Showing papers in "Journal of Rheology in 2001"

Elasto-capillary thinning and breakup of model elastic liquids

Abstract: We study the elasto-capillary self-thinning and ultimate breakup of three polystyrene-based ideal elastic fluids by measuring the evolution in the filament diameter as slender viscoelastic threads neck and eventually break. We examine the dependence of the transient diameter profile and the time to breakup on the molecular weight, and compare the observations with simple theories for breakup of slender viscoelastic filaments. The evolution of the transient diameter profile predicted by a multimode FENE-P model quantitatively matches the data provided the initial stresses in the filament are taken into account. Finally, we show how the transient uniaxial extensional viscosity of a dilute polymer solution can be estimated from the evolution in the diameter of the necking filament. The resulting “apparent extensional viscosity” profiles are compared with similar results obtained from a filament stretching rheometer. Both transient profiles approach the same value for the steady state extensional viscosity, which increases with molecular weight in agreement with the Rouse–Zimm theory. The apparent discrepancy in the growth rate of the two transient curves can be quantitatively explained by examining the effective stretch rate in each configuration. Filament thinning studies and filament stretching experiments thus form complementary experiments that lead to consistent measures of the transient extensional viscosity of a given test fluid.

The effects of interparticle interactions and particle size on reversible shear thickening: Hard-sphere colloidal dispersions

Abstract: A comparison between the effects of two colloidal stabilizing methods (electrostatic versus Brownian) on the reversible shear thickening transition in concentrated colloidal suspensions is explored Five suspensions of monodisperse silica are synthesized via the Stober synthesis and dispersed in an index matched organic solvent to minimize van der Waals interactions The residual surface charge is neutralized with nitric acid (cHNO3≈01 M) resulting in a near hard-sphere interaction that is confirmed by small angle neutron scattering measurements across a range of volume fractions Rheological measurements demonstrate the effects of neutralization on the low shear and high shear rheology, which show that the onset of shear thickening moves to lower applied shear stresses and scales inversely with particle size cubed, in agreement with theory Quantitative comparisons of both the low shear viscosity and the critical stress for shear thickening to predictions for hard spheres and literature data demonstrate

Differential constitutive equations for polymer melts: The extended Pom–Pom model

Abstract: The Pom‐Pom model, recently introduced by McLeish and Larson @J. Rheol. 42, 81‐110~1998!#, is a breakthrough in the field of viscoelastic constitutive equations. With this model, a correct nonlinear behavior in both elongation and shear is accomplished. The original differential equations, improved with local branch-point displacement, are modified to overcome three drawbacks: solutions in steady state elongation show discontinuities, the equation for orientation is unbounded for high strain rates, the model does not have a second normal stress difference in shear. The modified extended Pom‐Pom model does not show the three problems and is easy for implementation in finite element packages, because it is written as a single equation. Quantitative agreement is shown with experimental data in uniaxial, planar, equibiaxial elongation as well as shear, reversed flow and step-strain for two commercial low density polyethylene ~LDPE! melts and one high density polyethylene ~HDPE! melt. Such a good agreement over a full range of well defined rheometric experiments, i.e., shear, including reversed flow for one LDPE melt, and different elongational flows, is exceptional. © 2001 The Society of Rheology. @DOI: 10.1122/1.1380426#

The molecular stress function model for polydisperse polymer melts with dissipative convective constraint release

Abstract: The molecular stress function theory for polymer melts is extended to include a new, dissipative convective constraint release process. First the Helmholtz free energy of tube segments with strain-dependent tube diameter is established neglecting constraint release, and it is demonstrated that the molecular stress is a function of the average logarithmic stretch under these conditions. Then convective constraint release is introduced as a dissipative process in the energy balance of tube deformation, which leads to a strain-dependent evolution equation for the molecular stress function. Constraint release is considered to be the consequence of different convection mechanisms for tube orientation and tube cross section. Our new, dissipative constraint release model emphasizes that tube kinematics are fundamentally different for rotational and nonrotational flows, and therefore distinguishes explicitly between simple shear and pure shear (planar extension). For the startup of simple shear and extensional flows, the predictions of our set of constitutive equations consisting of a history integral for the stress tensor and a differential evolution equation for the molecular stress function with only two nonlinear material parameters are in excellent agreement with experimental data of a polydisperse high-density polyethylene (HDPE) and a polydisperse low-density polyethylene (LDPE) melt. Also, stress relaxation after step-shear strain is described for both the HDPE and the LDPE melt.

An interlaboratory comparison of measurements from filament-stretching rheometers using common test fluids

Abstract: Following development of a filament-stretching extensional rheometer at Monash University, similar rheometers have been designed and built in other laboratories. To help validate the basic technique, a collaborative program was undertaken to compare results from several instruments. First, three test fluids prepared at the University of California at Berkeley were characterized in steady and transient shear flows there and at the Massachusetts Institute of Technology (M.I.T.), and then tested in extensional rheometers at M.I.T., Monash and the University of Toronto. Each fluid is a constant-viscosity solution of narrow-molecular-weight-distribution polystyrene dissolved in oligomeric polystyrene. The solute molecular weights are 2.0, 6.5, and 20 million g/mol, and the polymer concentration in each fluid is 0.05 wt. %. From linear viscoelastic measurements, the Zimm relaxation times of the fluids are found to be 3.7, 31, and 150 s, respectively. The scaling of relaxation times with molecular weight indicat...

Dynamics of dilute and semidilute DNA solutions in the start-up of shear flow

Abstract: We have investigated the dynamics of dilute (10−5C*) and semidilute (⩽6C*) DNA solutions both in steady and in the start-up of shear flow by combining fluorescence microscopy, bulk rheological measurements, and Brownian dynamics simulations. First, the microscopic states, i.e., the conformational dynamics of single DNA molecules in solution during the start-up of shear flow, were examined by fluorescence microscopy. To investigate the macroscopic response resulting from the changes in the microscopic state, the bulk shear viscosity of the same DNA solutions was also measured. While the transient dynamics of individual molecules is highly variable, an overshoot in the ensemble-averaged molecular extension is observed above a critical Wi following an overshoot in shear viscosity for both dilute and semidilute DNA solutions. These two overshoots are further analyzed and explained on a physical basis from our simulation findings. Based on the physical picture, we have derived a simple scaling to predict the s...

Microscopic theory of convective constraint release

Abstract: We develop a microscopic description of the contribution to stress relaxation in entangled polymer melts of convective constraint release, which is the release of entanglement constraints due to the effects of convective flow on chains surrounding a given chain. Our theory resolves three of the main shortcomings of the Doi–Edwards model in nonlinear rheology, in that it predicts (1) a monotonically increasing shear stress as a function of shear rate, (2) shear stress independent of molecular weight at sufficiently high shear rates, and (3) only modest anisotropies in the single chain scattering function, in agreement with experiment. In addition, our approach predicts that a stress maximum and resulting shear-banding instability would occur for living micelle solutions, as observed.

A simple constitutive equation for entangled polymers with chain stretch

Abstract: We propose a simple way of including chain stretch effects in convective constraint release theories for entangled polymers. The main idea is that the characteristic time of orientational relaxation depends in a series-parallel way on all three relevant mechanisms, i.e., reptation, constraint release (thermal and convective), and Rouse relaxation. As usual, a separate equation describes chain stretch, which however is assumed not to be affected by constraint release. The model is further simplified by writing the orientational equation in differential form. For step strains, the successful damping function of the Doi–Edwards theory is exactly preserved. Predictions in steady shear also favorably compare with typical data of nearly monodisperse polymers.

Novel viscosity equations for emulsions of two immiscible liquids

Abstract: Starting from the Taylor equation [Taylor, G. I., Proc. R. Soc. London Ser., A 138, 41–48 (1932)] for the viscosity of very dilute emulsions, new viscosity equations are developed for concentrated emulsions using the effective medium approach. According to the equations derived in the paper, the viscosity of concentrated emulsions increases with the increases in the volume fraction of the dispersed phase and the viscosity ratio (K) of the dispersed phase to the continuous phase. In the limit of K→∞, the equations reduce to the well-known formulas for solids-in-liquid suspensions. The proposed equations are evaluated in light of a large body of experimental data for concentrated emulsions, covering a broad range of dispersed-phase to continuous-phase viscosity ratio (3.87×10−4⩽K⩽3.25×105).

Droplet breakup in concentrated emulsions

Abstract: In this paper we report an experimental study on the conditions for droplet breakup in concentrated emulsions under simple shear flow. We present a set of experiments where the ratio between drop and matrix viscosity was varied from 0.1 to 22 and the volume fraction ranged from 0% to 70%. It was observed that the critical shear rate for breakup decreased by more than an order of magnitude for the most concentrated emulsions. Further, drops with viscosity ratio of 22 were seen to rupture in simple shear as soon as the emulsion concentration was raised to 40%. All these effects were conveniently explained by means of a mean field model which assumes simply that breakup of a droplet in a concentrated emulsion is determined by the average emulsion viscosity rather than the continuous phase viscosity.

Rheology of non-Brownian rigid fiber suspensions with adhesive contacts

Abstract: An experimental investigation is undertaken into the shear-thinning behavior of suspensions of non-Brownian rigid fibers in Newtonian fluids. In particular, the influence of the shear stress and the fiber concentration is investigated. The shear stress is adjusted by varying both the shear rate and the solvent viscosity. In the semidilute concentration regime, where direct mechanical contacts between fibers are rare, the suspension is found to be nearly Newtonian over the stress range investigated. In the concentrated regime, the suspension becomes shear thinning below a certain shear rate. The shear thinning increases with concentration and decreases with solvent viscosity. Although shear-thinning behavior of fiber suspensions has often been reported in the literature, its physical origins are not well understood. Our experiments are interpreted in terms of the formation and breakage of fiber flocs due to the competition between hydrodynamic and colloidal forces. Our interpretation is confirmed by measurements of the adhesive forces between two individual fibers.

Visualizing the elimination of sharkskin through fluoropolymer additives: Coating and polymer–polymer slippage

Abstract: We developed a capillary rheo-optics technique to visualize how fluoropolymer polymer processing additives (PPA) eliminate a surface distortion called “sharkskin” in the extrudate of linear low-density polyethylene (LLDPE) The measurements were carried out in a transparent sapphire tube located at the exit of a twin-screw extruder Depth-resolved optical microscopy was used to measure both the polymer velocity profiles and to image the coating process of the PPA onto the capillary wall In the absence of PPA, no slippage occurs between the capillary wall and the polyethylene; sharkskin was observed at all flow rates Upon addition of the PPA to the LLDPE, the PPA migrates to the capillary wall where it sticks and induces slippage between itself and the LLDPE, concomitant with the elimination of sharkskin The interface between the PPA and LLDPE is characterized by long stripes in the flow direction Large values of the polymer–polymer slippage parameter were found which indicate that the fluoropolymer and LLDPE are fully disentangled at their interface The PPA acts by dramatically reducing the extensional deformation of the LLDPE at the exit surface

Yield stress for particle suspensions within a clay dispersion

Abstract: This article focuses on suspensions of coarse particles within a clay dispersion. The behavior of such suspensions is generally dictated by the colloidal fine fraction, notably its yield stress. The dependence of this yield stress on the solid concentration (in coarse particles) is examined. It has been experimentally shown that adding coarse particles usually induced an increasingly marked enhancement of yield stress. However, in some cases, adding a small amount of coarse particles led to a decrease in bulk yield stress. We propose two mechanisms responsible for variations in bulk yield stress. First, at low concentrations, depletion of clay particles may be sufficient to induce an increase in the bulk yield stress. Two values for the depletion layer thickness have been found depending on the coarse particle type. At large concentrations, the substantial increase in bulk yield stress has been ascribed to the development of a coarse particle network within the dispersion. In this case, yielding results from the breakdown of indirect (lubricated) contacts between particles.

Electrorheology of filled silicone elastomers

Abstract: Electrorheological (ER) silicone elastomers containing particles based on silica (SiO2) and titania minerals (BaTiO3, Ba2Ti9O20, and BaTiO3/CaZrO3) were prepared and characterized. An electrical field was applied to align the particles during the cure of the silicone prepolymer. For the silicone/silica compositions, a prominent Maxwell–Wagner dispersion in the dielectric response suggested that surface conductivity of the silica particles dominated the polarization. Alignment of the particles increased the overall dielectric permittivity as well as the magnitude of the Maxwell–Wagner dispersion. Their ER response exhibited a negative deviation from a quadratic dependence on field intensity at high fields, and was accompanied by nonlinear conductivity. A highly nonlinear enhancement of the ER effect with increasing particle concentration was observed. For the silicone/titania elastomers as a class, the ER response increased with the particle’s permittivity. In the case of the silicone/BaTiO3 elastomer, the...

Rheology of aging, concentrated, polymeric suspensions: Application to pasty sewage sludges

Abstract: The rheological behavior of pasty sewage sludges from different origins and at different ages of fermentation has been studied on the basis of careful rheometrical tests. These materials appear to be basically yield stress fluids whose flow curves can be fairly well represented by a Herschel–Bulkley model. The yield stress existence is evidenced by a clear transition between a viscoelastic and a viscous behavior at a critical shear stress. The rheological parameters are shown to mainly depend on the organic fraction and the time of fermentation. By extracting the main solids components (minerals, proteins, lipids, carbohydrates) we show that the behavior evolution is governed by the synthesis of volatile fatty acids.

Structure formation in moderately concentrated viscoelastic suspensions in simple shear flow

Abstract: We report experimental results on the evolution of the particle microstructure for noncolloidal particles that are suspended in a viscoelastic medium. For dilute suspensions consisting of particles having a monodisperse particle size distribution subjected to steady and oscillatory shear flows, as well as suspensions having a bidisperse size distribution subjected to steady shear, we verify previous results reported in the research literature. We also present new results of the particle microstructure for a dilute bidisperse system that was subjected to oscillatory shear. For moderately concentrated suspensions we report the formation of particle strings that correspond to a quantitative reduction in the simultaneously measured shear stress.

A slotted plate device for measuring static yield stress

Abstract: A slotted-plate device was constructed with a balance and a linear-motion platform to directly measure static yield stresses of suspensions by moving the plate in the suspension in a similar mode as is done in the well-known Wilhelmy-plate technique for measuring surface tension. Wall effects associated with the original plate yield-stress instrument [De Kee et al. (1980)] were minimized by opening a series of slots on the plates. Yield-stress experiments were conducted on both high-concentration (40, 50, 60, and 70 wt % TiO2) and low-concentration (2, 3, and 5 wt % bentonite) aqueous suspensions. The new setup avoids the disadvantages of the vane instrument, possible secondary flow between the blades as well as a nonuniform stress distribution along a virtual cylindrical surface. Yield stress values of TiO2 suspensions were compared with the values obtained via a variety of other methods, including indirect extrapolation from steady-shear data, vane creep testing, and vane stress-ramp measurements using ...

Nonlinear rheology of telechelic polymer networks

Abstract: We report on the nonlinear rheology of aqueous solutions of telechelic associating polymers in the network regime. Different telechelic polymers with a poly(ethylene oxide) middle chain and semiperfluorinated end caps C8F17(CH2)11 were synthesized and characterized with respect to their functionalization. At a telechelic concentration c=4%, the aqueous solutions studied are highly viscoelastic and close to Maxwellian fluids. Due to the strong hydrophobicity of the end caps, the network relaxation time can be very long compared to that of polymers with fully hydrogenated hydrophobes. We have exploited these long relaxation times to explore the time-resolved stress responses obtained in start-up experiments. The steady shear stress versus shear rate curves exhibit two main and stable branches separated by a discontinuity. The first branch of the flow curve is Newtonian and shear thickening, whereas the second branch is shear thinning. At the onset of shear thickening, we show that the viscosity increase is related to the hardening of the network, i.e., to an increase of the effective elastic modulus. In the shear-thinning branch, extremely slow transients are observed and the steady state is reached for deformations as large as several hundreds of strain units. It is suggested that these slow relaxations, as well as the discontinuity in the shear stress, are the signatures of the onset of an inhomogeneous flow resulting from the breakdown or rupture of the network.We report on the nonlinear rheology of aqueous solutions of telechelic associating polymers in the network regime. Different telechelic polymers with a poly(ethylene oxide) middle chain and semiperfluorinated end caps C8F17(CH2)11 were synthesized and characterized with respect to their functionalization. At a telechelic concentration c=4%, the aqueous solutions studied are highly viscoelastic and close to Maxwellian fluids. Due to the strong hydrophobicity of the end caps, the network relaxation time can be very long compared to that of polymers with fully hydrogenated hydrophobes. We have exploited these long relaxation times to explore the time-resolved stress responses obtained in start-up experiments. The steady shear stress versus shear rate curves exhibit two main and stable branches separated by a discontinuity. The first branch of the flow curve is Newtonian and shear thickening, whereas the second branch is shear thinning. At the onset of shear thickening, we show that the viscosity increase is ...

The effect of long chain branches on the shear flow behavior of polyethylene

Abstract: The effects of long chain branching (LCB) on the viscosity and first normal stress coefficient of metallocene polyethylene (mPE) are described. LCB increased the zero shear viscosity and the susceptibility to shear thinning. The first normal stress coefficient increased with degree of LCB at all rates. The materials with LCB followed the Cox–Merz rule and the Gleissle mirror relations while the linear material followed only the Cox–Merz rule. These results indicate that the behavior of long chain branched mPE in simple shear can be predicted using only linear viscoelastic properties.

Effect of compatibilization on the breakup of polymeric drops in shear flow

Abstract: A block copolymer may be added as a compatibilizer during polymer processing in order to promote intimate mixing of thermodynamically immiscible homopolymers. The action of this compatibilizer can only partially be attributed to its effect on the interfacial tension between the immiscible homopolymers. Here the additional contributions of the compatibilizer are directly probed by measuring the capillary number during coalescence experiments. Model blends consisting of polyisobutylene (PIB) and polydimethylsiloxane (PDMS), compatibilized with various amounts of a PIB–PDMS diblock copolymer, are used for this purpose. The mean capillary number of the droplets is determined from the mechanical frequency response of the blends. With increasing amounts of compatibilizer, a systematic increase in steady shear capillary number is seen, to values well above the critical capillary number for droplet breakup of uncompatibilized systems. This indicates that a simple decrease in interfacial tension is not the only ef...

Simulation of melt spinning including flow-induced crystallization. Part III. Quantitative comparisons with PET spinline data

Abstract: The mathematical model for melt spinning of Doufas et al. [Doufas, A. K. et al., J. Rheol. 43, 85–109 (1999); J. Non-Newtonian Fluid. Mech. 92, 27–66 (2000); 92, 81–103 (2000)] coupling the polymer microstructure (molecular orientation, chain extension, and crystallinity) with the macroscopic velocity/stress and temperature fields is tested against low- and high-speed spinline experimental data of PET melts. The model includes the combined effects of flow-induced crystallization (FIC), viscoelasticity, filament cooling, air drag, inertia, surface tension, and gravity and simulates melt spinning from the spinneret down to the take-up roll device (below the freeze point). As is the case with nylon systems, model fits and predictions are shown to be in very good quantitative agreement with spinline data for the fiber velocity, diameter, and temperature fields at both low- and high-speed conditions, and, with flow birefringence data available for high speeds. Our model captures the necking phenomenon for PET quantitatively and the associated extensional softening which is shown to be related to nonlinear viscoelastic effects and not to the release of latent heat of crystallization. Although crystallization is quite slow under low-speed spinning conditions, the model captures the occurrence of the freeze point naturally, and is thus a significant improvement over existing melt spinning models that enforce the freeze point at the glass transition temperature. In this article we demonstrate the robustness of our microstructural FIC model to melt spinning of quite slow crystallizers in the quiescent state, while the robustness for faster crystallizers was shown previously [Doufas, A. K. et al., J. Non-Newtonian Fluid. Mech. 92, 27–66 (2000); 92, 81–103 (2000)].The mathematical model for melt spinning of Doufas et al. [Doufas, A. K. et al., J. Rheol. 43, 85–109 (1999); J. Non-Newtonian Fluid. Mech. 92, 27–66 (2000); 92, 81–103 (2000)] coupling the polymer microstructure (molecular orientation, chain extension, and crystallinity) with the macroscopic velocity/stress and temperature fields is tested against low- and high-speed spinline experimental data of PET melts. The model includes the combined effects of flow-induced crystallization (FIC), viscoelasticity, filament cooling, air drag, inertia, surface tension, and gravity and simulates melt spinning from the spinneret down to the take-up roll device (below the freeze point). As is the case with nylon systems, model fits and predictions are shown to be in very good quantitative agreement with spinline data for the fiber velocity, diameter, and temperature fields at both low- and high-speed conditions, and, with flow birefringence data available for high speeds. Our model captures the necking phenomenon for PET ...

Simulation of film blowing including flow-induced crystallization

Abstract: The two-phase microstructural flow-induced crystallization model developed by the authors is applied to the simulation of film blowing. In order to isolate the effects due to crystallinity, a simplified “quasicylindrical” approximation is used for the momentum equations, which neglects the effect of axial curvature in the axial direction. The present simulations include the combined effects of flow-induced crystallization, viscoelasticity, and bubble cooling. In all cases studied, the location of the frost line is predicted naturally as a consequence of flow-induced crystallization. The effects of inflation pressure, melt extrusion temperature, and take-up ratio on the bubble shape are predicted to be in agreement with experimental observations. The combination of these processing conditions determines the shape of the bubble, i.e., whether the bubble contracts or expands. An important feature of our model is the prediction of the locked-in system stresses at the frost line that are related to the physical and mechanical properties of the film.The two-phase microstructural flow-induced crystallization model developed by the authors is applied to the simulation of film blowing. In order to isolate the effects due to crystallinity, a simplified “quasicylindrical” approximation is used for the momentum equations, which neglects the effect of axial curvature in the axial direction. The present simulations include the combined effects of flow-induced crystallization, viscoelasticity, and bubble cooling. In all cases studied, the location of the frost line is predicted naturally as a consequence of flow-induced crystallization. The effects of inflation pressure, melt extrusion temperature, and take-up ratio on the bubble shape are predicted to be in agreement with experimental observations. The combination of these processing conditions determines the shape of the bubble, i.e., whether the bubble contracts or expands. An important feature of our model is the prediction of the locked-in system stresses at the frost line that are related to the physica...

Normal stresses and free surface deformation in concentrated suspensions of noncolloidal spheres in a viscoelastic fluid

Abstract: Concentrated suspensions of noncolloidal spheres in a constant viscosity elastic fluid were characterized rheologically using rotating plate viscometers and profilometry of the suspension surface deflection near a rotating rod It was found that the relative viscosity was quantitatively consistent with a previously determined correlation for suspensions based on Newtonian fluids Moreover, the first normal stress difference N1 was found to be positive and the second normal stress difference N2 negative Although the magnitude of N1 and N2 increased with the solids volume fraction φ, in general the ratio |N1/N2| decreased as loading increased Analysis of the normal stress data suggests that the rheological contribution of the solids microstructure was in large part independent of that of the dissolved polymers at high solids loading (φ⩾03) The magnitude of N2 at high concentrations approached that measured for similar suspensions in Newtonian fluids, while the magnitude of N1 could be attributed to the

Experimental observation and numerical simulation of transient “stress fangs” within flowing molten polyethylene

Abstract: We report experimental observations and matching numerical simulation for the time-dependent start-up flow of two molten polyethylenes (PEs) within a slit entry and exit geometry. For the case of a low density polyethylene (LDPE), an unexpected transient, birefringence "stress fang" was observed downstream of the slit exit. The stress fang consisted of a localized region of stress concentration. The stress fang, however, was not observed for a linear low density polyethylene (LLDPE) sample subjected to the same processing condition. A matching time-dependent numerical simulation of the flow is also presented. Using a split Lagrangian–Eulerian method for simulating transient viscoelastic flow with the multimode pom–pom constitutive equation, the general features of the stress fangs were predicted for the LDPE. In addition, the simulation did not predict stress fangs for the LLDPE. The paper demonstrates that for this particular case the pom–pom model can successfully discriminate the complex flow behavior of different PEs, and shows that the presence (or otherwise) of a stress fang is sensitive to the particular rheology of the polymer that arises from long chain branching.

Ligament creep behavior can be predicted from stress relaxation by incorporating fiber recruitment

Abstract: Structural modeling and morphological crimp analysis were used to investigate if collagen fiber recruitment could account for our previous finding, that ligament creep behavior cannot be predicted using inverse stress relaxation behavior directly. Ligament creep behavior was accurately predicted using our simple nonlinear structural model that incorporated collagen fiber creep and collagen fiber recruitment. Collagen fiber creep was modeled using the inverse stress relaxation function and estimated fiber modulus. Collagen fiber recruitment was modeled with a linear variation in crimp over an idealized rectangular ligament cross section. Concomitantly, significant differences in collagen crimp patterns were observed as a result of creep testing; however; no significant changes were observed as a result of relaxation testing. This morphological evidence supported the model assumptions that fiber recruitment occurs during creep and that stress relaxation behavior results from the viscoelastic response of an unchanging group of fibers. Not only was the prediction improved compared to the inverse stress relaxation behavior alone, the model demonstrated that fiber recruitment increased the load-bearing area of the ligament over time and correspondingly stress was redistributed, reducing stress on the fibers initially loaded. These findings may have important implications for both models and experiments on ligament structure and function at low loads.

Shear thickening in steady and superposition flows effect of particle interaction forces

Abstract: The phenomenon of shear thickening has been studied in sterically stabilized colloidal suspensions. Temperature has been used to vary the solvent quality of the suspending medium for the grafted polymer in the stabilizer layer. A decrease in temperature causes the polymer coat to shrink. This results in a lower viscosity in the shear thinning region but also in a lower critical shear rate and shear stress at the onset of shear thickening. The critical shear stress increases with increasing particle volume fraction, at least for the volume fractions studied here. In large amplitude oscillatory flow, strain hardening for both storage and loss moduli sets in at the same peak shear rate that causes shear thickening in steady state flow. Parallel superposition also has been used to probe the effect of the stabilizer layer. The frequency dependence of the superposition moduli changes drastically once shear thickening sets in. Surprisingly, this produces a simple and constant shape for the moduli-frequency curve...

Micellar structure changes in aqueous mixtures of nonionic surfactants

Abstract: Rheology and small-angle neutron scattering are used to probe the structure of nonionic surfactant mixtures in water. Small amounts of a C14 diol (Surfynol® 104) cause enormous structural and rheological changes when added to aqueous solutions of an ethylene oxide-propylene oxide-ethylene oxide triblock copolymer (Pluronic® P105). The C14 diol is only soluble up to 0.1 wt % in pure water, but can be added in large quantities to aqueous solutions of the copolymer. The hydrophobic diol incorporates into the existing copolymer micelles and causes a cascade of changes in the micelle structure, with resultant changes in rheology. Particularly striking is the spherical to worm-like micelle transition, where the viscosity changes by a factor of more than 104.

Rheology of fiber suspensions in viscoelastic media: Experiments and model predictions

Abstract: The rheological behavior of suspensions of short glass fibers in different fluids has been studied. Transient tests on presheared samples of fiber suspensions in Boger fluids showed that orientation of fibers not only depends on the strain, but also on the rate-of-strain. The experimental results show that upon increasing fiber concentration and/or fiber aspect ratio, the steady shear material functions of fiber suspensions increase at low shear rates, whereas at high shear rates, these material functions approach those of the matrix and become almost independent of fiber characteristics. A rheological model based on the modified Jeffery equation for the fiber motion and a Hookean energy model, formulated within the GENERIC framework, for the matrix has been developed to quantitatively predict experimental data for suspensions of fibers in different fluids. A quantitative comparison of experimental data with model predictions shows the ability of the model to predict the rheological behavior of fiber suspensions in viscoelastic media.

Deficiencies of FENE dumbbell models in describing the rapid stretching of dilute polymer solutions

Abstract: A wide variety of bead-spring kinetic theory models have been proposed to explain the stress growth and hysteretic behavior of dilute polymer solutions in uniaxial extension. We analyze the Kramers chain, a fine-scale model for polymer dynamics, in order to assess the validity of the coarser-grained bead-spring models in these deformations. Whereas the spring force is a simple function of the dumbbell length for the FENE spring, we find that the relationship between the ensemble-averaged end-to-end force and the extension for a Kramers chain depends on the kinematic history to which it has been subjected. In a quiescent fluid, the Kramers chain force–extension relationship is identical to the FENE force law. However, during start up of elongational flow, the ensemble-averaged end-to-end force for a given (end-to-end) length of the molecule increases with strain until steady state is reached. If the extensional flow is suddenly stopped, the Kramers chain force–extension relationship relaxes back to the FEN...

Step strain flow: Wall slip effects and other error sources

Abstract: The traditional technique for the experimental characterization of the shear stress relaxation modulus by applying a step shear strain was investigated using flow visualization. A high-speed camera was employed in conjunction with cone-and-plate and parallel-disk fixtures of a Rheometric Scientific Advanced Rheometric Expansion System rheometer. In the nonlinear region the true shear strain imposed on a polyethylene melt deviates considerably from the targeted strain. The main source of the deviation is the wall slip of the polymer melt. The presence of wall slip reduces significantly the range of strains for which the strain-dependent relaxation modulus can be determined for the linear polyethylene melt. Errors associated with the control of the motion of the tool which introduces the shear strain are also documented.