Showing papers in "Rheologica Acta in 2011"

Melt strengthening of poly (lactic acid) through reactive extrusion with epoxy-functionalized chains

Abstract: Poly (lactic acid) is an industrially mature, bio-sourced and biodegradable polymer. However, current applications of this eco-friendly material are limited as a result of its brittleness and its poorly melt properties. One of the keys to extend its processing window is to melt strengthen the native material. This paper considers the chain extension as a valuable solution for reaching such an objective. An additive based on epoxy-functionalized PLA was employed during reactive extrusion. The reaction times as a function of chain extender ratios were determined by monitoring the melt pressure during recirculating micro-extrusions. Once residence times were optimized, reactive extrusion experiments were performed on a twin screw extruder. Size exclusion chromatography provided information about the molecular weight distributions (MWD) of the modified PLAs and revealed the creation of a high molecular weight shoulder. The rheological experiments highlighted the enhancement of the melt properties brought about by the chain extension. Shear rheology revealed some enlarged and bimodal relaxation time spectra for the extended materials which are in accordance with the MWD analysis. Such a modification directly amplified the shear sensitivity of modified PLAs. Regarding the rheological temperature sensitivity, it was found to be decreased when the chain extender content is raised as shown from the Arrhenius viscosity fit. The reduction of the polar interactions from neat to highly chain-extended PLAs is here proposed to explain this surprising result. Chain extension was also found to impact on the elongational melt properties where strain hardening occurred for modified PLAs. Investigation of the chain extension architecture was made from the rheological data and revealed a long-chain branched topology for the modified PLAs.

Issues in the flow of yield-stress liquids

Abstract: Yield-stress liquids are materials that are solid below a critical applied stress and flow like mobile liquids at higher stresses. Classical descriptions of yield-stress liquids, which have been the basis for asymptotic and computational studies for five decades, are inadequate to describe many recent experimental observations, and it is clear that the time dependence of microstructure must be taken into account in the description of many real yield-stress liquids.

Microstructure and magnetorheology of graphite-based MR elastomers

Abstract: This study focuses on the magnetorheology of graphite-based magnetorheological elastomers (Gr MREs). By introducing graphite to conventional MREs, the Gr MREs with various graphite weight fractions are fabricated. Both steady-state and dynamic tests were conducted to study rheological properties of the samples. For dynamic tests, the effects of magnetic field, strain amplitude and frequency on both storage modulus and loss modulus were measured. The influence of graphite weight fraction on mechanical performances of these samples was summarized. Also, the microstructures of isotropic and anisotropic Gr MREs were observed. In anisotropic MREs, the graphite powders disperse in matrix randomly. The graphite particles lead to an increment of initial mechanical properties and a decrement of the MR effect.

Extensional rheology of human saliva

Abstract: We have developed an oscillatory cross-slot extensional rheometer capable of performing measurements with unprecedentedly small volumes of test fluids (∼10–100 μL). This provides the possibility of studying exotic and precious or scarce bio-fluids, such as synovial fluid. To test our system, we have looked at a relatively abundant and accessible biological fluid, namely human saliva; a complex aqueous mixture of high molecular weight mucin molecules and other components. The results represent our first attempts to by this technique and as yet we have only sampled a small dataset. However, we believe we have produced the first successful quantitative measurements of extensional viscosity, Trouton ratio, and flow-induced birefringence made on saliva samples. The results significantly add to the scant literature on saliva rheology, especially in extension, and demonstrate the important role of saliva extensibility in relation to function.

Bentonite slurries—zeta potential, yield stress, adsorbed additive and time-dependent behaviour

Abstract: Bentonite clay is a vital ingredient of drilling mud. The time-dependent and high shear thinning yield stress behaviour of drilling mud is essential for maintaining wellbore stability and to remove cuttings, cool and clean the drill bit of debris. As-prepared 3, 5 and 7 wt.% bentonite clay slurries displayed time-dependent behaviour where the yield stress (measured after quick stirring) decreased with time of rest. An equilibrium value is reached after 24 h. Despite the low solids concentration, the yield stress is already relatively high and is displayed at all pH level. The yield stress is maximum at pH 2 and minimum at pH ∼ 7. This yield stress is due to the formation of gel structure by the swelling clay particles. However the addition of phosphate additives such as (PO3)19 − , (P3O10)5 − and (P2O7)4 − completely dispersed the clay slurries at pH above 6. At pH below 6, yield stress is still present but is 3-folds smaller than that of the pure bentonite slurry. With phosphate additives, the magnitude of the critical zeta potential at the complete dispersion pH is ca 48 mV. However for the pure bentonite, the slurry remained flocculated at zeta potential of >50 mV in magnitude. Interestingly, (P2O7)4 − anions is more effective than the other two phosphate additives in reducing the yield stress at low pH, ∼ 2.0.

Rheology of Ziegler–Natta and metallocene high-density polyethylenes: broad molecular weight distribution effects

Abstract: The linear viscoelastic properties of two series of Ziegler–Natta and metallocene HDPEs (ZN-HDPEs and m-HDPEs, respectively) of broad molecular weight distribution (MWD) have been studied. Correlations between zero-shear viscosity and molecular weight and molecular weight distribution show that the breadth of the MWD for m-HDPEs plays a role. Other interesting correlations between the crossover modulus and steady-state compliance with MWD of both these classes of polymers have also been derived. Finally, the steady-shear viscosities from capillary rheometry are compared with LVE data to check the applicability of the empirical Cox–Merz rule. It is shown that the original Cox–Merz rule is applicable for the ZN-HDPEs, while it apparently fails for the m-HDPEs. However, once the capillary data for m-HDPEs are corrected for slip effects, the applicability of the Cox–Merz rule is validated for their case as well.

Scaling behavior of the elastic properties of non-dilute MWCNT–epoxy suspensions

Abstract: In this paper, the network structure of multiwalled carbon nanotube (MWCNT)–epoxy suspensions was investigated under the influence of flow history and temperature using the scaling behavior of the linear viscoelastic properties of the concentrated suspensions above their gel point. It is shown that the suspensions have a self-similar fractal structure with the dimension of about 2.15, characteristic of weakly flocculating suspensions and their elasticity originates from inter- and intra-floc links of nanotubes. From the scaling behavior of the flow-induced storage modulus and the critical strain for the limit of linearity, it is shown that the fractal dimension and so the superstructure of the network did not change significantly under the influence of the flow history due to the initial compact structure of the network before pre-shearing. The time–temperature superposition principle was verified for the CNT suspensions and the shift factor was accounted for by an Arrhenius equation. The reduced storage and loss moduli of the suspensions using the complex modulus of the neat epoxy were shown to increase with temperature revealing more inter-particle interactions as the temperature was raised. However, it was impossible to conclude on the changes of the fractal dimensions with temperature.

Mircorheology and jamming in a yield-stress fluid

Abstract: We study the onset of a yield stress in a polymer microgel dispersion using a combination of particle-tracking microrheology and shear rheometry. On the bulk scale, the dispersion changes from a predominantly viscous fluid to a stiff elastic gel as the concentration of the microgel particles increases. On the microscopic scale, the tracer particles see two distinct microrheological environments over a range of concentrations—one being primarily viscous, the other primarily elastic. The fraction of the material that is elastic on the microscale increases from zero to one as the concentration increases. Our results indicate that the yield stress appears as the result of jamming of the microgel particles, and we infer a model for the small-scale structure and interactions within the dispersion and their relationship to the bulk viscoelastic properties.

The influence of temperature on rheological properties of blood mixtures with different volume expanders—implications in numerical arterial hemodynamics simulations

Abstract: During the complicated cardiac surgery on a non-beating heart with cardiopulmonary bypass, protection of the heart is accomplished by injecting cold cardioplegic solutions. In most forms of circulatory shock, it is necessary to immediately restore the circulating volume. Intravenous solutions of volume expanders, such as hydroxyethyl starch and dextrans, are used to increase the volume of fluid in the circulating blood. In this work, blood samples of six donors were obtained and used to prepare mixtures with different volume expanders in concentrations ranging from 10 to 50 vol./vol.%. The flow curves of all mixtures in the temperature range from 4°C to 37°C were constructed and fitted to the Herschel–Bulkley model, in order to extract the shear thinning and yield stress parameters. To assess the influence of the observed changes in the rheological properties of blood on the hemodynamics in arterial vasculature, a realistic three-dimensional rigid-wall computational model was constructed from MRI images of the right carotid bifurcation obtained in vivo from a healthy male volunteer. The time-varying flow field was numerically computed using the Newtonian model as well as the Herschel–Bulkley model with the Papanastasiou regularization. The numerical simulations indicate only moderate changes in the time-averaged flow field that become accentuated when the instantaneous flow field is considered. We also found that although the influence of temperature, hematocrit, and volume expanders on hemodynamics is significant, this can primarily be attributed to the changes in the nominal viscosity of the flow medium.

How polymeric solvents control shear inhomogeneity in large deformations of entangled polymer mixtures

Abstract: This work aims to elucidate how molecular parameters dictate the occurrence of inhomogeneous cohesive failure during step strain and large amplitude oscillatory shear (LAOS) respectively in entangled polymer mixtures. Based on three well-entangled polybutadiene (PB) mixtures, we perform simultaneous rheometric and particle-tracking velocimetric (PTV) measurements to illustrate how the slip length controls the degree of shear banding. Specifically, the PB mixtures were prepared using the same parent polymer (M w ∼ 106 g/mol) at 10 wt.% concentration in respective polybutadiene solvents (PBS) of three different molecular weights 1.5, 10, and 46 kg/mol. After step strain, the entangled PB mixture with PBS-1.5 K displayed interfacial failure whereas the PB mixture with PBS-10 K showed bulk failure, demonstrating the effectiveness of our strategy to suppress wall slip by controlling PBS’ molecular weight. Remarkably, the PBS-46K actually allows the elastic yielding to occur homogeneously so that no appreciable macroscopic motions were observed upon shear cessation. PBS is found to play a similar role in LAOS of these three PB mixtures. Finally, we demonstrate that in case of the slip-prone mixture based on PBS-1.5 K the interfacial failure could be drastically reduced by use of shearing plates with considerable surface roughness.

Normal stress differences in large-amplitude oscillatory shear flow for the corotational “ANSR” model

Abstract: Using the integral form of a nonlinear corotational model, we derive explicit analytical expressions for the zeroth, second, and fourth harmonics of the second normal stress difference in large-amplitude oscillatory shear (LAOS). This model yields an arbitrary normal stress ratio (ANSR) in any simple shearing deformation, including LAOS. This corotational ANSR model adds one parameter to the corotational Maxwell model, a time constant λ 0 controlling the ratio Ψ2/Ψ1 for both the real and imaginary parts of each harmonic of the normal stress difference. The explicit analytical expressions for all harmonics of the alternating shear stress and first normal stress difference responses in LAOS match those obtained previously for the corotational Maxwell model. We evaluate the corotational ANSR model for the case of a single Maxwell relaxation time fluid.

Correlation between thermal and rheological studies to characterize the behavior of bitumen

Abstract: The objective of this work is a comprehensive thermo-rheological study of pure bitumen. The bitumen is a complex material consisting of asphaltenes dispersed in a maltene matrix. As a consequence, its flow behavior is characterized by the presence of a yield stress, which depends on temperature below 50°C. Applying the Cox–Merz rule, a master curve of viscosity can be obtained over a wide range of shear rates for temperatures above 50°C. It can be accurately modeled by a Carreau–Yasuda law with a yield stress. This specific rheological behavior can be explained by the changes induced by the temperature on the microstructure, evidenced by modulated differential scanning calorimetry measurements.

X-ray scattering measurements of particle orientation in a sheared polymer/clay dispersion

Abstract: We report steady and transient measurements of particle orientation in a clay dispersion subjected to shear flow. An organically modified clay is dispersed in a Newtonian polymer matrix at a volume fraction of 0.02, using methods previously reported by Mobuchon et al. (Rheol Acta 46: 1045, 2007). In accord with prior studies, mechanical rheometry shows yield stress-like behavior in steady shear, while time dependent growth of modulus is observed following flow cessation. Measurements of flow-induced orientation in the flow-gradient plane of simple shear flow using small-angle and wide-angle X-ray scattering (SAXS and WAXS) are reported. Both SAXS and WAXS reveal increasing particle orientation as shear rate is increased. Partial relaxation of nanoparticle orientation upon flow cessation is well correlated with time-dependent changes in complex modulus. SAXS and WAXS data provide qualitatively similar results; however, some quantitative differences are attributed to differences in the length scales probed by these techniques.

Wall material and roughness effects on transmittable shear stresses of magnetorheological fluids in plate–plate magnetorheometry

Abstract: A direct comparison of plate–plate magnetorheometry results for nonmagnetic (titanium/brass) and ferromagnetic plates is presented, using a modified Anton Paar magnetocell MRD180/1T. Necessary corrections to derive the true flux density in the magnetorheological fluid (MRF) from the online Hall probe reading and to account for the gap opening effect caused by normal forces on shear stress and flux density are addressed. The measured shear stress versus magnetic flux density characteristics agree in the low flux density regime <0.1 T but yield distinctly higher transmittable shear stresses for ferromagnetic plates at elevated flux densities (49% increase at 1 T for 90% by weight carbonyl iron powder (CIP) and 84% for 85% by weight CIP). Remarkably, the normal force, if corrected for its magnetostatic part, remains independent of the type of plates up to about 0.6 T. We address the role of normal forces, of magnetic interactions between CIP and wall, as well as the role of wall roughness in a solid body friction model. A systematic variation of wall properties and materials was achieved by introducing both a modular rotor and stator, which ease the variation of the walls in contact to the MRF. The transmittable shear stress of nonmagnetic plates (e.g., brass) may be increased up to the level of ferromagnetic disks by a higher wall roughness or by grooves. No shear stress increase is obtained for grooves in ferromagnetic plates, which is explained by the different local flux density modulation at the grooves for ferromagnetic compared to nonmagnetic plates. Finally, we address the effect of ferromagnetic and nonmagnetic coatings on brass and steel disks, and show that, e.g., a layer of CIP on brass efficiently increases the transmittable shear stress.

Strain stiffening of non-colloidal hard sphere suspensions dispersed in Newtonian fluid near liquid-and-crystal coexistence region

Abstract: Concentrated hard sphere suspensions often show an interesting nonlinear behavior, called strain stiffening, in which the viscosity or modulus starts to increase at critical strain amplitude. Sudden increase of rheological properties is similar to shear thickening; however, the particle dynamics in the strain stiffening under oscillatory shear flow does not necessarily coincide with the mechanism of shear thickening under step shear flow. In this study, we have systematically investigated the nonlinear rheology of non-colloidal (>1 μm) hard sphere suspensions dispersed in Newtonian fluid near liquid-and-crystal coexistence region in order to better understand the strain stiffening behavior. The suspensions near liquid-and-crystal coexistence region are known to locally form the closed packing structure. The critical strain amplitude which is the onset of strain stiffening was different for the storage and loss modulus. But they converged to each other as the suspension forms a more crystalline structure. The critical strain amplitude was independent of medium viscosity, imposed angular frequency, and particle size, but was strongly dependent upon particle volume fraction. The onset of strain stiffening was explained in terms of shear-induced collision due to particle motion in the closed packing structure. Nonlinear stress wave-forms, which reflect the micro-structural change, were observed with the onset of strain stiffening. During the strain stiffening, enhanced elastic stress before and after flow reversal was observed which originates from changes in the suspension microstructure. Nonlinearity of the shear stress in terms of Fourier intensity was extremely increased up to 0.55. Beyond the strain stiffening, the suspension responded liquid-like and the nonlinearity decreased but the elastic shear stress was still indicating the microstructure rearrangement within a cycle.

Hysteresis cycles and fatigue criteria using anelastic models based on fractional derivatives

Abstract: The constitutive equation and the fatigue of anelastic media are described by using fractional order derivatives. The stress–strain relation, based on a generalization of the Kelvin–Voigt model, describes typical hysteresis cycles with the stress increasing as the number of cycles increases, a phenomenon known as cyclic hardening and observed in many materials such as, for instance, steel. Criteria are established to find the number of cycles which may cause fatigue for a strain with a given amplitude and frequency. They are based on the yield and fatigue stresses, on the melting temperature through the dissipated energy, and on the strain energy. In all the cases, it is seen that the number of cycles to failure is inversely proportional to the amplitude and to the frequency of the applied strain. Comparison to experimental data indicates that the model satisfies, at least qualitatively, the behavior of real materials under cyclic loading.

Flow behaviour of highly concentrated emulsions of supersaturated aqueous solution in oil

Abstract: A set of highly concentrated water-in-oil emulsions with supersaturated dispersed phase were investigated in this work to verify and/or develop the models that have been presented both in the literature and in this work. The material used to form emulsions consisted of supersaturated oxidiser solution, hydrocarbon oil and PIBSA-based surfactants. The interfacial characteristics for different surfactant types were first examined. Then, the rheology of samples was studied, and different scaling methods and fitting of experimental data were studied. On the basis of flow curve measurements and observed $\tau _\emph{v} \sim \dot {\gamma }^{1/2}$ scaling, a modified version of Windhab model was suggested which showed excellent fitting of experimental results. The linear dependences of τ y0/σ versus 1/d 32 for studied emulsions showed non-zero intercept which implies a non-linear dependence (resulting from interdroplet interaction) to fulfil the zero-intercept requirement. It was established that the zero intercept condition was fulfilled in the $\tau _{y0} \sim \sigma /d_{32}^2 $ scaling, although the experimental results for different surfactants were not superimposed.

Analytic derivation of the Cox–Merz rule using the MLD “toy” model for polydisperse linear polymers

Abstract: A general constitutive formalism, the “naive” polydisperse MLD model, has been developed by Mead et al. (Macromolecules 31:7895–7914, 1998) and Mead (Rheol Acta 46:369–395, 2007) at both the tube coordinate level and the mathematically simplified “toy” level independent of the tube coordinate. The model includes constraint release generated by convection-driven chain retraction (which is equivalent to “convective constraint release” (CCR)), reptation, and tube contour length fluctuations. The properties of the mathematically simplified naive polydisperse “toy” MLD model are explored in linear and nonlinear steady shear flows where we analytically derive the Cox–Merz rule relating the steady shear viscosity to the modulus of the linear viscoelastic dynamic viscosity. The Cox–Merz rule relating the linear viscoelastic material properties and the nonlinear material properties is shown to be a direct consequence of convective constraint release. The specific feature of CCR that leads to this result is that the relaxation rate due to convective constraint release is proportional to the shear rate, \(\dot{{\gamma }}\), independent of molecular weight. The viability of this well-known empirical relationship is a direct consequence of a coincidence in the mathematical structure of the linear viscoelastic material properties and convective constraint release. There is no physical analogy or relationship between the molecular relaxation mechanisms operative in linear (diffusive relaxation) and nonlinear (convective relaxation) flow regimes. The polydisperse MLD model predictions of the individual molecular weight component contributions to the flow curve, and interpretations thereof, are effectively identical to those first postulated by Bersted (J Appl Polym Sci 19:2167–2177, 1975, J Appl Polym Sci 20:2705–2714, 1976). Following the theoretical developments, a limited experimental study is executed with a commercial polydisperse polystyrene melt. Nearly quantitative agreement between the polydisperse MLD theory and experimental measurements of steady-shear viscosity and dynamic moduli is achieved over a wide range of shear rates.

Elongational and shear flow in polymer-clay nanocomposites measured by on-line extensional and off-line shear rheometry

Abstract: Rheological behaviour of polymer nanocomposites has been usually characterized by rotational as well as capillary rheometry, which are both time and cost consuming. We have already published that reinforcement in polymer-clay nanocomposites can be estimated very fast using extensional rheometer in combination with a capillary rheometer. It has been proven that the magnitude of melt strength can be correlated with that of tensile strength, i.e. 3D physical network made of layered silicate and polymer matrix, which is responsible for material reinforcement, can be monitored directly using extensional rheometry. Therefore, additional time for samples preparation by press or injection moulding as well for long measurements by tensile testing is not required any more. In this contribution, results of extensional rheometry measured directly during compounding process are presented. In this manner, further reduction in time required for material characterization has been achieved. The samples have been prepared by advanced compounding using a melt pump and special screw geometries. With the use of on-line extensional rheometry and off-line rotational rheometry, different nanocomposites have been tested and the effect of processing conditions (screw speed and geometry in the twin-screw extruder) on elongational and viscoelastic properties has been investigated. It has been found that the level of melt strength measured by extensional rheometry correlates with a high accuracy with dynamic rheological data measured by rotational rheometry. It was hereby confirmed that the network structure made of silicate platelets in polymer melt is reflected in both elongational and shear flow in the same way.

Bread dough rheology: an improved damage function model

Abstract: We describe an improved damage function model for bread dough rheology. The model has relatively few parameters, all of which can easily be found from simple experiments as discussed in this paper. Small deformations in the linear region are described by a gel-like power-law memory function. Then, we consider a set of large non-reversing deformations—stress relaxation after a step of shear, steady shearing and elongation beginning from rest and biaxial stretching. With the introduction of a revised strain measure which includes a Mooney–Rivlin term, all of these motions can be well described by the damage function described previously. For reversing step strains, larger amplitude oscillatory shearing and recoil we present a discussion which shows how the damage function model can be applied in these cases.

Instrument compliance effects revisited: linear viscoelastic measurements

Abstract: Torsional compliance of the torque transducer can be an important issue in linear viscoelastic (LVE) measurements when the sample stiffness is high relative to the instrument stiffness. We evaluated compliance effects of the ARES 2K-FRT on LVE measurements by systematically comparing the results of the frequency sweep mode obtained with 25 and 8 mm plates, respectively. In addition to the transducer, the test fixtures do contribute significantly to the system compliance. Without correction, the upper limit for the complex modulus ∣ G ∗ ∣ is approximately close to 4 × 105 Pa at 10% uncertainty, when using 25 mm plates. This limit is lower than the plateau modulus \(\left( {G_N^0 } \right)\) of most polymers. Therefore, instrument compliance can lead to significant errors for \(G_N^0 \) and wrong scaling for G″ in the plateau and Rouse regions. The respective roles of transducer and tool compliances are discussed. The FRT transducer compliance is corrected in real time in the instrument firmware. Tool compliance is a common problem for all rheometers when measuring stiff samples.

Modeling macromolecular movement in polymer melts and its relation to nonlinear rheology

Abstract: We present direct evidence of the macromolecular network behavior at high deformation rates based on macroscopic simulation of these systems by a group of elastics as a model of flexible-chain polymer concentrated solutions or melts. It was shown that at low deformation rates, the disentanglement process really takes place providing a possibility to irreversible deformations (flow), while at high deformation rates, the dominating effect is the formation of large inhomogeneous structures (“grains” or “bundles”) consisting of flocks of entangled chains. This is a model of the deformation induced flow-to-rubbery transition, which makes the irreversible flow impossible. The attempt to increase the deformation rate leads to the rupture of elastics. So, we constructed a model for the deformation-induced fluid-to-rubber transition at high rates and confirmed it by direct measurements of elastic-to-plastic strain ratio as a function of deformation rate.

On the determination of elastic properties of composites of polycarbonate and multi-wall carbon nanotubes in the melt

Abstract: In this work, the elastic properties of melts of polycarbonate (PC)/multi-wall carbon nanotubes (MWCNT) composites were studied by means of rotational and capillary rheometry. Linear viscoelastic shear oscillations combined with simultaneous electrical measurements, creep recovery experiments in shear and extrudate swell measurements were performed. The application of the fractional Zener model for the phenomenological description of the viscoelastic properties of PC/MWCNT composites in the linear regime is discussed. The modulus of the spring of the fractional Zener model is a measure of elasticity and increases with carbon nanotubes concentration. The results of creep recovery experiments reveal that the microstructure strongly influenced the viscosity and the reversible deformation of the nanocomposites. Below the rheological percolation threshold, agglomeration of carbon nanotubes generally led to an increase of the reversible deformation up to a maximum value. Above the rheological percolation threshold, a larger concentration of carbon nanotubes caused a decrease of the recoverable deformation. As revealed by capillary rheometry, the extrudate swell of the PC/MWCNT composites at shear rates above 100 s − 1 was lower than the extrudate swell of neat polycarbonate which indicates the decrease of reversible deformation caused by the addition of MWCNT in capillary flows (large stress regime). The experimental data are discussed in the context of the current understanding of the rheological properties and the microstructure of polymer composites with carbon nanotubes.

Computations with viscoplastic and viscoelastoplastic fluids

Abstract: This numerical study focuses on regularised Bingham-type and viscoelastoplastic fluids, performing simulations for 4:1:4 contraction–expansion flow with a hybrid finite element–finite volume subcell scheme. The work explores the viscoplastic regime, via the Bingham–Papanastasiou model, and extends this into the viscoelastoplastic regime through the Papanastasiou–Oldroyd model. Our findings reveal the significant impact that elevation has in yield stress parameters, and in sharpening of the stress singularity from that of the Oldroyd/Newtonian models to the ideal Bingham form. Such aspects are covered in field response via vortex behaviour, pressure-drops, stress field structures and yielded–unyielded zones. With rising yield stress parameters, vortex trends reflect suppression in both upstream and downstream vortices. Viscoelastoplasticity, with its additional elasticity properties, tends to disturb upstream–downstream vortex symmetry balance, with knock-on effects according to solvent-fraction and level of elasticity. Yield fronts are traced with increasing yield stress influences, revealing locations where relatively unyielded material aggregates. Analysis of pressure drop data reveals significant increases in the viscoplastic Bingham–Papanastasiou case, O (12%) above the equivalent Newtonian fluid, that are reduced to 8% total contribution increase in the viscoelastoplastic Papanastasiou–Oldroyd case. This may be argued to be a consequence of strengthening in first normal stress effects.

Dispersion state and rheology of hectorite particles in water over a broad range of salt and particle concentrations

Abstract: The relationship between the rheological properties of deionized aqueous suspensions of hectorite particles and the dispersion states of the particles has been studied with a broad range of salt and particle concentrations The shear viscosity of the hectorite suspensions decreases drastically after exhaustively deionizing the suspensions with ion-exchange resins By means of DLS measurements, it is clarified that the average size of the flocs of hectorite particles decreases and reaches the Stokes diameter of the individual particle as the degree of deionization advances This fact strongly supports the idea that the electrical double layer around the hectorite particles expands significantly in the exhaustively deionized state and the particles are well-dispersed individually and do not form a three-dimensional network structure composed of particles, whereas such a network structure forms in the presence of a large amount of salt In the case of exhaustively deionized state, the suspension forms a glassy state, at high particle fractions The results show the importance of the electrical double layer that causes a strong repulsive force among the particles on the particle dispersion state, especially in the exhaustive deionization area below 10 − 4 M, and on the rheological properties; the hectorite suspension can be considered a Newtonian liquid in the deionized state, but it becomes elastic-solid in the presence of salt above a certain concentration confirmed by normal stress measurements

Using rheometry to determine nucleation density in a colored system containing a nucleating agent

Abstract: A new suspension-based rheological method was applied to experimentally study the crystallization of a nucleating agent (NA) filled isotactic polypropylene. This method allows for determination of point nucleation densities where other methods fail. For example, optical microscopy can fail because nucleation densities become too high to be counted (materials with effective NA) or crystallites are not easily visible (colored materials), while differential scanning calorimetry does not allow the effect of flow to be studied. Both quiescent and mild-shear-induced crystallization were investigated. The results show that the addition of a nucleating agent increases the nucleation density by six decades for quiescent crystallization. The effect of shear on crystallization in the presence of a nucleating agent was assessed, and it is demonstrated that, at least for this system, the effect of shear is much smaller than the effect of the nucleating agent.

Non-linear dynamics of semi-dilute polydisperse polymer solutions in microfluidics: effects of flow geometry

Abstract: The non-linear dynamics of a semi-dilute (c/c* = 15) polydisperse polyethylene oxide (PEO) solution in microfluidics are studied experimentally using benchmark contraction–expansion flow geometries with three contraction–expansion ratios (4:1:4, 8:1:8 and 16:1:16) and two narrow channel lengths (L c/D h = 53 and 5.3, where L c is the length of the narrow channel and D h is its hydraulic diameter). Complex flows over a range of elasticity numbers (El), Weissenberg numbers (Wi) and Reynolds numbers (Re) are characterized using micro-particle image velocimetry ( $\upmu$ -PIV) and pressure drop measurements. The evolution of vortex formation and dynamics has been visualized through a step-flow-rate experiment. Various flow dynamics regimes have been quantified and are presented in a Wi–Re diagram. The experimental results reveal that the contraction ratio can result in qualitatively different vortex dynamics of semi-dilute polymer solutions in microfluidics, whereas the length of the narrow channel merely affects the dynamics at a quantitative level. A single elasticity number, if defined by the size of the narrow channel, is not sufficient to account for the effects of contraction ratio on the non-linear vortex dynamics.

Thermorheological behavior of polyethylene: a sensitive probe to molecular structure

Abstract: Recent investigations have shown that different topographies in polyethylene (PE) lead to either thermorheological simplicity (linear and short-chain branched PE) or two different types of thermorheologically complex behavior. Low-density polyethylene (LDPE) has a thermorheological complexity, which can be eliminated by a modulus shift, while long-chain branched metallocene PE (LCB-mPE) has a temperature dependent shape of the spectrum and thus a total failure of the time-temperature superposition principle. The reason for that behavior lies in the different relaxation times of linear and long-chain branched chains, present in LCB-mPE. The origin of the thermorheological complexity of LDPE might be the temperature dependence of the miscibility of the different molar mass fractions that differ in their content of short chain branches.

Coalescence in thermoplastic olefin (TPO) blends under shear flow

Abstract: In this article, the capability of the Lee–Park (LP) model (Lee and Park, J Rheol 38:1405–1425, 1994) in predicting the extent of drop coalescence under transient shear has been evaluated. Thermoplastic olefin blends of polypropylene (PP) and three types of metallocene catalyzed ethylene copolymers (EC) with different melt viscosities were investigated. The interfacial tension between the PP and the ECs was determined by means of linear viscoelastic measurements using a simplified version of the Palierne (Rheol Acta 29:204–214, 1990) model as well as the Choi and Schowalter (Phys Fluids 18:420–427, 1975) equation. Flow-induced coalescence was investigated by shearing the samples at a very low shear rate of 0.01 s − 1. The size evolution and orientation of the dispersed droplets under shear were correlated with the transient rheological data. To account for the non-affine deformation, an additional slip parameter (Lacroix et al., J Non-Newton Fluid Mech 86:37–59, 1999) was introduced into the LP model. The modified model (LPL model) was found to predict well the morphological state of all blends in conjunction with the rheological data, whereas in most of the cases, the LP model significantly underestimated the interfacial area (Q). Coalescence was favored by a decrease of the viscosity of the dispersed phase. Smaller viscosity droplets increased the interfacial mobility and, hence, reduced the drainage time promoting the coalescence of two approaching droplets.

Deformation and orientation of single droplets during shear flow: combined effects of confinement and compatibilization

Abstract: Combined effects of geometrical confinement and compatibilization on the deformation and orientation of single droplets during steady-state shear flow are investigated in a counter-rotating cell by means of microscopic observations. The model system consists of polydimethylsiloxane droplets of varying sizes and viscosities in a polyisobutylene matrix. To this system, a premade polyisobutylene–polydimethylsiloxane block copolymer is added as compatibilizer in different concentrations. For each droplet, the equilibrium interfacial tension is determined by comparing droplet axes with the predictions of the confined Minale model for uncompatibilized droplets at the appropriate degree of confinement. Although large reductions in interfacial tension are seen for all compatibilized droplets, it is shown that the effect of compatibilization on droplet deformation and orientation can efficiently be taken into account in the equilibrium capillary number. This way, for all viscosity ratios and confinement ratios, steady-state data for compatibilized and uncompatibilized droplets coincide, and agree well with the predictions of the confined Minale model at sub-critical conditions. For near-critical capillary numbers, compatibilization slightly reduces droplet deformation and postpones breakup, irrespective of the degree of confinement.