Showing papers in "Rheologica Acta in 1997"

Modelling the thixotropic behaviour of dense cohesive sediment suspensions

Abstract: A dense cohesive sediment suspension, which contains primarily clay particles, is a thixotropic non-ideal Bingham fluid with a true yield stress. Its time-dependent rheological behaviour can be described by the structural kinetics theory in which the yield stress is taken as a measure for the structural parameter. This theory allows the construction of a more general equation of state, which is independent of the rate equation. The applicability of the model is demonstrated by examples of the prediction of constant structure curves and of transient behaviour. The thixotropy model is incorporated into a Navier-Stokes solver to stimulate the flow behaviour in a Couette viscometer.

Strain hardening of fibrin gels and plasma clots

Abstract: Biological macromolecules have unique rheological properties that distinguish them from common synthetic polymers. Among these, fibrin has been studied extensively to understand the basic mechanisms of viscoelasticity as well as molecular mechanisms of coagulation disorders. One aspect of fibrin gel rheology that is not observed in most polymeric systems is strain hardening: an increase in shear modulus at strain amplitudes above 10%. Fibrin clots and plasma clots devoid of platelets exhibit shear moduli at strains of approximately 50% that are as much as 20 times the moduli at small strains. The strain hardening of fibrin gels was eliminated by the addition of platelets, which caused a large increase in shear storage modulus in the low strain linear viscoelastic limit. The reduction in strain hardening may result from fibrin strand retraction which occurs when platelets become activated. This interpretation is in agreement with recent theoretical treatments of semi-flexible polymer network viscoelasticity.

Effects of coalescence and breakup on the steady-state morphology of an immiscible polymer blend in shear flow

Abstract: The steady-state morphology of an immiscible polymer blend in shear flow has been investigated by optical microscopy techniques. The blend is composed by poly-isobutylene (PIB) and poly-dimethylsiloxane (PDMS) of comparable viscosity. Experiments were performed by means of a home-made transparent parallel plate device. The two plates can be independently counterrotated, so that sheared droplets of the dispersed phase can be kept fixed with respect to the microscope point of view, and observed for long times. The distribution of drops and their average size were measured directly during flow at different shear rates and for different blend compositions. It was found that the average drop size in steady-state conditions is a decreasing function of the applied shear rate, and does not depend on blend composition for volume fractions up to 10%. Experiments have proved that, in the shear rate range which could be investigated, the stationary morphology is controlled only by coalescence phenomena, droplet breakup playing no role in determining the size of the dispersed phase. More generally, it has been shown that the steady-state morphology is a function not only of the physical parameters of the blend and of the shear rate, but also of the initial conditions applied to the blend. The steady-state results reported in this paper constitute the first direct experimental confirmation of theoretical models which describe the mechanisms of shear-induced drop coalescence.

Linear viscoelastic behavior of molten polymer blends: A comparative study of the Palierne and Lee and Park models

Abstract: The linear viscoelastic properties of several molten blends with immiscible components of different viscosity ratio have been investigated. All the blends show a morphology of emulsion type. At low frequencies, the behaviors of these blends are essentially governed by the interface. The Palierne (1990) model is shown to well predict the linear behavior of all the blends. The Lee and Park model (1994), developed to take into account the relationship between the rheological behavior and morphological changes under large strain flows, is also shown to well describe the storage and loss moduli of the blends by adjusting a single fitting parameter. Based on the weighted relaxation spectra, a comparison of both model predictions is made focussing on the time associated to the interface. An approximate method is then proposed to evaluate the interface parameter introduced in the Lee and Park model. At high frequency, discrepancies are observed for the Lee and Park predictions when the viscoelastic properties of both components are considerably different. The description of the bulk properties of the blend, i.e., the mixing rule used by Lee and Park, is modified to obtain a better description of the high frequency data.

Physical gelation under shear for gelatin gels

Abstract: Physical gelation is the process of crosslinking which reversibly transforms a solution of polymers into a gel. The crosslinks of the network have a physical origin (hydrogen bonding, Van der Waals forces... ) and therefore are sensitive to variations of temperature, pH, ionic content, etc. (non-permanent crosslinks). Physical and chemical gelation have been extensively studied in quiescent conditions, where rheology experiments have been performed to follow the network formation without disturbing the process. In this study we consider gelation of a well known physical, thermoreversible, gel (gelatin gel), which proceeds under flowing conditions. The gelling solution is submitted to a shearing, with imposed, permanent shear stresses or imposed, permanent, shear rates. Under flow, a competition arises between the formation of clusters by physical crosslinking and their disruption by the shear forces. This investigation defines the flowing conditions which either allow or impede gel formation. In particular, a critical shear rate \(\dot \gamma *\), related to the gelation temperature and gelatin concentration, is identified which separates the two regimes. A microscopic model is proposed, based on the analysis of flow curves and dynamic measurements, which describes the structure of the gelling solution: microgel particles grow to a maximum size which depends on the flow. When the volume fraction of particles is high enough, percolation between particles occurs suddenly and a yield stress fluid is formed (particulate gel). The differences between gels made in quiescent conditions and gels made under flow are underlined.

Transient filament stretching rheometer II: Numerical simulation

Abstract: The Lagrangian specification is used to simulate the transient filament stretching rheometer. Simulations are performed for a dilute PIB-solution modeled as a four mode Oldroyd-B fluid and a semidilute PIB-solution modeled as a non-linear single integral equation. The simulations are used to investigate a sequence of filament stretching rheometers. The questions of special and temporal homogeneity of the experiment are investigated. The simulations are compared with data from a specific rheometer by Tirtaadmadja and Sridhar.

Structural transitions in wormlike micelles

Abstract: We investigate the effect of excess salt and simple shear on the dynamics and structure of semi-dilute aqueous solutions of cetyltrimethylammonium bromide and sodium salicylate. Small-amplitude oscillatory rheological measurements suggest a structural evolution from an entangled to a multi-connected network as the salt concentration is increased. Steady-shear measurements, however, show a significant departure from the Cox-Merz rule. At low salt concentrations, this departure occurs at high shear rates with η* ∞ ω−0.92±0.08 and η ∞ γ−0.51±0.06 and is attributed to the formation of large shear-induced structures. The critical shear rate γ c at which the Cox-Merz rule fails approximates the inverse of the terminal relaxation time, τ. At high salt concentrations, however, the departure occurs at both low and high shear rates and is attributed to the formation of a multi-connected network. Small-angle light scattering (SALS) under shear was used to probe the mesoscopic structure and revealed novel scattering patterns exhibiting two-fold symmetry at low salt concentration and four-fold symmetry at high salt concentration. The SALS patterns were in qualitative agreement with the formation of large scale anisotropic structures at high shear rates and a multi-connected network at high salt concentrations.

Oscillatory and simple shear flows of a flour-water dough: a constitutive model

Abstract: We reported some dynamic and viscometric data on an Australia strong flour-water dough. In oscillatory shear flow experiments, we found the linear viscoelastic strain limit is extremely low, of O(10−3), consistent with other published data on doughs. The relaxation spectrum derived from the dynamic data is broad, indicating the “blend” nature of dough. In the start-up of a simple shear flow, we found the shear stress increases nonlinearly with time to a peak value and then decreases rapidly, with no steady-state response. The concept of steady-state viscosity is not very meaningful here, unless the strain at which the measurements are taken is also specified. The stress peaks are strain-rate dependent; but they occur at a strain of O(10), for the strong flour/water dough used, over four decades of strain rates. The experimental data were used to construct a phenomenological model for dough, consisting of an hyperelastic term (representing the elastic gluten network of permanent cross-linked long chain polymers), and a viscoelastic contribution (representing the suspension of starch globules and other long-chain components in dough that are not parts of the permanent cross-linked gluten network). The model predictions compared favourably with experimental data in oscillatory and shear flows.

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

Some examples of possible descriptions of dynamic properties of polymers by means of the coupling model

Abstract: Thanks to the research efforts of Prof. John D. Ferry and others over the last several decades, the viscoelastic properties of polymers have been extensively determined. From this accumulated wisdom, polymer viscoelasticity has become a mature field of research. This basic knowledge of polymer viscoelasticity has made it possible to discern the deviations from the apparently established general rules that one of us (DJP) have found, continuing the tradition of exhaustive experimental measurement started by Prof. Ferry. From many experimental studies on polymers carried out in different laboratories, it has also become clear that these viscoelastic anomalies are general and not exceptional features. Therefore, they pose significant problems in the quest of a truly satisfactory understanding of polymer viscoelasticity. The Coupling Model (CM) has been used to rationalize a number of deviations from thermorheological simplicity. In the realm of polymer viscoelastic behavior, we consider first the local segmental motion that is responsible for the glass temperature and show that the CM provides a consistent description in either the modulus or the compliance representation. Next, we elucidate several viscoelastic anomalies which originate from the different viscoelastic mechanisms being thermorheologically complex. Finally, we revisit the original formulation of the terminal relaxation of entangled polymer chains using the CM. The neglect of the lateral nature of the constraints imposed on one chain by other chains in the original formulation leads to failure in explaining the shape of the terminal relaxation, although it is successful in other aspects. A new formulation, which includes the lateral nature of the constraints and its subsequent mitigation when the terminal relaxation is reached, has restored consistency of the prediction with the terminal relaxation of a monodisperse polyisoprene melt probed dielectrically. The results can describe also the experimental data of dilute polyisoprene probes in polybutadiene matrices and in networks.

Stick-slip transition in capillary flow of linear polyethylene: 3. Surface conditions

Abstract: Effects of surface topology and energy on the stick-slip transition were studied in capillary flow of highly entangled polyethylene (PE) melts. Surface roughness was shown to increase the critical stress of the stick-slip transition because of the increased resistance to interfacial disentanglement. Lowering the surface energy of a smooth die wall by treatment with a fluorocarbon completely eliminates the stick-slip transition and produces massive interfacial slip at PE/wall boundary down to a stress level of 0.05 MPa. On the other hand, considerable roughness on the same low energy surface can produce a stick hydrodynamic boundary condition and restore the stick-slip transition despite the weak PE/wall interfacial interactions. Additionally, a slip-slip transition was found in the die with a nearly non-adsorbing wall that appears to involve a secondary chain-debonding process.

Non-linear flow properties of viscoelastic surfactant solutions

Abstract: This paper gives a quantitative description of the viscoelastic properties of aqueous solutions of entangled rod-shaped micelles. The experimental data are compared with the theoretical predictions of a special constitutive equation which is based on the concept of deformation-dependent tensorial mobility. In the regime of small deformations, shear stresses or shear rates, the dynamic features of the viscoelastic solutions are characterized by the equations of a simple Maxwell material. These phenomena are linked to the average lifetime of the micellar aggregates and the rheological properties are controlled by kinetic processes. At these conditions one observes simple scaling laws and linear relations between all theological quantities. At elevated values of shear stresses or deformations, however, this simple model fails and non-linear properties as normal stresses, stress overshoots or shear-thinning properties occur. All these phenomena can be described by a constitutive equation which was first proposed by H. Giesekus. The experimental results are in fairly good agreement with the theoretical predictions, and this model holds for a certain, well defined value of the mobility factor α. This parameter describes the anisotropic character of the particle motion. In transient and steady-state flow experiments we always observed α = 0.5. Especially at these conditions, the empirically observed Cox-Merz rule, the Yamamoto relation and both Gleisle mirror relations are automatically derived from the Giesekus model. The phenomena discussed in this paper are of general importance, and can be equally observed in different materials, such as polymers or proteins. The viscoelastic surfactant solutions can, therefore, be used as simple model systems for studies of fundamental principles of flow.

Transient filament stretching rheometer. I : Force balance analysis

Abstract: The filament stretching device which is used increasingly as an apparatus for measuring extensional properties of polymeric liquids is analysed. A force balance that includes the effects of inertia and surface tension is derived. The force balance may be used to correct for the effects of inertia and surface tension, provided online measurements of the filament surface shape are available. In addition, the question of initial asymmetry due to gravity is addressed.

Extensional viscosity from entrance pressure drop measurements

Abstract: Extensional rheological properties are important in characterization and processing of polymeric liquids. The use of entrance pressure drop to obtain extensional viscosity is particularly attractive because it can be applied to both low and high viscosity liquids using the Bagley correction obtained from a conventional capillary rheometer. Low density polyethylene of three different melt index values, including IUPAC-X (a different batch of IUPAC-A), and a high density polyethylene were tested using a commercial capillary rheometer. The entrance pressure drop (ΛP en ) was obtained with a “zero-length” orifice die with an abrupt contraction. The contraction ratio was 12:1. Predictions from several approximate analyses to calculate the uniaxial extensional viscosity ηu (using an axisymmetric contraction) from ΔP en were compared. These comparisons are summarized in the appendices. Due to the transient nature of contraction flows, η u is also a function of the strain (ɛ). This was examined by comparing η u from ΔP en (Cogswell's analysis was chosen for convenience) with transient extensional viscosity (η u +) at different magnitudes of ɛ from fiber-windup technique (Padmanabhan et al., 1996). η + at ɛ≈ 3 was found to be close to η u from ΔP en (using Cogswell's analysis) for two LDPE samples that had fiber-windup data available. The magnitude of the strain in the contraction did not vary with strain rate.

Stability of viscoelastic flow around periodic arrays of cylinders

Abstract: Low Reynolds number flow of Newtonian and viscoelastic Boger fluids past periodic square arrays of cylinders with a porosity of 0.45 and 0.86 has been studied. Pressure drop measurements along the flow direction as a function of flow rate as well as flow visualization has been performed to investigate the effect of fluid elasticity on stability of this class of flows. It has been shown that below a critical Weissenberg number (Wec), the flow in both porosity cells is a two-dimensional steady flow, however, pressure fluctuations appear above Wec which is 2.95±0.25 for the 0.45 porosity cell and 0.95±0.08 for the higher porosity cell. Specifically, in the low porosity cell as the Weissenberg number is increased above Wec a transition between a steady two-dimensional to a transient three-dimensional flow occurs. However, in the high porosity cell a transition between a steady two-dimensional to a steady three-dimensional flow consisting of periodic cellular structures along the length of the cylinder in the space between the first and the second cylinder occurs while past the second cylinder another transition to a transient three-dimensional flow occurs giving rise to time- dependent cellular structures of various wavelengths along the length of the cylinder. Overall, the experiments indicate that viscoelastic flow past periodic arrays of cylinders of various porosities is susceptible to purely elastic instabilities. Moreover, the instability observed in lower porosity cells where a vortex is present between the cylinders in the base flow is amplifieds spatially, that is energy from the mean flow is continuously transferred to the disturbance flow along the flow direction. This instability gives rise to a rapid increase in flow resistance. In higher porosity cells where a vortex between the cylinders is not present in the base flow, the energy associated with the disturbance flow is not greatly changed along the flow direction past the second cylinder. In addition, it has been shown that in both flow cells the instability is a sensitive function of the relaxation time of the fluid. Hence, the instability in this class of flows is a strong function of the base flow kinematics (i.e., curvature of streamlines near solid surfaces), We and the relaxation time of the fluid.

Stress relaxation as a microstructural probe for immiscible polymer blends

Abstract: Relaxation has been investigated in immiscible blends that consist of slightly viscoelastic components Both the shear and normal stresses have been measured after cessation of steady shear flow as well as after transient shear histories The latter can generate a fibrillar structure which can relax by either retraction or break-up via end-pinching or Rayleigh instabilities Each of these three relaxation mechanisms is reflected in the shape of the stress curves, from which also the corresponding structural time scales can be deduced The experimental results have been used to evaluate the Doi-Ohta and Lee-Park models for immiscible blends The scaling relations by Doi-Ohta are confirmed by the experimental results, but none of the existing models can correctly predict the complex relaxation behaviour observed for a highly deformed droplet phase In the present study an alternative approach has been proposed The stress relaxation due to fibril break-up via Rayleigh instabilities has been predicted successfully by combining physical models for the structural changes with the basic approach of the Doi-Ohta model

Generating line spectra from experimental responses. Part V : Time-dependent viscosity

Abstract: The computer algorithm for determining line spectra from experimental data, described in earlier publications in the form in which it is applicable to data obtained in response to excitations as step functions of time and to sinusoidally oscillating excitations, is modified here to allow line spectra to be obtained from data generated in response to the imposition or the removal of a constant rate of strain.

Modeling spurt and stress oscillations in flows of molten polymers

Abstract: The paper presents an approach for modeling polymer flows with non-slip, slip and changing non-slip — slip boundary conditions at the wall. The model consists of a viscoelastic constitutive equation for polymer flows in the bulk, prediction of the transition from non-slip to sliding boundary conditions, a wall slip model, and a model for the compressibility effects in capillary polymer flows. The bulk viscoelastic constitutive equation contains a hardening parameter which is solely determined by the polymer molecular characteristics. It delimits the conditions for the onset of solid, rubber-like behavior. The non-monotone wall slip model introduced for polymer melts, modifies a slip model derived from a simple stochastic model of interface molecular dynamics for cross-linked elastomers. The predictions for the onset of spurt, as well as the numerical simulations of hysteresis, spurt, and stress oscillations are demonstrated. They are also compared with available data for a high molecular weight, narrow distributed polyisoprene. By using this model beyond the critical conditions, many of the qualitative features of the spurt and oscillations observed in capillary and Couette flows of molten polymers, are described.

On the use of stretched-exponential functions for both linear viscoelastic creep and stress relaxation

Abstract: The use of the stretched-exponential function to represent both the relaxation function g(t)=(G(t)-G ∞)/(G 0-G ∞) and the retardation function r(t) = (J ∞+t/η-J(t))/(J ∞-J 0) of linear viscoelasticity for a given material is investigated. That is, if g(t) is given by exp (−(t/τ)β), can r(t) be represented as exp (−(t/λ)µ) for a linear viscoelastic fluid or solid? Here J(t) is the creep compliance, G(t) is the shear modulus, η is the viscosity (η−1 is finite for a fluid and zero for a solid), G ∞ is the equilibrium modulus G e for a solid or zero for a fluid, J ∞ is 1/G e for a solid or the steady-state recoverable compliance for a fluid, G 0= 1/J 0 is the instantaneous modulus, and t is the time. It is concluded that g(t) and r(t) cannot both exactly by stretched-exponential functions for a given material. Nevertheless, it is found that both g(t) and r(t) can be approximately represented by stretched-exponential functions for the special case of a fluid with exponents β=µ in the range 0.5 to 0.6, with the correspondence being very close with β=µ=0.5 and λ=2τ. Otherwise, the functions g(t) and r(t) differ, with the deviation being marked for solids. The possible application of a stretched-exponential to represent r(t) for a critical gel is discussed.

Experimental studies of interfacial instabilities in multilayer pressure-driven flow of polymeric melts

Abstract: The interfacial deformation and stability of two-(A-B) as well as three-layer symmetric (A-B-A) and asymmetric (A-B-C) pressure-driven flow of viscoelastic fluids has been investigated. Flow visualization in conjunction with digital image processing has been used to observe and measure the rate of encapsulation and interfacial stability/instability of the flow. Specifically, the encapsulation behavior as well as stability/instability of the interface and the corresponding growth or decay rate of disturbances as a function of various important parameters, namely, number of layers and their arrangement, layer depth ratio, viscosity and elasticity ratio as well as disturbance frequency, have been investigated. Based on these experiments, we have shown that the encapsulation phenomena occurs irrespective of the stability/instability of the interface and in cases when both encapsulation and instability occur simultaneously their coupling leads to highly complex and three-dimensional interfacial wave patterns. Moreover, it has been shown that the simple notion that less viscous fluids encapsulate more viscous fluids is incorrect and depending on the wetting properties of the fluid as well as their first and second normal stresses the reverse could occur. Additionally, in two- and three-layer flows it has been shown that by placing a thin, less viscous layer adjacent to the wall longwave disturbances can be stabilized while short and intermediate wavelength disturbances are stabilized when the more elastic fluid is the majority component. Furthermore, in three-layer flows it has been demonstrated that in the linear instability regime no dynamic interaction between the two interfaces is possible for short and intermediate wavenumber disturbances. However, in the nonlinear stability regime dynamic interactions between interfaces have been observed in this range of disturbance wavenumbers leading to highly chaotic flows. Finally, in the parameter space of this study no subcritical bifurcations were observed while supercritical bifurcations resulting in waves with a pointed front and a gradual tail were observed.

Elongational flow opto-rheometry for polymer melts : 1. Construction of an elongational flow opto-rheometer and some preliminary results

Abstract: Polymer melt elongation is one of the most important procedures in polymer processing. To understand its molecular mechanisms, we constructed an elongational flow opto-rheometer (EFOR) in which a high precision birefringence apparatus of reflection-double path type was installed into a Meissner's new elongational rheometer of a gas cushion type (commercialized as RME from Rheometric Scientific) just by mounting a small reflecting mirror at the center of the RME's sample supporting table. The EFOR enabled us to achieve simultaneous measurements of tensile stress σ(t) and birefringence Δn(t) as a function of time t under a given constant strain rate $$\dot \varepsilon _0$$ within the range of 0.001 to 1.0s−1. σ(t) can be monitored upto the maximum Hencky strain ɛ(t) of 7 as attained, in principle, with RME, while the measurable range of the phase difference in the birefringence was 0 to 250 π (0 to 79 100 nm for He-Ne laser light) within the accuracy of ±0.1 π (±31.6 nm) up to ɛ(t) ∼ 4. The performance was tested on an anionically polymerized polystyrene (PS) and a low density polyethylene (LDPE). For both polymers σ(t) first followed the linear viscoelasticity rule in that the elongational viscosity, $$\eta _E (t) \equiv \sigma (t)/\dot \varepsilon _0$$ , is three times the steady shear viscosity, 3η o(t), at low shear rate $$\dot \gamma$$ , but the η E (t) tended to deviate upward after a certain Hencky strain $$\varepsilon (t) = \dot \varepsilon _0 t$$ was attained. The birefringence Δn(t) was a function of both Hencky strain $$\varepsilon (t) = \dot \varepsilon _0 t$$ and strain rate $$\dot \varepsilon _0$$ in such a way that the stress-optical law holds with the stress-optical coefficient C(t) = Δn(t)/δ(t) being equal to the ones reported from shear flow experiments. Interestingly, however, for PS elongated at low strain rates the C(t) vs σ(t) relation exhibited a strong nonlinearity as soon as σ(t) reached steady state. This implies that the tensile stress reaches the steady state but the birefringence continues to increase in the low strain-rate elongation. For the PS melt elongated at high strain rates, on the other hand, C(t) was nearly a constant in the entire range observed. For LDPE with long-chain branchings, σ(t) exhibited tendency of strain-induced hardening after certain critical strain, but C(t) was nearly a constant in the entire range of σ(t) observed.

The effect of parallel combined steady and oscillatory shear flows on blood and polymer solutions

Abstract: Human blood at physiological volume concentration exhibits non-Newtonian and thixotropic properties The blood flow in the microcirculation is pulsatile, initiated from the heart pulse and can be considered as superposition of two partial flows: a) a steady shear, and b) an oscillatory shear Until now steady and viscoelastic behavior were separately investigated Here we present the response to the combination of steady and oscillatory shear for human blood, a high molecular weight aqueous polymer solution (polyacrylamide AP 273E) and an aqueous xanthan gum solution The polyacrylamide and xanthan solutions are fluids that model the rheological properties of human blood In general, parameters describing blood viscoelasticity became less pronounced as superimposed steady shear increased, especially at low shear region and by elasticity, associated with reduction in RBC aggregation The response of polymer solutions to superposition shows qualitative similarities with blood by elasticity, but their quantitative response differed from that of blood By viscosity another behavior was observed The superposition effect on viscous component was described by a modified Carreau equation and for the elastic component by an exponential equation

Extensional rheology of concentrated poly(ethylene oxide) solutions

Abstract: The shear and extensional rheology of three concentrated poly(ethylene oxide) solutions is examined. Shear theology including steady shear viscosity, normal stress difference and linear viscoelastic material functions all collapse onto master curves independent of concentration and temperature. Extensional flow experiments are performed in fiber spinning and opposed nozzles geometries. The concentration dependence of extensional behavior measured using both techniques is presented. The zero-shear viscosity and apparent extensional viscosities measured with both extensional rheometers exhibit a power law dependence with polymer concentration. Strain hardening in the fiber spinning device is found to be of similar magnitude for all test fluids, irrespective of strain rate. The opposed nozzle device measures an apparent extensional viscosity which is one order of magnitude smaller than the value determined with the fiber spinline device. This could be attributed to errors caused by shear, dynamic pressure, and the relatively small strains developed in the opposed nozzle device. This instrument cannot measure local kinematics or stresses, but averages these values over the non-homogenous flow field. These results show that it is not possible to measure the extensional viscosity of non-Newtonian and shear thinning fluids with this device. Fiber spin-line experiments are coupled with a momentum balance and constitutive model to predict stress growth and diameter profiles. A one-mode Giesekus model accurately captures the plateau values of steady and dynamic shear properties, but fails to capture the gradual shear thinning of viscosity. Giesekus model parameters determined from shear rheology are not capable of quantitatively predicting fiber spinline kinematics. However, model parameters fit to a single spinline experiment accurately predict stress growth behavior for different applied spinline tensions.

Imperfect lubricated squeezing flow viscometry for foods

Abstract: Commercial mayonnaise and mustard samples placed in a wide, shallow Teflon container were compressed by a wide Teflon plate to induce an ‘imperfect’ lubricated squeezing flow. A dominant squeezing flow regime could be clearly identified as a linear region in the log F(t) vs log H(t) relationship, F(t) and H(t) being the momentary force and specimen height respectively. The slope of the relationship enabled the estimation of the flow index, n, and the consistency coefficient K. The n values of the mayonnaise were on the order of 0.6–0.85 and those of the mustard about 0.7. The corresponding K values were on the order of 6–13 and 4–5 kPasn respectively. Considering the crudeness of the array the measurements were highly reproducible and sensitive enough to detect differences (mayonnaise) or establish similarities (mustard) in products of different brands. The calculated flow index was practically independent of the plate's radius and of the consistency coefficient, which had a weak dependency on the latter. The calculated elongational viscosity vs biaxial strain rate relationship could also be used to compare the different products and brands. At 0.01 s−1 the elongational viscosity of the maynonnaise was on the order of 150 kPas, and of the mustard 60 kPas.

Fracture phenomena in shearing flow of viscous liquids

Abstract: In start-up of steady sheafing flow of two viscous unen- tangled liquids, namely low-molecu- lar-weight polystyrene and a-D-glu- cose, the shear stress catastrophi- cally collapses if the shear rate is raised above a value corresponding to a critical initial shear stress of around 0.1-0.3 MPa. The time de- pendence of the shear stress during this process is similar for the two liquids, but visualization of samples in situ and after quenching reveals significant differences. For a-D-glu- cose, the stress collapse evidently results from debonding of the sam- ple from the rheometer tool, while in polystyrene, bubbles open up within the sample, as occurs in cavi- tation. Some similarities are pointed out between these phenomena and that of "lubrication failure" reported in the tribology literature.

Dynamic study of gelatin gels by creep measurements

Abstract: We have investigated the dynamical properties of gelatin gels using creep measurements. A commercial apparatus (Carrimed CSL500) was modified in order to increase the deformation of the gel and to take advantage of the inertia of the system. When a step stress is applied, the very first response of these materials is an oscillating strain owing to a coupling of the high elasticity of the gelatin gels and the inertia of the apparatus. From these damped oscillations, we have extracted the elastic and loss moduli as a function of frequency, which allows us to widen the frequency range (toward high frequencies) of measurement. After subtraction of the oscillations, we have obtained the compliance funtion from which, using Ferry's formalism, we can calculate the relaxation time distribution function over a very large time range (10−3–104 s). We show that the dynamics of gelatin gels is governed by two very different characteristic times. We interpret the faster relaxation time as relaxation at the scale of the gel network mesh-size, while the slower time we assign to relaxations involving the lifetime of the gelatin gel cross-links. It is now possible to use creep measurements as an alternative to the forced oscillatory function determination, as the same data can be obtained but, more quickly, and over a large frequency range. This gives us more indication of the gel's structure (gel network behaviour, kinetics of ageing) than all the laborious methods previously necessary.

Rheo-NMR study of a non-flow-aligning side-chain liquid crystal polymer in nematic solution

Abstract: The flow behavior of a highly concentrated solution of a nematic side-chain liquid crystal polymer in a low molecular mass nematic solvent is investigated by deuterium nuclear magnetic resonance with simultaneous measurement of the shear viscosity in a cone-and-plate NMR viscometer. The director orientation under shear in the magnetic field is determined from the quadrupole splitting of the NMR spectra. The orientation as a function of shear rate is analyzed in terms of the Ericksen-Leslie-Parodi theory, yielding the Leslie coefficients μ 2 and μ 3 and thus the flow alignment parameter λ. From the combined analysis of orientation and viscosity as a function of shear rate a total of four independent viscosity parameters is obtained for the nematic solution. The value determined for the flow-alignment parameter, λ≈0.2, and the analysis of the data based on Brochard's theory show that the polymer is of the non-flow-aligning type and has a slightly prolate shape.

Molecular origin of viscoelasticity and chain orientation of glassy polymers

Abstract: Dynamic birefringence and dynamic viscoelasticity of poly(4-methyl styrene) and poly(4-t-butyl styrene) were measured to investigate the molecular origin of viscoelasticity around the glass transition zone. The data were analyzed with a modified stress-optical rule: The birefringence and the stress were separated into two component functions of different molecular origins. One component is related to the orientation of the main chain axis and the other one to the rotation of the repeating units about the main chain axis. The strain dependence of the two characteristic orientation functions in the glassy zone was estimated and the orientation mechanism of repeating units was discussed.

Sliding plate rheometry of planar oriented concentrated fiber suspension

Abstract: The rheology of concentrated planar fiber suspensions is investigated. A new experimental technique for fiber suspensions based on a sliding plate rheometer incorporating a shear stress transducer is developed. It is shown that this instrument works well for the tested material systems. The rheological behavior in steady shear is subsequently investigated. The results can be largely explained by a combination of frictional and hydrodynamic interaction. Despite this evidence of friction no yield stress could be detected for the investigated shear rates. It was also found that the fiber aspect ratio did not influence the steady shear viscosity.

Branched viscoelastic surfactant solutions and their response to elongational flow

Abstract: Several years ago, Munstedt and Laun reported on the influence of branching on the elongational flow properties of polymer chains (Munstedt and Laun, 1981). They concluded that, in addition to the molecular weight distribution, the degree of branching strongly affects the degree of strain thickening of the elongational viscosity in such a way that the maximum in this material function increases with branching. In a recent paper by Lin, a ternary system of dodecyldimethylamine oxide-sodium laureth sulphate-sodium chloride surfactant solutions was investigated by CryoTEM and rheology (Lin, 1996). He reported a linear relation between the added sodium chloride and the branching of the wormlike micelles. In this paper we present an investigation of these surfactant solutions in elongational flow. Our results indicate that for branched micellar systems the presence of branching enhances the maximum of the elongational viscosity in the same manner as in the case of polymer melts.