Showing papers on "Rheometer published in 1998"

Fresh concrete: A Herschel-Bulkley material

Abstract: Some preliminary results of an experimental program on the rheological behavior of fresh concrete are presented. In the rheological tests, performed with a plane-to-plane rheometer, it appears tha the relationship between torque and rotation speed is not exactly linear. The fresh concrete behavior is better described by the Herschel-Bulkley model: $$\tau = \tau \prime _0 + a \dot \gamma ^b $$ ; τ and $$\dot \gamma $$ are the shear stress and the strain gradient applied to the specimen, respectively. τ′0,a andb are three material parameters describing the concrete behavior. Among other advantages, this new description avoids the problem of negative yield stress encountered with the Bingham model.

Shear thickening in low-concentration solutions of wormlike micelles. I. Direct visualization of transient behavior and phase transitions

Abstract: Shear thickening of low-concentration solutions of wormlike micelles is investigated using simultaneous rheological and visualization measurements. Shear-induced structures (SISs) are directly visualized in transparent Couette cells using a laser light scattering technique similar to dark-field microscopy. From these measurements, four different regimes of behavior are identified. In regime I, which occurs below a critical shear stress σc, the shear rate increases monotonically with stress and no shear thickening or SISs are observed. In regime II, which occurs for stresses greater than σc but less than σs, SISs nucleate inhomogeneously and grow from the inner cylinder of the Couette cell. In this regime, the steady state shear rate initially decreases with increasing stress and then increases again as the stress is raised. The steady state in regime II is characterized by two coexisting states separated by a cylindrical interface (concentric with the Couette cylinders). Near the inner cylinder, viscous S...

Entanglement, Elasticity, and Viscous Relaxation of Actin Solutions

Abstract: We have investigated the viscosity and the plateau modulus of actin solutions with a magnetically driven rotating disk rheometer. For entangled solutions we observed a scaling of the plateau modulus versus concentration with a power of 7/5. The measured terminal relaxation time increases with a power 3/2 as a function of polymer length. We interpret the entanglement transition and the scaling of the plateau modulus in terms of the tube model for semiflexible polymers.

Rheology of Sulfonated Polystyrene Solutions

Abstract: We report the apparent viscosity of aqueous solutions of a well-characterized synthetic polyelectrolyte, the sodium salt of polystyrene sulfonate. Using two rheometers we measure the apparent viscosity over more than five decades of shear rate to determine the Newtonian viscosity and the onset of shear thinning, which is inversely proportional to the relaxation time. We study five decades of polyelectrolyte concentration, from the dilute to the entangled regimes, and three decades of added salt (NaCl) concentration, on a single polyelectrolyte sample. Much of the viscosity data in the literature are shown to be in the shear thinning regime and are not indicative of the Newtonian viscosity. Our data for viscosity and longest relaxation time are qualitatively consistent with a recent scaling theory. There are systematic deviations from this simple theory, including the failure of the well-established Fuoss law, which is shown to be obeyed by the apparent viscosity only at higher shear rates.

The Yielding of Waxy Crude Oils

Abstract: The yielding process of statically cooled waxy crude oil was examined in detail. Three direct measurementsa controlled stress test, a creep−recovery test, and an oscillatory testwere employed using a controlled stress rheometer to investigate the yielding of two distinctly different statically cooled waxy crude oils. The results showed that yielding of waxy crude oil occurs by an initial elastic response, followed by viscoelastic creep and a final fracture. A model with an elastic-limit yield stress, a static yield stress, and a dynamic yield stress was introduced to describe the yielding process. The three yield stresses were determined by means of different techniques. The effect of the time scale in different measurements on the three yield stresses was studied. It was shown from both the creep−recovery and oscillatory tests that the elastic-limit yield stress was independent of the time scale. The static yield stress was found to be dependent on the time scale in the three tests. The dynamic yield str...

Slip velocity and slip layer thickness in flow of concentrated suspensions

Abstract: The rheological characterization of highly filled suspensions consisting of a Newtonian matrix (hydroxyl-terminated polybutadiene), mixed with two different sizes of aluminum powder (30% and above by volume) and two different sizes of glass beads (50% and above by volume), was performed using a parallel disk rheometer with emphasis on the wall slip phenomenon. The effects of the solid content, particle size, type of solid particle material, and temperature on slip velocity and slip layer thickness were investigated. Suspensions of small particles of aluminum (mean diameter of 5.03 μm) did not show slip at any concentration up to the maximum packing fraction. However, suspensions of the other particles exhibited slip at the wall, at concentrations close to their maximum packing fraction. In these suspensions, the slip velocity increased linearly with the shear stress, and at constant shear stress, the slip velocity increased with increasing temperature. The slip layer thickness increased proportionally with increasing size of the particles for the glass beads. Up to a certain value of (filler content/maximum packing fraction), ϕ/ϕm, the slip layer thickness divided by the particle diameter, δ/DP, was 0, but it suddenly increased and reached a value that was independent of ϕ/ϕm and the temperature. On average, the ratio of δ/DP was 0.071 for aluminum and 0.037 for glass beads. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 515–522, 1998

Numerical simulation of extensional deformations of viscoelastic liquid bridges in filament stretching devices

Abstract: Large extensional deformations of viscoelastic fluid columns in filament stretching rheometers are studied through numerical simulations up to Hencky strains of greater than e=4. The time-dependent axisymmetric calculations incorporate the effects of viscoelasticity, surface tension, fluid inertia, plus a deformable free surface and provide quantitative descriptions of the evolution in the filament profile, the kinematics in the liquid column and the resulting dynamic evolution in the viscous and elastic contributions to the total stress. In addition to investigating the variation in the apparent Trouton ratio expected in experimental measurements using this new type of extensional rheometer, we also investigate the generic differences between the response of Newtonian and viscoelastic fluid filaments described by the Oldroyd-B model. For small strains, the fluid deformation is governed by the Newtonian solvent contribution to the stress and the filament evolution is very similar in both the Newtonian and viscoelastic cases. However, in the latter case at large strains and moderate Deborah numbers, elastic stresses dominate leading to strain-hardening in the axial mid-regions of the column and subsequent drainage of the quasi-static liquid reservoir that forms near both end-plates. These observations are in good qualitative agreement with experimental observations. For small initial aspect ratios and low strains, the non-homogeneous deformation predicted by numerical simulations is well described by a lubrication theory solution. At larger strains, the initial flow non-homogeneity leads to the growth of viscoelastic stress boundary layers near the free surface which can significantly affect the transient Trouton ratio measured in the device. Exploratory design calculations suggest that mechanical methods for modifying the boundary conditions at the rigid end-plates can reduce this non-homogeneity and lead to almost ideal uniaxial elongational flow kinematics.

Human blood shear yield stress and its hematocrit dependence

Abstract: Human blood cells build a percolating physical gel all over the sample when at rest. This gel is progressively broken when it is continuously sheared in the bulk. It can slip at the wall, a phenomenon that depends on the roughness of the surface. Hence, smooth and rough walls were used to investigate the rheometrical shear properties of blood. A Couette-type rheometer with cylindrical walls allowed the shear rate to be varied in the range 10−3–10 s−1 and the hematocrit in the range 0.53–0.95. Calibration was performed with standard silicon oil. The stress measured at low shear rates with rough walls seemed, indeed, to tend to a constant yield stress value. The value of the shear stress at the shear rate of 10−3 s−1 was taken as a realistic approximation of the yield stress of blood. This yield stress was measured for different values of cell concentration. Data were fitted and showed to be proportional to the cube of the concentration over the range of hematocrit studied. However, variability between the ...

Time-periodic flow induced structures and instabilities in a viscoelastic surfactant solution

Abstract: Using various optical and mechanical techniques we report on a shear induced structure (SIS) that results in instabilities during flow. The solution in this study is an equimolar solution of cetylpyridinium chloride and sodium salicylate. In this investigation we further probe the hypothesis of Rehage et al. that an SIS occurs [1] . We find that not only does an SIS occur but as the solution is subjected to stronger flows it becomes turbid before forming ring-like structures which alternate in intensity both spatially and temporally. Since the rings alternate periodically it is evident that the flow not only forms these structures but is also capable of destroying them as well. Simultaneously measuring birefringence and mechanical properties we observe the same periodic oscillations in the birefringence. On the other hand these ring-like structures are present in the parallel plate, cone and plate, and Taylor Couette flow cells. These rings are similar in appearance to those seen for elastic instabilities in Boger fluids seeded with mica flakes. In a constant stress rheometer the shear rate exhibits overshoots and fluctuations again resembling the behavior of elastic instabilities in Boger fluids.

Nonlinear shear and extensional rheology of long-chain randomly branched polybutadiene

Abstract: We present nonlinear shear and uniaxial extensional measurements on a series of polybutadienes with varying amounts of long-chain, random branching. Startup of steady shear experiments is used to evaluate the damping function of the melts. The damping function is found to show a trend toward decreased dependency on strain with increasing branching content. Interior chains, which are believed to be responsible for changing the damping function, are calculated to comprise less than 3 wt % of the melt. Extensional measurements are used to investigate the role of branching in strain hardening. We show that samples with increased branch contents do show larger deviations of the transient Trouton ratio from the linear viscoelastic limit of three. However, we also show that the extensional data can be fit using parameters determined solely by the shear measurements. Furthermore, we show that the changes in the damping function seen in shear have little impact on extensional behavior. The extensional behavior of the melt is found to be most affected by changes in the relaxation spectra which can result from both branching and increases in the high end of the molecular weight distribution. This statement runs contrary to the often expressed view that strain hardening behavior in extension is exclusively produced by branching.

Deformation of a Capsule in a Spinning Drop Apparatus

Abstract: This paper studies the rheological properties of ultrathin aminomethacrylate membranes that are synthesized at the interface between oil and water. We have measured the kinetics of the cross-linking reactions, the zero shear modulus, and the nonlinear rheological properties of those two-dimensional networks. The measurements were performed at a planar interface using a special surface rheometer. Once stabilized, the membranes exhibit rubber-elastic properties. In addition, we have formed small emulsion droplets using the same type of interfacial polymerization. In order to get more information on the mechanical properties of the resulting microcapsules, we have developed a new experimental technique that consists in measuring the deformation of a capsule in a spinning drop apparatus. A theoretical analysis of the deformation process allows an interface apparent elastic Young modulus to be calculated. From a comparison of the surface shear modulus with the Young modulus, it is then possible to estimate the surface Poisson ratio, which is here approximately zero. These results point to the existence of area compressible anisotropic membranes.

Using instrumental inertia in controlled stress rheometry

Abstract: We describe a new method for characterizing the non-linear behavior of complex fluids at both small and large deformations. For creep measurements, we use the coupling between the instrumental inertia and the material‘s elasticity to follow the rheological behavior of a solution of iota carrageenan both above and below the yield stress. It is shown that this coupling selectively excites one particular frequency of the relaxation spectrum. An analytical calculation is used to quantify the non-linear behavior near the yield stress. The “free“ oscillations observed during the first few seconds allow us to choose the most appropriate mechanical model. Comparison with experiment shows that even above the yield stress, a linear model can still give independently reliable information about the changes in each element of the mechanical model. A comparison of free and forced oscillations in controlled stress rheometry shows both experimentally and theoretically the conditions under which the use of free oscillations is advantageous.

Can extensional viscosity be measured with opposed-nozzle devices?

Abstract: Opposed-nozzle devices are widely used to try to measure the extensional viscosity of low-viscosity liquids. A thorough literature survey shows that there are still several unanswered questions on the relationship between the quantity measured in opposed-nozzle devices and the “true” extensional viscosity of the liquids. In addition to extensional stresses, opposed nozzle measurements are influenced by dynamic pressure, shear on the nozzles, and liquid inertia. Therefore the ratio of the apparent extensional viscosity that is measured to the shear viscosity that is independently measured is greater than three even for Newtonian liquids. The effect of inertia on the extensional measurements is analyzed by computer-aided solution of the Navier-Stokes system, and by experiments on low-viscosity Newtonian liquids (1 mPa s

Yield stress measurements of aqueous foams in the dry limit

Abstract: This paper reports measurements of yield stress of aqueous foams approaching the dry foam limit using a pendulum device. Traditionally, the vane rheometer has been used to measure the yield stress in liquids that exhibit wall slip. However, using the simple and inexpensive pendulum technique, shear rates many orders of magnitudes lower can be achieved. The pendulum was used to observe the change in yield stress for the foam as the gas fraction and bubble size increased. The local gas fraction in the foam was found by measuring the sonic velocity, and the bubble size was determined photographically. Strong support is found for the existence of a true yield stress in aqueous foams at the dry foam limit. Yield stress results, once scaled by σ/〈R〉, agree well with data from previous studies.

Rheology of cement paste and concrete

Abstract: Much has been learned in recent years about the rheology of cement paste and how it relates to microstructure. Such progress has been made possible in part by the use of specialized instruments for measuring dynamic rheological properties, developed to characterize viscoelastic materials. This paper reviews recent studies of cement paste in our laboratory in which dynamic rheological properties are used to follow the effects of cement hydration. Not as much has been learned about the rheology of concrete. Measuring flow behavior of concrete presents interesting challenges; concrete rheometers have been developed, but they are not widely used. It appears likely that concrete rheology is controlled by rheology of its paste, although such links have not yet been directly established. This paper also presents preliminary results from our laboratory on concrete flow behavior. Concrete rheology provides important information about its workability, and rheological parameters have important advantages over slump when characterizing workability.

Effect of shear on cubic phases in gels of a diblock copolymer

Abstract: The effect of shear on the orientation of cubic micellar phases formed by a poly(oxyethylene)–poly(oxybutylene) diblock copolymer in aqueous solution has been investigated using small-angle x-ray scattering (SAXS) and small-angle neutron scattering (SANS). SAXS was performed on samples oriented in a Couette cell using steady shear, and SANS was performed on samples subject to oscillatory shear in situ in a rheometer with a shear sandwich configuration. A body-centered-cubic (bcc) phase observed for gels with concentrations greater than 30 wt % copolymer was found to orient into a polydomain structure, with the close-packed {110} planes both parallel and perpendicular to the shear plane. For gels with 30 wt % copolymer or less, a face-centered-cubic (fcc) phase was observed, and this was also observed on heating the more concentrated gels that formed a bcc phase at room temperature. The fcc phase could be oriented to form a highly twinned structure, with a significant deviation from the ABCABC… stacking sequence of the ideal structure due to random sequences resulting from slip of {111} hexagonal-close-packed planes. For the lower concentration gels, a transition from hard to soft gel on increasing temperature was found to be characterized by a change in the susceptibility of the sample to macroscopic shear orientation, as probed using SAXS. The hard gel could be oriented by shear into a twinned fcc structure, whereas the soft gel comprised a fcc phase with a small grain size, which could not be sheared to form a macroscopically oriented domain. Shear only homogenized the sample, producing a powder SAXS pattern from a fcc structure.

Experimental and conceptual problems in the rheological characterization of wheat flour doughs

Abstract: Wheat flour dough is an industrially important material and a better understanding of its rheological behavior could have long ranging impact on the agricultural and the food processing industries. However, rheological characterization of dough is proving to be difficult due to a range of testing issues and anomalies in flow behavior. In a cone-and-plate rheometer wheat flour doughs “roll-out” of the gap before steady state viscosities can be established, as discussed by Bloksma and Nieman (1975). However, the mirror image of the transient viscosity-time plot obtained using a cone-and-plate viscometer has been used to obtain an estimate of steady shear viscosity behavior (Gleissle, 1975). To check this transient methodology for doughs, a second method, in addition to cone-and-plate transient flow, for determination of the shear viscosity, was needed. For this, capillary extrusion was chosen. Both a piston-driven and pressure driven capillary rheometer were employed. End corrections were determined to provide information on both the shear viscosity and, following Binding (1988), the extensional viscosity of the doughs. There are few data available on end corrections for doughs, though published data by Kieffer indicate that the corrections are unexpectedly very high. In this present work it was found that the end correction experiments were very difficult and imprecise in part due to the time-dependent nature of the doughs and difficulties in preparing replicate batches required to compare dies of differing L/R values. Further it was unexpectedly found that the samples, though prepared by normal mixing procedures to the “optimum” level, were so heterogeneous that large fluctuations in the pressure at constant output rate (in the piston-driven rheometer) and in output rate at constant pressure (in the pressure-driven instrument) were observed. These fluctuations could be eliminated by overmixing of the doughs, but overmixed doughs are of little practical interest. Although the problems encountered in this work were significant, it was encouraging that even these preliminary studies indicate that rheological measurements are effective in differentiating between spring and winter wheats. Defining a constitutive model for dough rheology still remains a major challenge, as results from one type of testing do not corroborate the findings from a different type of testing.

Effects of enzymes and oxidizing agents on shear stress relaxation of wheat flour dough: additions of protease, glucose oxidase, ascorbic acid, and potassium bromate

Abstract: Measuring shear stress relaxation with an established research rheometer after slowly applying large strain proved useful for characterizing the effects of different chemical and enzymatic additives. Baking tests done with and without added ascorbic acid indicated that the method can be used for predicting effects of such additives on breadbaking quality. The relaxation process for dough is discussed in terms of two flow processes, one occurring at short time periods and the other at longer time periods. The slowly applied large strain shear stress relaxation method provides information about stress relaxation behavior during the second flow process. The effects of two enzymes (protease and glucose oxidase) and two chemical additives (ascorbic acid and potassium bromate) were studied. The results are presented as the stress relaxation curve with the corresponding rate plot and the initial value of the stress. Addition of the oxidizing agents increased the second flow process times and decreased t...

Poly(1-vinyl-2-pyrrolidinone) hydrogels as vitreous substitutes: a rheological study

Abstract: In order to develop an artificial vitreous, a large series of hydrogels have been previously produced by polymerization of 1-vinyl-2-pyrrolidinone, with or without crosslinking. Based on the assumption that a functional vitreous substitute should possess viscoelastic properties after its delivery, a number of selected gels were characterized rheologically by both oscillatory shear stress analysis and shear creep analysis, using a controlled stress rheometer in the cone/plate configuration. The experiments demonstrated a dramatic effect of injecting the gels through small-gauge needles, as many lost their viscoelasticity to become free-flowing fluids, probably because of the cleavage of chains and crosslinks. It was also found that the increase of comonomer content (2-hydroxyethyl methacrylate) and of crosslinking level generally had a strengthening effect. However, the effects of hydrophilic crosslinking agents (diallyl ether and divinyl glycol) were irregular. Eventually, only four hydrogels in this series showed viscoelastic characteristics after injection through a 30-gauge (0·13mm diameter) needle, maintaining behaviour typical of crosslinked networks and warranting further assessment as potential vitreous substitutes. © 1998 SCI.

Sliding plate rheometer studies of concentrated polystyrene solutions: Nonlinear viscoelasticity and wall slip of two high molecular weight polymers in tricresyl phosphate

Abstract: Two high molecular weight, narrow distribution polystyrenes dissolved in tricresyl phosphate were studied using a sliding plate rheometer. Shear stress and birefringence measurements in step strain allowed the determination of N3 for both solutions. Start-up of steady simple shear and large amplitude oscillatory shear measurements revealed that the critical shear stress for slip for both solutions is around 2500 Pa and that slip is dynamic in nature, allowing rheological measurements above the critical stress to be made at sufficiently high frequencies. The critical stress value was used in conjunction with data from the literature to develop a relationship between critical stress and concentration. Large-amplitude oscillatory shear stress measurements revealed that the solutions exhibit a limiting behavior at high strain rate amplitudes in the intermediate frequency range when the dependence on strain rate amplitude becomes small. Birefringence measurements showed that at high frequencies, where the time scale of the deformation is short, the oscillatory component of N3 vanished. This was found to occur at lower frequencies for the lower molecular weight solution. In contrast, the average value of the third normal stress difference, reflected in the zeroth harmonic, was found to vary little between the two solutions.

Elastic membranes in viscous shear flow

Abstract: Liquid capsules with an elastic membrane surface in simple shear flow are considered beyond the regime of small deformations. A simple model is introduced to give a mathematical description of an elastic membrane in viscous flow. The question of whether there is a steady state of a system of a single membrane in external shear flow is studied. It is found by analytical considerations that the possible steady-state flows are restricted by symmetry. The evolution of the membrane in time is found by numerical calculations. For a single spherically symmetric membrane, I find by numerical simulations, a steady-state shape and its dependence on the shear strength. For nonspherically symmetric membranes we see that there is no steady-state shape in general, but by numerical simulations I find that the shape changes can become periodic. This leads to a new alternative explanation of previous experimental results. The stress tensor of the membranes and the effective viscosity of a dilute system of elastic membranes immersed in a liquid is calculated. I find an agreement with former analytical calculations for small shear, and obtain shear thinning behavior when the shear rate is increased.

Measurement of foam modulus via a vane rheometer

Abstract: A vane rheometer was used to measure the modulus of foam systems. Modulus development in a reactive polyurethane foam system was captured using a commercial stress-controlled rheometer with a four-blade vane geometry test fixture. The results were compared to those from a temperature programmed flooded parallel plate method. The vane geometry method was shown to be a convenient and accurate way to measure the physical gelation time of the reactive foaming system, without the complexity of the temperature-program method. Shaving cream was used as a calibration material to show the validity of the vane method for foam modulus measurements. For open cell foams, the error in the vane method increases due to the additional compressibility of the foam. The nature and magnitude of this error was analyzed via numerical simulation.

Elastohydrodynamic Film Forming With Shear Thinning Liquids

Abstract: Recent advances in high pressure rheometry have elucidated the shear response of liquid lubricants at the high shear stress characteristic of the traction generating region of lubricated concentrated contacts. These new measurement techniques are used to characterize the shear response of shear thinning liquids at low (<10 MPa) shear stress. A recently developed numerical scheme for calculating film thickness is extended to accommodate sliding. Film thickness predictions are compared with measurements using shear thinning liquids including a polymer/mineral oil blend, a highly elastic liquid, and synthetic base oils. Useful insights are provided concerning the effects of pressure-viscosity behavior for Newtonian liquids, sliding, and starvation for non-Newtonian liquids and the relevant shear stress for film forming.

A resonance shear force rheometer modeled as simple oscillating circuit

Abstract: A novel resonance method for studying the viscoelasticity of very thin liquid films and elastic materials is developed using a shear force apparatus. The shear stress created by an oscillating piezo unit attached to leaf springs is recorded as the lateral displacement by capacitance probe. The oscillation frequency is varied around the resonance frequency of the mechanical system in order to trace the amplitude and the phase of the resonance peak. Two reference states are obtained: the resonance of free oscillations in air and one under constrain introduced by the cantilever spring in contact with the shear mechanical unit. The presence of a liquid film changes these resonance states depending on the film thickness and the cantilever load. A simple mechanical model is proposed entrapping the contribution of different parts in effective spring, mass, and damping constants. The model separates the effect of the liquid film from the background oscillation of the mechanical parts. The method is applied here to investigate the elastic contact between two solid surfaces.