Showing papers on "Rheometer published in 1990"

Cone-and-plate rheometry of yield stress fluids. Study of an aqueous gel

Abstract: This work particularly focuses on the rheometric study of a physical gel exhibiting a yield stress. The measurements were carried out in a cone—plate configuration using two different types of rheometer working under controlled torque or under controlled velocity. Shear creep, constant shear rate, and stress relaxation tests have been performed. Measurements of apparent viscometric properties were conducted at the same time as observation of the strain field in the sample. Observing the strain field enables us to confirm the reliability of the interpretation of the results and also to estimate the true shear rate in the fluid. It is shown how the determination of shear rheological properties can be affected by anomalous phenomena such as fracture and slip at the wall. The influence of roughness of the tool surfaces and of evaporation shows up. The results presented in this study show how some rheometrical measurements of the yeild stress and the microstructural interpretations given, may be erroneous. Some recommendations are made in order to improve current rheometrical tests and their interpretation. A log—log graph with typical shear stress-shear rate measurements and their corresponding strain fields is given: it should be used as a guideline in yield stress fluids rheometry. In addition it is made clear that visual observation of the sheared sample is a key technique. A protection which completely eliminates evaporation is suggested. It is shown that the measurement of residual stress in stress relaxation tests may be a convenient means of determining the value of the yield stress.

The vane‐in‐cup as a novel rheometer geometry for shear thinning and thixotropic materials

Abstract: We have addressed the question of the suitability of the vane‐in‐cup as a rheometer geometry. A numerical simulation of this geometry was conducted for a power‐law fluid and the results compared with a similar study for a conventional bob‐in‐cup geometry. The comparison indicates that for a sufficiently shear‐thinning fluid (of shear‐thinning index less than 0.5) the fluid within the periphery of the vane blades is essentially trapped there and turns with the vane as a solid body. Calculation of the shear stress at the cup wall indicates that this quantity is equal in both geometries for a given rotational rate of the spindle. Thus the torque required to turn the spindle would be the same and identical flow curves would be predicted. This prediction was tested on two fluids thought to possess a yield stress: a 5.5% sodium carboxymethylcellulose (CMC) solution and a 4.2% Veegum PRO clay suspension. Equivalent flow curves were obtained at very low stresses/shear rates but a sudden, catastrophic viscosity loss was found for both fluids with the bob at shear rates which were still quite low. Such a loss was observed with the vane as well, but at much higher shear rates. It is suggested that this phenomenon is a form of apparent slip due to the formation of a thixotropic layer at the bob/vane surface. The much flatter stress profile obtained in the vane geometry is reasoned to postpone the formation of this layer. Rheological data obtained with the vane appear to be a faithful representation of these materials and show the absence of a true yield stress.

The Benefits of Basic Rheometry in Studying Dough Rheology

Abstract: Cereal Chem. 67(4):31 1 -317 Bread doughs are viscoelastic bodies with explicit, nonlinear shear thin- and temperature-dependent dough characteristics are discussed. In addining and thixotropic behaviors. The commonly used empirical and descrip- tion, attention is drawn to the feasibility of using basic rheometry in tive rheological methods determine the consistency and extensibility of the processing and product quality testing of flours. Finally a recording doughs by applying large deformation forces in a single-point measure- baking test is introduced and discussed as a promising method for moniment. They are therefore not suitable to describe dough flow properties. toring not only how dough changes structurally during heating and cooling Flow properties can be determined by basic Theological methods in both but also how those changes are influenced by various amounts and propdestructive and nondestructive applications. The use of a viscometer for erties of relevant flour constituents and flour additives. steady state flow and a rheometer for dynamic measurements of the strain Besides chemistry, rheometry is a necessary and powerful technique for explaining and predicting the quality of cereal foods. Consequently, it has found use in a wide range of practical and scientific studies addressing the needs of breeders, traders, processors, and, particularly, researchers. During the past 60 years, a variety of instruments based on various principles and techniques has been developed and applied to the study of dough rheology. Comprehensive reviews of the instrumentation and techniques used in food rheology have appeared recently (Van Wazer et al 1963, Rasper 1976, Voisey and DeMan 1976, Bagley 1983). Based on the principles employed, techniques have been classified as empirical, descriptive, and basic (or fundamental) (Scott Blair 1958). Although most of the important knowledge of and experience in dough rheology has been obtained by the use of basic rheometry (Muller 1975), the instruments and methods belonging to the empirical and descriptive classes have found more acceptance and widespread usage. Basic rheometry, applied to more complicated food systems such as dough, has been found to be laborious, time-consuming, and often incapable of producing simple answers (Muller 1973). These are the main reasons for the somewhat poorer acceptance of basic rheometry than its empirical counterpart.

A converging channel rheometer for the measurement of extensional viscosity

Abstract: The development of an extensional rheometer for polymer solutions is described. The test section is a converging channel through which a test fluid is pushed at high Reynolds numbers, and several pressure drops along the channel are measured with flush-mounted transducers. The high Reynolds numbers ensure a core flow of purely extensional motion and the channel is shaped to produce a constant rate of extension. Analysis of the stress field shows that the pressure drop is equal to the normal stress difference τzz - τrr in the core, plus other terms which are calculated. The calculations are based on an analytical solution for inelastic power-law fluids, and the calculations were verified by finite element computations. Extensional viscosity measurements were made for a solution of a copolymer of PMMA in an organic solvent. The data show that extensional viscosity increases with extensional rate to about the 3/2 power and, at a fixed extensional rate, extensional viscosity increases roughly as the total fluid strain.

Liquid crystal domains and thixotropy of filamentous actin suspensions.

Abstract: The thixotropic properties of filamentous actin suspensions were examined by a step-function shearing protocol. Samples of purified filamentous actin were sheared at 0.2 sec-1 in a cone and plate rheometer. We noted a sharp stress overshoot upon the initiation of shear, indicative of a gel state, and a nearly instantaneous drop to zero stress upon cessation of shear. Stress-overshoot recovery was almost complete after 5 min of "rest" before samples were again sheared at 0.2 sec-1. Overshoot recovery increased linearly with the square root of rest time, suggesting that gel-state recovery is diffusion limited. Actin suspensions subjected to oscillatory shearing at frequencies from 0.003 to 30 radians/sec confirmed the existence of a 5-min time scale in the gel, similar to that for stress-overshoot recovery. Flow of filamentous actin was visualized by polarized light observations. Actin from 6 mg/ml to 20 mg/ml showed the "polycrystalline" texture of birefringence typical for liquid crystal structure. At shear rates less than 1 sec-1, flow occurred by the relative movement of irregular, roughly ellipsoidal actin domains 40-140 microns long; the appearance was similar to moving ice floes. At shear rates greater than 1 sec-1, domains decreased in size, possibly by frictional interactions among domains. Eventually domains flow in a "river" of actin aligned by the flow. Our observations confirm our previous domain-friction model for actin rheology. The similarities between the unusual flow properties of actin and cytoplasm argue that cytoplasm also may flow as domains.

Flow of ordered latex suspensions: yielding and catastrophic shear thinning

Abstract: The flow properties of ordered latex suspensions are investigated using a constant-stress rheometer. The suspensions, studied over a volume-fraction range 0.38–0.62 and at ionic strength 10–2, 10–3 and 10–4 mol dm–3, show four distinct responses to increasing stress. At low stresses the suspensions behave like elastic solids holding a strain for times greater than 104 s and displaying complete strain recovery upon release of stress. At higher stresses, the suspensions show non-recoverable strain upon application of the stress, but steady-state rates of deformation are not reached for times greater than 104 s. Over a narrow stress range, the suspensions begin to flow reaching a steady-state rate of deformation in times less than 500 s. The stress becomes independent of steady-state shear rate in this region suggesting the onset of flow occurs at a dynamic yield stress which marks the onset of the low-stress, shear-thinning region. The high-stress, shear-thinning region is entered at a critical stress where the steady-state viscosity drops 1–3 orders of magnitude. Scaling of the long-time zero-stress modulus, yield stress and critical stress on volume fraction and suspending medium ionic strength are discussed in light of structural studies reported in the literature.

The shear and extensional flow properties of M1

Abstract: Steady shear, oscillatory shear and extensional flow experiments are carried out on the test liquid designated M1. The steady shear experiments cover a shear rate range up to 8 × 104 s−1 and the oscillatory shear experiments involve frequencies up to 40 Hz. Three different extensional flow experiments are involved, namely contraction flow, spinning and the open-syphon technique. A search is made for internal consistency between the various rheometrical tests and the continuum mechanics requirements in the limit of low shear rates and frequencies are confirmed.

NMR investigation of chain deformation in sheared polymer fluids

Abstract: The use of NMR to monitor the behavior of entangled polymers subjected to shear, is proposed as a new experimental technique for studying the microscopic response to shear-induced anisotropy in bulk polymers. A cone and plate rheometer designed to operate in a standard NMR probe is described. Sheared melts of poly(isobutylene) and poly(dimethylsiloxane) are studied. A shear-induced phase transformation in poly(vinyl methyl ether)-water mixtures is also demonstrated

Flow of Newtonian liquids in opposed‐nozzles configuration

Abstract: Flow into or out of two opposed nozzles immersed in a liquid offers a means of measuring an extension‐dominated response of a liquid up to extension rates as high as 1000 s−1 and more. We evaluate the concept as it applies to Newtonian liquids, by solving the Navier–Stokes system for complete fields of stress and extension rate. We examine possibilities of inferring nearly extensional stress over some internal area from a measured force (torque, actually) on one nozzle assembly, of resolving that stress into its pressure and viscous parts, and of having nearly uniform extension rate over the same area so that an extensional viscosity can be reliably deduced; the choice of the area and the control volume of which it is part proves pivotal. This examination leads to an instrument design optimization problem which we solve approximately for the near‐best nozzle bore, separation, and exterior shape, and for the near‐best choice of control volume. Results are compared with experimental findings and implication...

Viscosity of concentrated suspensions : an approach based on percolation theory

Abstract: A different approach to study the viscosity of concentrated suspensions is proposed. It is based on percolation theory. It is suggested that at the critical concentration of fillers, φ c a macroscopic interconnected cluster (secondary structure) of fillers is formed. This cluster extends from one wall of the rheometer ot the other. It is the properties of this macroscopic cluster which determine the viscosity for φ≥φ c

Shear and extensional flow of polyacrylamide solutions

Abstract: As part of an EEC Science Stimulation programme on extensional viscosity two major conferences were organised on the subject. The second of these was devoted to the results obtained on a standard fluid, M 1. The data obtained in shear flow was remarkably consistent from laboratory to laboratory. Extensional flow results presented quite a different picture. Using a series of nonequilibrium techniques such as the spinline rheometer, opposing jet, falling drop and converging flow, extensional viscosity results were obtained which differed by as much as two to three orders of magnitude. Nevertheless, it was apparent that consistancy did exist between similar techniques. It is in the context of this information that the measurements described below have been made. The shear and extensional flow properties of partially ionised polyacrylamide in solution at concentrations ranging from 5 ppm were measured. The method of solution preparation was found to have a profound effect on the behaviour of the solutions in shear flow. The influence of salt concentration and pH was investigated and is discussed in the context of molecular shape in solution. Extensional flow measurements, using the spinline rheometer, show that the solutions are strongly strain thickening even at concentrations as low as 5 ppm. These results are considered in the light of polymer entanglement and association in the strong flow field.

Measurement of the extensional viscosity of M1 in a converging channel rheometer

Abstract: Measurements of the extensional viscosity of M1 were made in a converging channel rheometer. The high viscosity of the fluid made it necessary to operate the instrument at Reynolds numbers lower than it was designed for, but analysis of the stress field showed that the normal stress difference in extension could still be detected from the measurement of pressure drop. Measurements of ηE were made at rates of extension up to 190 s−1, for temperatures from 21 to 40 ° C, and at three locations along the channel, i.e. for increasing strain. The data show that ηE depends strongly on strain and increases approximately linearly with the rate of extension ∈ . .

Rheology of polycarbonate/linear low density polyethylene blends

Abstract: The flow behavior of linear low density polyethylene blended with polycarbonate (LLDPE/PC) was studied at 245°C using an Instron Capillary Rheometer and a Rheometrics Mechanical Spectrometer. The capillary measurements were repeated several times for each crosshead speed and capillary. The averaged values were corrected for shear heating as well as the pressure, entrance-exit, and power-law fluid effects. In spite of the utmost care, blend results were erratic with a standard deviation of 25 to 35 percent. Analysis of the capillary data suggested a telescopic flow with the lower viscosity component of the blend migrating toward the capillary wall. The experimental difficulties resulted from the flow and time induced variations of blend morphology. By contrast, the dynamic shear test results were found to be rapid and reproducible with a standard deviation for the complex viscosity of blends not exceeding four percent. The shear moduli of blends indicated the presence of an apparent (time dependent) yield stress, originating from interaction between domains of the dispersed phase. At frequencies exceeding a critical value, shear coalescence of the dispersed phase was observed near the rim of the rheometer plates.

Rheological characterization of the fluid M1 and of its components

Abstract: An opposing jet rheometer was used to measure the apparent elongational viscosity of fluid M1. The behaviour in simple shear was characterized by using different types of rotational rheometers. From the dynamic moduli in oscillatory shear the steady-state compliance and a characteristic relaxation time were calculated. The steady-state viscosity and the primary normal stress difference versus shear rate, as well as the temperature dependence of the zero shear viscosity, were measured. A study of flow instabilities becoming apparent above a critical shear stress was performed. The flow instabilities cause irreversible degradation. The molecular weight and the Mw/Mn ratio of the PIB component were deduced from measurements of the melt viscosity and the melt compliance, respectively. The relaxation time of the solvent could be evaluated both from oscillatory shear and normal stress measurements taking into account fluid inertia effects.

4 Rheology of Well Cement Slurries

Abstract: Publisher Summary This chapter discusses the rheology of well cement systems. The chapter presents a review of the relevant rheological models and concepts, followed by a discussion specific to particle-laden fluids. The rheological behavior of a cement slurry is optimized effectively to remove drilling mud from the annulus. The appropriate cement slurry design is a function of many parameters, including the wellbore geometry, casing hardware, formation integrity, drilling mud characteristics, presence of spacers and washes, and mixing conditions. The accurate and reliable rheological characterization of oil well cement slurries presents a problem for the industry. The coaxial cylinder viscometer, which is commonly used for measuring the rheological properties of cement slurries can suffer from severe limitations as a result of particle migration, end effects, or slippage at the rheometer wall. Equations describing the flow of cement slurries in pipes and annuli have focused on two widely used rheological models-the power law and the Bingham plastic models.

Optical anisotropy of a thermotropic liquid‐crystalline polymer in transient shear

Abstract: A rheo-optical apparatus, based on a linear shear rheometer, has been constructed to study the deformation of liquid–crystalline polymers. This apparatus uses optical techniques such as flow birefringence, small-angle light scattering, and optical microscopic image analysis. The rheological responses were simultaneously measured under varying temperatures and deformation conditions. The modified Debye-Bueche equation for scattering, in the nonspherically symmetrical form, was adapted to analyze small-angle light-scattering data. The orientation correlation lengths, determined by this method, reveal the deformation mechanism in nematic melts. Flow birefringence results are in agreement with the proposed mechanism.

Elongational flow behavior of ABS polymer melts

Abstract: Elongational flow behavior of ABS polymer melts with various rubber‐particle contents was investigated by using a Meissner‐type uniaxial extensional rheometer. The effects of strain rate and rubber particles on the elongational behavior are discussed. A linear viscoelastic behavior was observed at sufficiently low strain rates e. The dependence of linear elongational viscosity η+E(t) on particle content is weak at short times but becomes stronger with time. Two types of mechanisms which cause the deviation of nonlinear viscosity function η+E(t,e) from η+E(t) at high strain rates in the short and long time regions were proposed. These deviations are respectively attributed to a volumetric resistance against the radial deformation of the samples because of the existence of particles, and an extension of matrix polymers in an elongational flow field.

On the influence of fluid inertia in oscillatory rheometry

Abstract: It is known that evaluating oscillatory rheological shear experiments in the usual way may lead to complete misinterpretation of the experimental data, especially for low viscosity fluids in the high frequency range. We used a modern type of oscillatory rheometer with a stiff transducer. When measuring Newtonian liquids artificial, second‐order fluid‐like data for the complex shear modulus were found. In order to explain those findings, it is essential to take into account the influence of fluid inertia. Accordingly a method, based on theoretical considerations, is suggested for the correction of experimental data for slightly elastic fluids in order to obtain the true viscoelastic material functions.

An investigation of the shear thickening and antithixotropic behavior of concentrated coal–water dispersions

Abstract: A study of the flow characteristics of a deflocculated, shear thickening, coal–water suspension indicated a significant antithixotropic behavior. The effect of applied shear rate, particle concentration, and shear zone dimensions were examined using a Couette flow viscometer. The dispersion was composed of refined coal particles with mass median diameter of 2.9 μm suspended in water with sufficient anionic dispersant. The rheological properties of eight dilutions of this suspension, from 47.7 to 54.5 vol %, were studied in detail. Shear stress was observed as a function of shear rate in ramp tests in which γ was increased from 10 to 1000 s−1, and as a function of time at several constant shear rates. All tests were conducted using three rotors to test the effect of annular gap width. The characteristics of the reversible, shear thickening with time phenomenon (antithixotropy) that occurs when sufficient shear rate is applied, is described in terms of three regions. Quantitative relationships that allow the determination of stress as a function of shear rate, particle concentration, and shearing time are developed for one of the rotors. An examination of the anisotropic nature of this phenomenon is also presented.

On the uniaxial extensional flow of polystyrene/polyethylene blends

Abstract: The uniaxial extensional flow behavior of polystyrene/low density polyethylene blends (PS/LDPE) was studied using a Rheometrics Extensional Rheometer, Model RER-9000, at 150°C within the range of the extension rates = 0.0005 to 1.0 (s−1). The measurements were repeated several times at each deformation rate to ascertain reliability. The effect of the suspending silicone oil was found to be negligible.

Flow birefringence studies in transient flows of a two roll mill for the test-fluid M1

Abstract: We have studied the test-fluid M1 in flows generated by a two-roll mill. These flows are linear, and two-dimensional, and the magnitudes of the strain-rates are greater than the vorticity. The polymeric solution is shown to degrade significantly, even for small values of the velocity gradient, as measured by the changes in the macroscopic relaxation time-scales. Consequently, the determination of macroscopic properties such as elongational viscosity based upon the overall pressure difference, or the total force as measured in a spin-line rheometer, may not be representative of the properties of the fresh, undegraded fluid. In this work, the degradation of the test-fluid has been studied as a function of the strength of the velocity gradient for different values of the ratio of the strain-rate to vorticity of the flow. Furthermore, the observed relaxation time-scales have been evaluated based on data for steady state flows, and several transient flow histories such as start-up and cessation of flows, and double-step flows. These flow histories provide us with some insight into the relaxation dynamics of the polymeric structure for different initial conformation states.

Rheological test apparatus and method using a helical screw rheometer

Abstract: A rheological test apparatus and method using a helical screw rheometer. The screw in the rheometer is driven by a hydraulic motor controlled by a proportioning valve. A pressure transducer provides a pressure signal at the outlet of the rheometer, and a solenoid valve is used to close the outlet downstream from the pressure transducer. In a test, fluid is flowed into the rheometer. With the screw stopped and the discharge closed, a static pressure is measured by the pressure transducer, and a computer means resets this to a zero reference value. The screw in the rheometer is then rotated at a constant speed with the discharge closed, and the dynamic pressure at the outlet is measured by the pressure transducer. Because the static pressure was zeroed, the dynamic pressure value is substantially equal to the differential pressure across the rheometer. The test is repeated for different speeds. In an alternate embodiment, the speed of the rheometer is varied over a period of time, and a series of dynamic pressure values are obtained which are substantially equal to a series of differential pressures across the rheometer for the corresponding speeds in the speed range. In both methods, the computer calculates, in response to the differential pressure values and speeds, the n' and k' parameters of a shear rate versus shear stress relationship of the fluid. Using these parameters, predictions of friction pressure loss and bottom hole treating pressure during a well fracturing treatment may be generated.