Showing papers on "Rheometer published in 1985"

The yield stress myth

Abstract: New experimental data obtained from constant stress rheometers are used to show that the yield stress concept is an idealization, and that, given accurate measurements, no yield stress exists. The simple Cross model is shown to be a useful empiricism for many non-Newtonian fluids, including those which have hitherto been thought to possess a yield stress.

The rheology of concentrated suspensions of spheres in simple shear flow by numerical simulation

Abstract: The newly developed simulation method known as Stokesian dynamics is used to investigate the rheological behaviour of concentrated suspensions. Both the detailed microstructure (e.g. pair-distribution function) and the macroscopic properties are determined for a suspension of identical rigid spherical particles in a simple shear flow. The suspended particles interact through both hydrodynamic and non-hydrodynamic forces. For suspensions with purely hydrodynamic forces, the increase in the suspension viscosity with volume fraction ϕ is shown to be caused by particle clustering. The cluster formation results from the lubrication forces, and the simulations of a monolayer of spheres show a scaling law for the cluster size: lc ∼ [1 − (ϕ/ϕm)½]−1, where ϕm is the maximum volume fraction that can shear homogeneously. The simulation results suggest that the suspension viscosity becomes infinite at the percolation-like threshold ϕm owing to the formation of an infinite cluster. The predicted simulation viscosities are in very good agreement with experiment. A suspension with short-range repulsive interparticle forces is also studied, and is seen to have a non-Newtonian rheology. Normal-stress differences arise owing to the anisotropic local structure created by the interparticle forces. The repulsive forces also reduce particle clustering, and as a result the suspension is shear-thickening.

Role of membrane viscosity in the orientation and deformation of a spherical capsule suspended in shear flow

Abstract: Red blood cells or artificial vesicles may be conveniently represented by capsules, i.e. liquid droplets surrounded by deformable membranes. The aim of this paper is to assess the importance of viscoelastic properties of the membrane on the motion of a capsule freely suspended in a viscous liquid subjected to shear flow. A regular perturbation solution of the general problem is obtained when the particle is initially spherical and undergoing small deformations. With a purely viscous membrane (infinite relaxation time) the capsule deforms into an ellipsoid and has a continuous flipping motion. When the membrane relaxation time is of the same order as the shear time, the particle reaches a steady ellipsoidal shape which is oriented with respect to streamlines at an angle that varies between 45° and 0°, and decreases with increasing shear rates. Furthermore it is predicted that the deformation reaches a maximum value, which is consistent with experimental observations of red blood cells.

Experimental evaluation of a dynamic yield criterion for granular fluid flows

Abstract: Data obtained in an annular shear cell are interpreted to evaluate a dynamic Coulomb yield criterion at the boundary separating flowing and stationary grains in a steady granular fluid shear flow. The ratio of the shear stress to the normal stress is nearly constant at the depth of no motion, suggesting a dynamic balance between the mobilizing effect of the applied shear stress and the stabilizing effect of the normal stress which results from the immersed weight of the moving grains. We suggest that the yield criterion can be applied to predict the immersed weight of grains undergoing bed load transport.

Shear Flow Properties of Semiconcentrated Fiber Suspensions

Abstract: Measurements are reported for the shear stress of a semiconcentrated suspension of rigid fibers in a Newtonian solvent in the startup of steady shear flow. There are n fibers each of length L per unit volume. A variety of different volume fractions of fibers was used; the fiber aspect ratio was varied as well. It is found that the viscous resistance of a randomly oriented fiber suspension is roughly (nL3) times the resistance observed when the particles lie in the planes of shear. The transient shear stress measurements show the rheological properties depend on the total strain and not on shear rate or time separately. Experiments were done in torsional flow between parallel plates separated by distance H to determine the effect of H/L on the measured properties. It is found that there is no effect for H/L>1. Two different models are presented to account for wall effects for H/L<1. Good agreement is found between computed and measured values.

High‐Shear Viscometry with a Rotational Parallel‐Disk Device

Abstract: A novel technique for measuring viscosity at high shear rates is described. The technique is an extension of a well‐established method, parallel‐disk viscometry, which has been limited traditionally to low shear rates ( 50,000 s−1) to be attained without disrupting the torsional flow field, and thus controlled measurements of viscosity could be made at those shear rates. The presence of flow disturbances such as shear heating, surface fracture, and radial migration, common impediments in high‐shear measurements, can be readily detected with a thixotropic loop program, and the effect of such disturbances can be separated from the actual rheological response of the fluid. Data are presented on some well‐defined fluids, both Newtonian and shear‐thinning, and the limitations of this technique are discussed.

Rheological Properties of Liquid Crystalline Copolyester Melts. II. Comparison of Capillary and Rotary Rheometer Results

Abstract: In an earlier paper capillary rheometer results were presented for two liquid crystalline copolyesters consisting of 60 mole % and 80 mole % p‐hydroxybenzoic acid (pHBA) and polyethyleneterephthalate (PET) In this paper obtained by means of cone‐and‐plate (CP) and parallel plate (PP) attachments of a Rheometrics Mechanical Spectrometer are compared with the capillary data In particular, it is observed that over the range of conditions studied, viscosity data obtained in the CP and PP agree with those obtained by means of the capillary rheometer Furthermore, there is no dependence of the viscosity on geometric or dimensional factors as is observed for low molecular weight liquid crystals under some conditions Although the viscosity is still shear rate dependent for shear rates as low as 5×10−2 sec−1, it is shown by stress relaxation experiments following the cessation of steady shear flow that no significant yield stresses exist The primary normal stress difference (N1) for the 60 mole % pHBA/PET copo

Constant Viscosity Elastic Liquids

Abstract: The rheology of non‐shear thinning elastic fluids is re‐examined to clarify some of the misconceptions that exist in regard to their behavior. It is demonstrated that these fluids are no more than dilute solutions, constructed by dissolving a high molecular weight polymer solute in a highly viscous Newtonian solvent. Organic and inorganic systems can be constructed and there is no magic recipe. The highly elastic non‐shear thinning fluid systems represent a class of real fluid behavior entirely consistent with molecular theory for dilute solutions and with the Oldroyd B constitutive equation. It is shown that the Oldroyd B constitutive equation is far superior to the convected Maxwell model for predicting the dynamic and steady shear properties of these materials. However the Oldroyd B model fails at higher shear rates and may be completely inadequate for representing the extensional properties of such materials. The paper concludes with some squeeze film flow experimental data for constant viscosity elas...

Nonlinear Shear Relaxation Modulus for a Linear Low‐Density Polyethylene

Abstract: The response of a polydisperse linear low‐density polyethylene to imposed step shear strains up to strains of 10 were measured. The measured stress is approximately factorable into a product of a time‐dependent term, which is just the linear relaxation modulus, and a strain‐dependent term, the damping function. The damping function can be fit by a simple analytic function derived from a molecular theory with a single ad hoc parameter ξ′; a larger value of ξ′ corresponds to more softening of the nonlinear modulus. A tabulation of ξ′ values from fits to experimental shear damping functions given here and published elsewhere indicates that ξ′ is decreased both by polydispersity and by long chain branching.

Shear and extensional flow of reinforced plastics in injection molding. I. Effects of temperature and shear rate with bulk molding compound

Abstract: Capillary flow studies on bulk molding compound (BMC) using an instrumented injection-molding machine are reported. The significance of extensional flow effects with fiber-reinforced materials is emphasized. The extensional flow behavior in converging dies is modeled, and a means of evaluating both extensional and shear viscosity from capillary flow data is proposed. Methods of correcting results for the effect of deformation heating are discussed. The shear and extensional flow behavior of BMC in the temperature region 18 to 58°C can be fitted to a simplified Arrhenius Law.

Rheological Properties of News Inks

Abstract: This work examines the effect of ink composition on steady‐state and time‐dependent behavior in simple shear, as well as on extensional flow behavior. The inks studied were formulated to represent a spectrum of compositions typical of North American news inks. Hydrocarbon pitch, a common dispersant, is found to lower the viscosity at low shear rates in simple shear and to cause a dramatic drop in the extensional stress in a uniaxial stretching test. The addition of a commercial resin commonly found in lithographic inks increased both the viscosity in simple shear and the extensional stress. With respect to time‐dependent behavior, not only is the duration of the shearing of an ink important but also the direction of repeated shear pulses and the time interval between such pulses. The first normal stress difference was found to be negative in certain shear rate ranges. The shear stress‐shear rate relationship of the inks examined has been successfully described by a four‐parameter model.

Rheological Properties of Slurry Fuels

Abstract: The rheological properties of four slurry fuels were studied and a variety of complex flow phenomena was observed. The slurry fuels contained ca. 50% by weight carbon black particles with an average particles size of 0.1 μm. In order to characterize the viscoelastic and plastic properties of the slurry fuels, stress growth and steady‐state experiments were performed over a wide range of shear rates. The slurry fuels were shear thinning under most conditions and were shear thickening at extreme conditions. They exhibited thixotropic behavior, with the degree of thixotropy depending on the specific slurry. Most of slurries had a yield stress ranging from 2 to 110 dyn/cm2. For the slurries which had a large yield stress, stress overshoot was observed. The stress overshoot increased with increasing shear rate. The maximum stress overshoot occurred at lower shear strains, the higher the shear rate. Temperature effects on the shear viscosity and yield stress were also studied. An unusual temperature dependency ...

Rheology of polymer melts in high shear rate

Abstract: Little is known of the rheology of polymer melts in the high shear rate up to 106 s−1 or more. A specially designed high-shear-rate rheometer was developed, by which the rheology of polymer melts for shear rates up to 108 s−1 can be investigated. Two non-Newtonian regions and a transition or the second Newtonian region were observed in the wide range of shear rates up to 107 s−1. The observed flow curves for various polymer melts are classified into three typical patterns. One is the flow curve typically shown of high-density polyethylene in which a clear second Newtonian region appears after the first non-Newtonian region. The second is the typical flow curve of polystyrene in which a “transition region” appears instead of the second non-Newtonian region. The third is the flow curve shown of acrylonitrile-styrene copolymer, which exhibits behavior between the two types. A generalized flow curve is proposed to explain the observed flow behaviors of various polymers over a wide range of shear rates. The flow behavior in high shear rate results from high orientation and scission of polymer molecules.

Shear elasticity and viscosity in colloidal crystals and liquids

Abstract: We have measured the shear elastic constants of colloidal crystals using a standing cylindrical resonance technique and the viscosity with a variety of rheometers. The elastic constants are well described by Debye-Huckel theory with a renormalized charge. The crystals melt upon addition of electrolyte and the resulting liquid has a viscosity which is strongly dependent upon the electrostatic interactions between the charged colloids. Extrapolation of the elastic constants into the liquid regime, together with a phenomenological relaxation time, allow us to predict the viscosity of the liquid. We also observe a transition from solid to liquid, "shear melting", upon application of sufficiently high stress. This transition is accompanied by an increase in viscosity in going from solid to liquid.

Wall Effects for Dilute Polymer Solutions in Arbitrary Unidirectional Flows

Abstract: General rheological results for a dilute solution of dumbbell model macromolecules in general unidirectional flows in the presence of plane walls (z=±d) are obtained. Quite generally, we show that all three normal stresses are unequal and that the second normal stress is independent of transverse position z. For simple shear flow a shear viscosity and a first normal stress coefficient result, which are independent of the strength of the flow. For channel flow within a narrow channel a first normal stress coefficient cannot be defined, the reason being that a nonzero flow‐induced first normal stress prevails at the channel axis, where the local shear rate is zero. The results are valid no matter what v(0), the actual center line velocity, is. This implies that, as in the shear flow case, the thickness of wall layers is independent of the strengths of the flow field.

Flow of viscoelastic fluids between plates rotating about distinct axes

Abstract: We discuss the flow of BKZ fluids in an orthogonal rheometer. Some analytical results are proved, and numerical solutions are obtained for the Currie model. These solutions show a boundary layer behavior at high Reynolds numbers and the possibility of discontinuous solutions or nonexistence at high Weissenberg numbers.

On-line rheometer for fermentation liquids

Abstract: INTRODUCTION An impeller measuring system for determining the rheologica] properties of non-homogeneous fermentation liquids has been developed (I). The applied measuring technique was verified using samples from a fermentation broth of Aureobasidium pullulans. In view of the importance of rheologieal properties as process variables, further investigations were carried out in an effort to develop an on-line rheometer for continuous measurements. The construction of the rheometer is based on the same measuring principles as described previously, as is the calibrating procedure (I).

Rheological and Pipeline Flow Behavior of Corn Starch Dispersions

Abstract: The rheological and pipeline flow behavior of corn starch dispersions has been studied. It was found that the flow characteristics of these systems gave dilatant (shear thickening) behavior until a critical shear rate was attained at which point Newtonian behavior commenced. This critical shear rate, as well as the behavior of power law index and consistency index were found to be functions of percent volume concentration. Additionally, the behavior of these rheological parameters was explained on the basis of an existing theory describing shear thickening. Pipeline flow data confirmed that dilatant (shear thickening) fluids followed the Metzner‐Reed friction factor relationships in the laminar region. The range of behavior was extended to fluids having n values of 2.92 and volume concentrations of 40.0 percent. Finally, it was found that transition and turbulent flow could not be attained for dilatant (shear thickening) corn starch suspensions. This principally occurred because the critical shear rates y...

A Rheometer for Characterizing Polymer Melts and Suspensions in Shear Creep and Recovery Experiments

Abstract: A rheometer capable of performing shear creep and recovery experiments has been developed by Rheometrics, Inc. The test geometry can be parallel plates, cone‐plate, or concentric cylinders. The torque is transmitted to the upper plate by a drag cup motor with low inertia and extremely smooth torque output. The driven shaft is supported both axially and radially by an air bearing of low friction assuring that all the torque is transmitted to the sample. Measurements of the angular displacement with a resolution of 10−5 rad over a complete revolution provide the output of strain. The lower torque limit after compensation for residual torques of the air bearing is ≃1 dyne cm. Results of creep and recovery experiments on polymer melts are presented. The viscosity and the steady‐state compliance, which are sensitive to small changes in molecular weight and molecular weight distribution, can easily be determined, thus making the rheometer a very suitable instrument for characterizing polymer melts. Finally, results on suspensions which exhibit yield stresses and thixotropic behavior are discussed, showing the potential of the creep and recovery experiments in characterizing coatings, inks, foods, etc.A rheometer capable of performing shear creep and recovery experiments has been developed by Rheometrics, Inc. The test geometry can be parallel plates, cone‐plate, or concentric cylinders. The torque is transmitted to the upper plate by a drag cup motor with low inertia and extremely smooth torque output. The driven shaft is supported both axially and radially by an air bearing of low friction assuring that all the torque is transmitted to the sample. Measurements of the angular displacement with a resolution of 10−5 rad over a complete revolution provide the output of strain. The lower torque limit after compensation for residual torques of the air bearing is ≃1 dyne cm. Results of creep and recovery experiments on polymer melts are presented. The viscosity and the steady‐state compliance, which are sensitive to small changes in molecular weight and molecular weight distribution, can easily be determined, thus making the rheometer a very suitable instrument for characterizing polymer melts. Finally, res...

Measurement of Fast Transient and Steady State Responses of Viscoelastic Fluids with a Sliding Cylinder Rheometer Executing Coaxial Displacements.

Abstract: A sliding cylinder rheometer (SCR) has been constructed for use with an existing materials test system (MTS) for the measurement of fast transient and steady‐state responses of viscoelastic fluids in simple shear. This MTS‐SCR combination capitalizes on the versatility and capability of the MTS programmable drive system, while the design of the SCR ensures the ideal simple shearing configuration. Experience with this new system indicates that SCR is not only less expensive to construct but also easier to use than the parallel‐plate rheometer (PPR). Instrumental compliance, material inertia, and mechanical lag in both the input and output components of the system are minimal. Reproducible shear stress data on a polyisobutylene‐in‐decalin solution have been obtained for various complicated flow programs.

Thixotropy of a coal water mixture

Abstract: The shear stress-shear rate behaviour of a thixotropic coal water mixture (CWM) was studied in coaxial rotating cylinder rheometer under a wide variety of rest and shear rate history. The transient stress data were used to construct equilibrium and constant-structure flow curves. These transient stress data, i.e. the stress relaxation data and the stress recovery data, were also used to evaluate the breakdown and buildup processes of the internal structure. A phenomenological thixotropy model which has the structural stress as a thixotropic structural parameter was proposed. This model can predict the thixotropic hysteresis loop precisely.

Shear modification of low density polyethylene

Abstract: A single screw extruder was used to impose different shear histories on a low density polyethylene with broad molecular weight distribution and high molecular weight tail that had very little long chain branching. This polymer exhibits relatively high melt elasticity and the viscoelastic properties of its melt are strongly affected by preshearing. Such changes are accomplished without significant changes in molecular weight distribution or chemical structure. Measured viscous and elastic properties of the melt are different from piston-driven and screw extruder capillary rheometers. Shear modification effects in single screw extruders are enhanced by decreasing melt temperature, increased screw rotation speeds, and higher screw compression ratios. Melt elasticity can be cycled between high and relatively low values, for the particular polymer, by annealing or shearing the polymer melt.

The Development of a True Biaxial Shear Tester

Abstract: To investigate the influence of the stress history on the yield limit a shear tester was developed, in which stresses or strains in x- and y-direction can be applied independently of each other. Since there are no shear stresses on the boundary surfaces of the sample these stresses are principal stresses and the Mohr stress circles are known for each state of stress. First results from shear tests with limestone show a significant influence of the different ways of consolidation on the yield limit, i. e. the flow properties of a bulk solid will depend on the stress history. The flow function, however, which is essential for silo design, proves to be invariable of the stress history.

Fractional Rates of Deformation

Abstract: An objective measure of fractional deformation rate is obtained by generalizing the order of the Rivlin‐Ericksen tensors from integer to real values. These extended measures of deformation rate are used to formulate a simple constitutive model for incompressible fluids. This model predicts a time‐dependent viscosity characteristic of antithixotropic fluids in steady shear flow. The constitutive model is contained in a theory for dissipative materials described by Goddard.

Foam Rheometer: Its Principle and Applications

Abstract: A Ithough polyurethanes are among the most versatile plastics available, the major portion of their production volume is processed as foams [1]. Among the foams the traditional volume leader continues to be flexible foam which principally is used for automotive applications and furniture. Rigid foam is primarily used for insulation. The research done on polyurethane foams has mostly concentrated on the chemical reactions between the components, and on morphology and mechanical properties of the final products. Rheological description of foaming systems has been limited to very empirical data such as cream time, initiation time, foaming pressure, rise time, rise height, and rise profile. This data can be obtained through experiments in a cup or by using MAX RiseRate System equipped with a pressure transducer [2,3]. However, this instrument has several serious drawbacks: