limited torque range of the rheometer making it necessary to use relatively small gap dimensions which introduce errors due to edge effects. Shear rates up to 40000 s−1 as typical for the application in dampers were investigated by a piston-driven capillary rheometer making use of a thermostated rectangular slit with superimposed magnetic field. A satisfactory agreement of the magnetorheological data with the concentric cylinder results is found in the overlapping shear rate range.

Shear history dependence of the viscosity of aggregated colloidal dispersions

Abstract: The shear history dependence of the viscosity of a depletion flocculated dispersion of colloidal spheres was studied using two different rheometrical geometries. The observed rheological behavior is found to depend on the geometry, due to effects of thixotropy and sedimentation. By comparing the results of a cone‐plate and a Couette geometry, we were able to obtain reliable data. The shear history dependence is explored by measuring a flow curve before and after subjecting the aggregated dispersion to a constant shear rate for one hour. The viscosity values of the flow curve after this hour turned out to be considerably lower than the initial flow curve. The results were interpreted with a microrheological model for fractal aggregation in shear flow. The drop in viscosity is attributed to a shear induced compaction of the aggregates. Combination of this model and the concept of compaction results in a satisfactory description of the experimental results.

Constant volume ring shear apparatus

Abstract: The paper describes a constant volume ring shear apparatus that allows the measurement of the undrained peak and residual shear strengths of cohesive soils The undrained peak and residual strengths are applicable to seismic stability evaluations of slopes comprised of or founded on cohesive soil The constant-volume ring shear apparatus is equipped with a mechanism to adjust the normal stress during shear and a new specimen container that allows undisturbed specimens to be trimmed directly into the container The normal stress is adjusted during shear such that the height of the soil specimen remains constant This results in a constant volume or undrained shear condition The results of constant volume ring shear tests on normally consolidated Drammen clay are compared with the results of undrained direct simple shear tests The comparison reveals that the undrained peak shear strength obtained using the constant volume ring shear and direct simple shear apparatuses are in agreement However, the constant-volume ring shear apparatus allows the measurement of the undrained residual strength because it permits unlimited continuous shear displacement

Talc-thermoplastic compounds: particle orientation in flow and rheological properties

Abstract: We describe an investigation of the orientation of talc particles in flow and the rheological properties of talc-thermoplastic compounds. The orientation of talc particles in samples removed from rheometers was determined using a wide angle X-ray diffraction technique. Talc particles, being disc-like in shape, orient with the disc normals perpendicular to the flow direction in the sandwich, cone-plate, parallel plate, capillary, and elongational rheometer. The level of orientation of particles is represented in terms of orientation factors. A broad range of experimental studies of the shear, elongational, and oscillatory flow behavior on thermoplastic melts with high loadings of talc particles is presented. The rheological measurements include the steady state shear viscosity, transient shear viscosity, elongational viscosity, complex viscosity, storage modulus, and loss modulus. Generally, the viscosity increases with increasing particle loading and with decreasing particle size. At low deformation rates, the stresses build up in an unbounded manner. There are stresses below which there is no flow indicating the existence of yield values. The implication of yield values in anisotropic particle compounds is considered. The compound as studied is considered a liquid crystalline particle array. The implication of the measurements to rheological modelling is considered.

Experimental observations on the pressure-dependent polymer melt rheology of linear low density polyethylene, using a multi-pass rheometer

Abstract: We report experimental data for a linear low density grade of polyethylene at elevated temperatures using a newly designed Multi-Pass Rheometer This rheometer is capable of measuring oscillatory viscoelastic data and steady shear capillary measurements on the same test fluid within an enclosed environment Data presented in this paper show that at low pressures there is reasonable self-consistency between the Multi-Pass data and separate oscillatory data obtained by using a Rheometrics Mechanical Spectrometer and steady shear data obtained from a Rosand capillary rheometer In addition, we report experimental data on the pressure dependence for both viscoelastic and steady shear data over the range of 1–230 bar The steady shear results appear to be consistent with previously published data The apparent viscosity and the viscoelastic data both show a linear increase of about 20% over the pressure range tested

Simultaneous measurement of viscoelastic changes and cell opening during processing of flexible polyurethane foam

Abstract: A parallel-plate rheomete was constructed and used to study the development of dynamic shear modulus and cell opening under forced adiabatic conditions for a series of flexible slabstock polyurethane foams. Typical industrial formulations were used. The plates were heated to follow the adiabatic temperature profile of a real foam bun during foaming. The rheometer overcomes difficulties encountered in other methods such as heat loss and bubble damage caused by the probe.

Closed loop viscosity control of reactive extrusion with an in‐line rheometer

Abstract: A novel in-line melt rheometer (ILR) measured viscosity to control ionomer neutralization during reactive extrusion. The ILR has the advantages of an in-line location to minimize measurement delay and a “partial Couette” flow geometry, which affords independent control of the shear rate. The reactive extrusion process for ethylene methacrylic acid (EMAA) ionomer neutralization was modeled as a first order plus dead time process. A comparison of the process responses to experimentally determined residence time distributions for the extruder showed that there was an appreciable inherent delay despite the advantage of the rheometer's in-line location. Proportional-integral and minimum variance control algorithms were implemented. The quality of control was generally good, but demonstrated the non-linearity of the neutralization process, suggesting that adaptive control techniques could improve control performance.

Relaxation behavior and the application of integral constitutive equations to wheat dough

Abstract: The small amplitude rheological properties of doughs of different compositions were measured using a dynamic rheometer. In the first study, the effect of water, flour, and added gluten on the dynamic properties was investigated. Frequency sweep was carried out in the range of 0.1 to 100 s -1 at an applied strain of 0.1%. Storage (G') and loss (G) moduli for each composition were recorded. The storage modulus versus frequency plots were a series of parallel curves indicating that the moisture-to-flour ratio was the most important parameter. The loss modulus versus frequency plot showed a falling-off after a frequency of 40 s -1 . A plot of tan δ versus frequency indicated a more elastic dough at shorter time scales (or higher frequencies). Data from the dynamic shear experiment were used to determine the parameters of the generalized Maxwell model. Depending on composition, the doughs exhibited unimodal or bimodal distributions. Stresses relaxed faster when the flour-to-water ratio was high. In the second study, the dynamic and transient properties ofthree commercial doughs were investigated. Stress-strain behavior of a short cookie dough showed strain softening at higher strains. Dough samples did not exhibit initial stress overshoot at the shear rates investigated. A time-strain separability, valid for many polymer melts, was observed for cracker doughs, which encouraged the use of the factorable KBKZ model. The integral constitutive Wagner equation was applied to the transient data and was found to give a reasonably good fit except at short times where stress maximum occurred. The Yamamoto model, on the other hand, gave a poor fit to the shear viscosity data.

Elongation and subsequent relaxation measurements on dilute polyisobutylene solutions

Abstract: The elongation and subsequent relaxation of dilute polyisobutylene solutions in mixtures of polybutene and kerosine and in mixtures of polybutene and toluene have been investigated using an extensional rheometer based on the principle of droplet stretching. Single exponential stretching functions have been applied and the resulting non-idealities of the experiment will be addressed. The results show that the transient Trouton ratio for different values of the applied axial extension rate between 1 and 6 s −1 scales with the strain for values larger than 3. The magnitude of non-ideal stretching effects on the Trouton ratio for strain values up to 3 is found to be fairly small. For strains larger than 5.5 the onset of steady-state values of the extensional viscosity is found, but true steady-state values are not obtained below the maximum strain of 7.2. The measurements provide an indication for increasing plateau values with increasing concentration and with increasing molecular weight of the polymer. The results also show that the relaxation after stretching up to a specific axial strain value with different extension rates scales with the product of the extension rate at the start of the relaxation process and the time.

Rheology of low nematic transition temperature thermotropic liquid crystalline copolyester HBA/HQ/SA

Abstract: The rheology of a liquid crystalline copolyester of hydroxybenzoic acid, hydroquinone, and sebacic acid (HBA/HQ/SA copolyester) was studied on both a rotational and a capillary rheometer. DSC studies show that the copolyester has a crystal-mesophasic and a broad mesophasic-isotropic transition at 170°C and 220°C. Optical texture observations show the mesophase is characterized by line defect textures, which are characteristic of a nematic structure. At 220°C, both isotropic and nematic phases coexist with the latter being the major. As temperature reaches 250°C, a clear dominance of isotropic phase is observed. At this temperature, the nematic phase of irregular shapes randomly disperses within the isotropic matrix. Subsequent rheological studies were thus conducted in crystal/nematic biphase, single nematic phase, nematic/isotropic biphase, and the near single isotropic phase. Dynamic strain sweep measurements show that a linear viscoelastic region exists at all temperatures tested. The maximum strain amplitude for the linear viscoelastic region is found to be highly structure dependent ; it is >100% in the nematic phase, ∼20% in the biphases, and only about 5% in the isotropic phase. The concurrence of curves obtained at different temperatures in a Cole-Cole plot of G' vs. G indicates similar structures in the nematic phase and biphases. Measurements of steady shear viscosity using a rotational rheometer and a roundhole capillary rheometer show that in the nematic phase the copolyester behaves as a shear thinning fluid for a wide shear rate range of 1 ∼ 10,000 s -1 , in which the power law index is about 0.6 ∼ 0.8, and the viscosity is 1 s -1 .

Modelling thixotropy using a novel structural kinetics approach: basis and application to a solution of iota carrageenan

Abstract: A new methodology for modelling the thixotropy of inelastic fluids is described and its application demonstrated. In order to validate the model, data from two measurement protocols are compared and shown to give coherent results. The structural kinetics approach uses a reduced structural variable, X, which has values between zero, when the structure is totally broken down, and one, when the structure is complete. The new approach is applied to the thixotropy of a solution of iota carrageenan, a well known food hydrocolloid. Solutions were sheared before testing in order to eliminate their viscoelasticity. The measurement protocols used were: (1) stress jumps from above and below each shear stress tested, and (2) triangular ramp-up/ramp down stress profiles, often called hysteresis loops. The first series ofmeasurements was used to determine the equilibrium structure and kinetics ofstructural build-up and breakdown at each shear stress. This data completely determines the parameters in the thixotropic model and allows prediction of the sample's response to any stress/time profile. Without using any additional parameters, this model was then used to predict the shape of the hysteresis loops in the second series of measurements. The agreement between the predictions and measurements was very good, provided that the experimental data was first corrected for the effects of inertia and the finite gap-width of the rheometer.

Transient extensional viscosity from a rotational shear rheometer using fiber‐windup technique

Abstract: The fiber‐windup technique, developed by Macosko and Lorntson [SPE Tech. Papers 19, 461–467 (1973)] for rotational shear rheometers, was re‐examined to obtain transient uniaxial extensional viscosity data. Macosko and Lorntson used a special force transducer to measure the tension in the filament being pulled. In contrast, in this study commercial rotational shear rheometers (Rheometrics Mechanical Spectrometer‐800 and Rheometrics Fluids Spectrometer‐II) with torque transducers were used to obtain the extensional viscosity. One end of the sample is clamped, while the other end is wound around a drum at a constant rotational speed, to achieve a given extension rate. Tests were conducted with a polyisobutylene sample at room temperature to verify reproducibility and were compared with published studies. Results for polyisobutylene and IUPAC‐X low density polyethylene were found to be in good agreement with published results. Hencky strains of up to 6 could be achieved. The technique can provide valuable ext...

Surface viscoelasticity of phospholipid monolayers at the air/water interface

Abstract: The surface viscoelastic properties of monolayers of two phospholipids DPPC (L-α-dipalmitoylphosphatidylcholine) and DMPE (L-α-dimyristoylphosphatidyl-ethanolamin), at the air-water interface have been investigated. Two techniques were used for the investigation. One involved use of an interfacial shear rheometer (torsion pendulum apparatus ISR1), to provide measurements of the shear viscosity ηs as a function of surface pressure, and the second, a modified LB trough with an oscillating barrier to generate periodie dilation and compression so as to measure the dilational elastic modulusE as a function of surface area.

Effect of molecular structure on rheological and crystallization properties of polyethylenes

Abstract: This article describes the development of reliable techniques to measure the isothermal crystallization rates (ICR) under quiescent as well as under small amplitude, oscillatory shear conditions. Quiescent crystallization rates were obtained using a differential scanning calorimeter. Those under small amplitude shear were obtained using Rheometrics rheometers. It is shown how a small amount of long-chain branching in high-density polyethylene homopolymer (HDPE) dramatically influences rheological properties and enhances ICR. For these HDPEs, the rate increases with the increase in long-chain branching. The general application of isothermal crystallization studies, however, should be done with great caution. This is because the fundamentals of isothermal crystallization require that it be done on the basis of a fixed undercooling with respect to the equilibrium melting temperature. Such a temperature is ill-defined for the commercial polymers having broad molecular weight distribution (MWD). Nonetheless, a practical procedure is outlined wherein the melting curve of a previously isothermally crystallized sample is used as a substitute for judging the equilibrium melting point and in deciding the selection of a proper crystallization temperature. Even this new procedure may not be applicable for polymers having heterogeneous short-chain branching distribution. © 1996 John Wiley & Sons, Inc.