Steady Shear Rheometry Research Articles

Experiments and numerical modelling of secondary flows of blood and shear-thinning blood analogue fluids in rotating domains

The transition from concentric primary flow to non-tangential secondary flow of blood was investigated using experimental steady shear rheometry and numerical modelling. The aims were to: assess the difference in secondary flow in a Newtonian versus shear-thinning blood analogue; and measure the secondary flow in the blood. Both experiments and numerical modelling showed that the transition from primary to secondary flow was the same in a Newtonian fluid and a shear-thinning blood analogue. Experiments showed whole blood transitioned to secondary flow at lower modified Reynolds numbers than the Newtonian fluid; and transition was haematocrit dependent with higher RBC concentrations transitioning at lower modified Reynolds numbers. These results indicate that modelling blood as a purely shear-thinning fluid does not predict the correct secondary flow fields in whole blood; non-Newtonian effects beyond shear-thinning behaviour are influential, and incorporating effects such as multiphase contributions and viscoelasticity, yield stress and thixotropy is necessary.

Understanding the rheology of kaolinite clay suspensions using Bayesian inference

Mud is a suspension of fine-grained particles (sand, silt, and clay) in water. The interaction of clay minerals in mud gives rise to complex rheological behaviors, such as yield stress, thixotropy, and viscoelasticity. Here, we experimentally examine the flow behaviors of kaolinite clay suspensions, a model mud, using steady shear rheometry. The flow curves exhibit both yield stress and rheological hysteresis behaviors for various kaolinite volume fractions (ϕk). Further understanding of these behaviors requires fitting to existing constitutive models, which is challenging due to numerous fitting parameters. To this end, we employ a Bayesian inference method, Markov chain Monte Carlo, to fit the experimental flow curves to a microstructural viscoelastic model. The method allows us to estimate the rheological properties of the clay suspensions, such as viscosity, yield stress, and relaxation time scales. The comparison of the inherent relaxation time scales suggests that kaolinite clay suspensions are strongly viscoelastic and weakly thixotropic at relatively low ϕk, while being almost inelastic and purely thixotropic at high ϕk. Overall, our results provide a framework for predictive model fitting to elucidate the rheological behaviors of natural materials and other structured fluids.

Characterization of Nonlinear Viscoelastic Properties of Enhanced Oil Recovery Polymer Systems Using Steady-Shear Rheometry

SummaryEnhanced oil recovery (EOR) polymer systems such as hydrolyzed polyacrylamide (HPAM) solutions flowing in porous media at high fluxes were reported to cause shear-thickening, a nonlinear viscoelastic (NLVE) phenomenon. Bulk rheological studies are usually performed to characterize the viscous and viscoelastic behaviors of polymer solutions in porous media. To characterize the viscoelastic properties, oscillatory shear rheology based on linear viscoelastic (LVE) data and extensional rheology based on NLVE data have been used. Although both extensional flow and high-speed shear flow can stretch and thicken polymer solutions in a nonlinear fashion, steady-shear rheometry has been used to measure only the viscous and thinning behavior of HPAM systems. In this paper, (a) the thickening ability of HPAM polymer systems formulated with contrasting salinity and molecular weight (MW) concentration is characterized in bulk shear rheometry to analyze if the thickening in the pure shear field correlates with the thickening behavior reported in porous media and other rheometry and (b) the thickening intensity of the polymer solutions of varying salinity concentration in the bulk shear field is compared with the reported mechanical degradation effects in the literature.The shear-thickening index obtained by fitting a power-law model to thickening data in steady-shear rheograms is higher for the high-saline, high-MW low-concentration HPAM systems that have inherently higher nonlinear viscoelasticity. Furthermore, the thickening behavior observed due to salinity variation in bulk steady shear contrasts with linear oscillatory shear behavior but conforms to the thickening behavior observed in porous media and in the extensional field. This signifies that polymer-EOR researchers performing comparative studies on low-salinity and high-salinity polymer floods, and having a shear rheometer at their disposal, must look beyond linear oscillatory rheology and make use of nonlinear steady-shear rheology. There is a direct relation between the shear-thickening index of saline HPAM solutions and their mechanical degradation intensity. This paper shows that the high-speed data in steady-shear rheometry that are usually ignored in EOR literature have useful information and the notion that HPAM solutions are purely thinning in the bulk shear field needs to be reconsidered.

Assessment of Piezoelectric Sensors for the Acquisition of Steady Melt Pressures in Polymer Extrusion

Piezoelectric sensors have made their way into polymer processing and rheometry applications, in particular when small pressure changes with very fast dynamics are to be measured. However, no validation of their use for steady shear rheometry is available in the literature. Here, a rheological slit die was designed and constructed to allow for the direct comparison of pressure data measured with conventional and piezoelectric transducers. The calibration of piezoelectric sensors is presented together with a methodology to correct the data from the inherent signal drift, which is shown to be temperature and pressure independent. Flow curves are measured for polymers showing different levels of viscoelasticity. Piezoelectric slit rheometry is validated and its advantage for the rheology of thermodegradable materials with viscosity below 100 Pa·s is highlighted.

Initial Droplet Size Impacts pH-Induced Structural Changes in Phase-Separated Polymer Dispersions.

The effect of pH change on the morphology of whey protein isolate (WPI)-pectin dispersions obtained from phase-separated systems after mild shear was studied. The purpose of this study was to examine the impact of mixing speed on the initial particle size of biopolymer complexes and their structure morphology after sequentially changing the pH. Therefore, solutions of WPI and pectin were combined at pH 6.1, allowed to phase separate and were then mildly homogenized at 50, 100, and 150 rpm, respectively, to form a dispersion containing differently sized WPI droplets in a surrounding pectin-rich phase. Each dispersion was then subjected to a pH change, such as 6.1 to 5.2 and 3.2, by slowly adding hydrochloric acid. The systems morphology, size, appearance, rheology, and storage stability was then characterized by optical microscopy, static light scattering, visual inspections, and steady shear rheometry to gain insights into the structural rearrangements. Results indicated substantial changes in the structure of the dispersion when the pH was changed. Formation of core-shell structures from the WPI droplets was observed at an intermediate pH. There, initial droplet size was found to affect structures formed, that is, core-shell type particles would only form if droplets were large (>1.5 μm) prior to pH change. Insights gained may be of importance to food manufacturers intending to create new structures from mixtures of proteins and carbohydrates.

Sequential modulation of pH and ionic strength in phase separated whey protein isolate – Pectin dispersions: Effect on structural organization

The effect of pH change, salt addition or sequences of pH change followed by salt addition or vice versa on the structure of a whey protein isolate – pectin dispersion obtained from a phase separated system after mild homogenization was studied. Solutions of whey protein isolate and pectin were combined at pH 6.1, allowed to phase separate and then mildly homogenized to form a dispersion consisting of whey protein isolate droplets in a surrounding pectin phase. The dispersion was then subjected to a pH change (6.1 → 5.2 → 3.2), to an addition of NaCl (0–200 mM) or to a combination thereof. The systems morphology, size, appearance, and rheology were then characterized by optical and confocal microscopy, static light scattering, turbidity measurements, visual inspections and steady shear rheometry to gain insights into structural rearrangements. Results indicated profound alterations in the structure of dispersion both when pH was changed and when NaCl was added. Formation of core–shell structures from the whey protein isolate droplets were observed when pH was decreased while clusters of droplets were formed when NaCl was added. The type of sequence (pH then salt or salt then pH) had no influence on the morphology of the structures created. Results were discussed in the context of changes in molecular interactions between the two biopolymers involved in transitions from segregative to aggregative phase separation processes. The gained insights may be of importance to food manufacturers intended on creating new structures from mixtures of proteins and carbohydrates.

Rheology of fresh cement paste with superplasticizer and nanosilica admixtures studied by response surface methodology

Thirteen cement pastes, keeping the water/cement ratio constant at w/c = 0.30, with different amounts of two admixtures were prepared. A superplasticizer (modified polycarboxylic ether polymer) was studied in the range of 0.14–1.00% (over cement weight), while a viscosity-modifying admixture (nanosilica aqueous dispersion) was tested at 0.50–3.00% range (over cement weight). Oscillatory and steady shear rheometry, as well as results of modified slump test and Marsh funnel flow time of 13 formulations were evaluated by response surface methodology, in order to find the optimum recipe that reduces the viscosity while keeping the paste workability. It was predicted that a formulation with 0.39% w/w of superplasticizer and 2.78% w/w of viscosity-modifying admixture would provide a paste with a fifth of the viscosity of control (cement paste without any admixture), but with an acceptable slump. This formulation was prepared and tested, and the plastic viscosity was 1.00 Pa-s, while the oscillatory yield stress was 330 Pa. The admixtures did not affect the compression strength (48 MPa average, after 28 days), compared with the control.

Impact of emulsifier-polysaccharide interactions on the stability and rheology of stabilised oil-in-water emulsions

In the present study, the interactions between negatively charged sodium stearoyl 2-lactylate (SSL) and positively charged chitosan were studied in solution and on oil-water interfaces. Phase diagrams of the SSL/chitosan systems were obtained at both pH 4.0 and 5.7 by preparing concentrated solutions and mixing them at different ratios. Optical microscopy revealed that the complexes formed at pH=4.0 were smaller than those formed at pH=5.7 and that the size of the complexes decreased as the SSL/chitosan ratio was reduced. Emulsions (10% w/w oil) were prepared at pH=5.7 by using the complexes of a constant SSL concentration (0.4% w/w) and at different SSL/chitosan weight ratios. The droplet size of the emulsions that were formed with the complexes was greater than those containing only SSL. Using complexes of low SSL/chitosan ratio as emulsifying agent resulted in the production of emulsions with enhanced viscosity as determined by steady shear rheometry. However, none of the samples showed a gel-like behaviour since in all cases the G’ (storage modulus) was lower that G” (loss modulus). The presence of polysaccharide at the interface resulted in an increased stability of the emulsions subjected to environmental stresses, such as heat treatment and a freeze thawing process.

Rheological and self-assembly behavior of cationized hydroxycellulose with ionic surfactants

A series of cationized hydroxycelluloses (cat-HEC) with different molar substitution have been synthesized. The forming interactions between sodium dodecyl sulfate (SDS) with cat-HEC and their temperature-dependent rheological behavior have been studied by steady-shear rheometry. Different molar substitution and the concentration of SDS can effect the rheological behavior of polymer matrices. Self-assembly behaviors of these supramolecules in water were studied through transmission electron microscopy (TEM). The results showed that different shapes were observed if increasing molar of SDS.

Steady shear rheometry of dissipative particle dynamics models of polymer fluids in reverse Poiseuille flow

Polymer fluids are modeled with dissipative particle dynamics (DPD) as undiluted bead-spring chains and their solutions. The models are assessed by investigating their steady shear-rate properties. Non-Newtonian viscosity and normal stress coefficients, for shear rates from the lower to the upper Newtonian regimes, are calculated from both plane Couette and plane Poiseuille flows. The latter is realized as reverse Poiseuille flow (RPF) generated from two Poiseuille flows driven by uniform body forces in opposite directions along two-halves of a computational domain. Periodic boundary conditions ensure the RPF wall velocity to be zero without density fluctuations. In overlapping shear-rate regimes the RPF properties are confirmed to be in good agreement with those calculated from plane Couette flow with Lees-Edwards periodic boundary conditions (LECs), the standard virtual rheometer for steady shear-rate properties. The concentration and the temperature dependence of the properties of the model fluids are shown to satisfy the principles of concentration and temperature superposition commonly employed in the empirical correlation of real polymer-fluid properties. The thermodynamic validity of the equation of state is found to be a crucial factor for the achievement of time-temperature superposition. With these models, RPF is demonstrated to be an accurate and convenient virtual rheometer for the acquisition of steady shear-rate rheological properties. It complements, confirms, and extends the results obtained with the standard LEC configuration, and it can be used with the output from other particle-based methods, including molecular dynamics, Brownian dynamics, smooth particle hydrodynamics, and the lattice Boltzmann method.

Universal Scaling of Linear and Nonlinear Rheological Properties of Semidilute and Concentrated Polymer Solutions

Using oscillatory and steady-shear rheometry on polystyrene (PS) solutions in tricresyl phosphate (TCP) with three nearly monodisperse molecular weights at six values of the reduced concentration c/ce, where ce is the entanglement concentration, we show that for each value of c/ce below 2.0 rheological functions are successfully superimposed using de Gennes “blob” scaling laws for the concentration-dependent plateau modulus GN0, and the equilibration time τe,scaling using the literature value of the solvent-quality exponent ν = 0.53 (Solomon, M. J.; Muller, S. J. J. Polym. Sci., Polym. Phys. 1996, 334, 181−192). However, once the polymer volume fraction exceeds the “swelling volume fraction” ϕs, above which the polymer takes a random walk configuration on all length scales even in a good solvent, this universal scaling breaks down and the polymer conformation appears to be governed by Colby−Rubinstein’s scaling laws for Θ solutions. We estimate that all polybutadiene solutions in phenyl octane (a good sol...

Influence of Pressure on Boron Cross-Linked Polymer Gels

Using steady-shear rheometry in combination with high-pressure 11B nuclear magnetic resonance spectroscopy (11B NMR), we have found that gels formed from water-soluble polymers containing vicinal hydroxyl groups cross-linked with various boron-containing compounds undergo significant structural changes that result in a pronounced loss of viscosity when placed under pressure. Importantly, gels from other cross-linking agents tested, including Ti(IV) and Zr(IV), did not show this loss in viscosity. The experimental study probed pressure-induced changes to both galactomannan and polyvinyl alcohol (PVA) gels cross-linked with either aryl boronic acids or alkali metal boron-containing salts using pressure conditions that ranged from atmospheric to 680 bar and temperatures that ranged from 20 to 65 °C. Significantly, the pressure-induced losses in viscosity and, to a somewhat lesser extent, the concomitant pressure-induced 11B NMR spectral changes were found to be reversed upon lowering the pressure.

Novel hydroxyethylcellulose‐graft‐poly acrylamide copolymer for separation of double‐stranded DNA fragments by CE

A novel separation medium, hydroxyethylcellulose-graft-polyacrylamide (HEC-g-PAM) synthesized by atom transfer radical polymerization (ATRP), used for dsDNA separation by CE is presented. The separation performance of HEC-g-PAM, which has the same graft density and different graft length, has been investigated in Tris-boric acid-EDTA (TBE) buffer solvent mixtures. The temperature-dependent rheological behavior of HEC-g-PAM was also studied by steady-shear rheometry. The results showed that dsDNA fragments between 72 and 1353 bp was achieved with a 30 cm effective capillary length at 150 V/cm using this type of graft copolymer as a separation medium in bare fused-silica capillaries, and separation improvement is obtained in HEC-g-PAM compared with HEC and poly(dimethylacrylamide (PDMA).

Structure–property relationships of nanocomposite-like polymer blends with ultrahigh viscosity ratios

We have investigated the structure–property relationships and the effects of a viscosity ratio on the rheological properties of nanocomposite-like polymer blends using oscillatory and steady shear rheometry and optical microscopy. These immiscible blends are consisted of ultrahigh viscous polybutadiene (PB1), high viscous polybutadien (PB2), and low viscous polydimethylsiloxane (PDMS). The PB1/PDMS blends with an ultrahigh viscosity ratio ( λ=162,000) exhibit non-Newtonian fluids behavior for Ω≥0.1 while the PDMS/PB2 blends ( λ=37) exhibit pseudo-Newtonian fluids behavior for Ω>0.6, where Ω is the weight fraction of PB1 or PB2 in the blends. The viscoelastic properties of the PB1/PDMS blends increase systematically with an increasing the weight fraction of PB1, and then exhibit plateau values above a certain maximum weight fraction ( Ω m) of PB1. In addition the viscoelastic properties of the PB1/PDMS blends are not affected by the change of blend morphology or phase inversion, where Ω m is larger than the phase inversion weight fraction ( Ω p). In contrast the viscoelastic properties of the PB2/PDMS blends follow a positive-deviation mixing rule and are significantly affected by phase inversion.

Influence of xyloglucan on gelatinization and retrogradation of tapioca starch

Dynamic and steady shear rheometry and differential scanning calorimetry (DSC) were used to investigate effects of xyloglucan (XG) on gelatinization and retrogradation of tapioca starch (TS). The viscosity of TS/XG pastes immediately after gelatinization increased with increasing XG content at the total polysaccharide concentration of 3.5%. Gelatinized TS alone showed pseudoplastic flow at low shear rates and dilatant behavior at higher shear rates (about >1 s −1), while mixtures with XG did not show dilatancy. Mechanical spectra of TS pastes containing XG were more liquid-like than those of TS pastes without XG. XG provides shear stability to the TS during storage. Increases in dynamic moduli during storage at 5 °C were suppressed in the presence of XG. In contrast, the retrogradation ratio determined based on DSC increased more rapidly in the presence of XG. These results suggest that XG forms a continuous liquid phase in a mixture to impart better mechanical stability during storage but to accelerate re-ordering of starch polysaccharides by effectively reducing the amount of water available for starch.

Comparative studies on solution characteristics of mannuronan epimerized by C-5 epimerases

Both dilute and concentrated solutions of bacterial Mannuronan (MANNA) and its epimerized products by AlgE1 at 5 and 24 h, named MANNAEp1t5h and MANNAEp1t24h, respectively, and AlgE4 (MANNAEp4) have been studied as a function of variables such as polymer concentration and ionic strength (NaCl) in order to investigate the macromolecular solution properties of these innovative polyuronic acids having the same charge density but different composition and sequence of β- d-mannuronic acid (-M-), α- l-guluronic acid (-G-) or MG-blocks. Measurements of intrinsic viscosity [ η] as a function of ionic strength, I, by capillary viscometry has led to an estimate of the Smidsrød-Haug parameter B, an index useful to characterize the stiffness of polymeric chains. The results are largely consistent with much of the published data relative to chain extension and conformational freedom around the torsional angles of the glycosidic linkages occurring in alginates. Steady shear rheometry provided information about the coil-overlapping parameter c*, which marks the transition from dilute to concentrated solution. The slopes of the double logarithmic plots of η sp vs. c[ η] both at low and high degrees of coil overlap suggest that all samples solutions behave like linear polymer entangled network systems. The value of c* is strictly influenced by the stiffness of the chains, and hence by the primary structure. Dynamic shear rheometry shows that the frequency dependence of dynamic viscosity is only partially superimposable to the shear rate dependence of viscosity. Such behaviour may be ascribed to the presence of semiflexible polymeric coils in a non-totally destructured entangled state. By solvent/non-solvent (H 2O/isopropanol) fractionation carried out on mannuronan, a set of samples with different average molecular weights and narrow polydispersities were obtained. Triple detection GPC allowed the evalutation of the Mark–Houwink–Sakurada parameters as well as of the characteristic ratio C ∞ for one of the fractionated MANNA samples. The chain persistence length was estimated by the wormlike chain model.

Rheological properties of gellan, κ-carrageenan and alginate polysaccharides: effect of potassium and calcium ions on macrostructure assemblages

The effect of added ions (0.5 mM K + or 0.25 mM Ca 2+) on the critical overlap ( C*) and gelation ( C o′) concentrations for gellan, κ-carrageenan and alginate polysaccharides were investigated using steady shear rheometry. Both C* and C o′ decreased for all solutions with added salts, a trend more pronounced for gellan and alginate with added Ca 2+, and κ-carrageenan with added K +. Aggregation near the sol–gel transition was modeled as a percolation-type transition. In addition, the frequency dependence of the viscoelastic storage ( G′) and loss ( G″) moduli for bulk and localized areas within a 0.7% (w/w) gellan sol were measured using a controlled stress and oscillating magnetic bead rheometer, respectively. Flow behavior of the bulk was in the terminal and plateau relaxation regions of the viscoelastic spectrum, whereas localized behavior was in the plateau and rubber–glass transition regions.

A novel thermogelling matrix for microchannel DNA sequencing based on poly-N-alkoxyalkylacrylamide copolymers.

We have developed a novel class of thermogelling polymer networks based on poly-N-alkoxyalkylacrylamides, and demonstrated their use as DNA sequencing matrices for high-throughput microchannel electrophoresis in capillary arrays. Polymers and copolymers of N-ethoxyethylacrylamide (NEEA) and N-methoxyethylacrylamide (NMEA) were synthesized by aqueous-phase free-radical polymerization and characterized by tandem gel permeation chromatography-multi-angle laser light scattering. These copolymer matrices exhibit "re-entrant"-type volume phase transitions, forming entangled networks with high shear viscosity at low (< 20 degrees C) and high (> 35 degrees C) temperatures, and undergoing a "coil-to-globular", lower critical solution temperature (LCST)-like phase transition over an intermediate temperature range (20-35 degrees C). Hence, matrix viscosity is relatively low at room temperature (25 degrees C), and increases rapidly above 35 degrees C. The material properties and phase behavior of these thermogelling polymer networks were studied by steady-shear rheometry. These matrices are easily loaded into capillary arrays at room temperature while existing as viscous fluids, but thermogel above 35 degrees C to form transparent hydrogels via a thermo-associative phase transition. The extent of the intermediate viscosity drop and the final viscosity increase depends on the composition of the copolymers. DNA sequencing by capillary array electrophoresis with four-color laser-induced fluorescence (LIF) detection shows that these thermogelling networks provide enhanced resolution of both small and large DNA sequencing fragments and longer sequencing read lengths, in comparison to appropriate control (closely related, nonthermogelling) polymer networks. In particular, a copolymer comprised of 90% w/w NMEA and 10% w/w NEEA, with a molecular mass of approximately 2 MDa, delivers around 600 bases at 98.5% base-calling accuracy in 100 min of electrophoresis.

Rheological properties of gellan solutions: effect of calcium ions and temperature on pre-gel formation

The critical overlap concentration ( C ∗) for dilute gellan dispersions was studied with respect to the effect of calcium ion concentration (0.25 and 0.5 mM Ca 2+) and temperature (5–25 °C), using steady shear rheometry. C ∗ was assumed to be associated with the formation of pre-gel structure at gellan concentrations close to the gelation threshold. As temperature decreased or Ca 2+ concentration increased, C ∗ declined. The temperature dependence of C ∗ in the presence and absence of added Ca 2+ could be described by the Eldridge–Ferry model, where heat of cross-link formation declined in a nearly linear manner with increasing levels of Ca 2+. This was interpreted as the result of a shift from a system dominated by hydrogen bonds to one mediated by ionic interactions. An alternative model, developed by Tang, Tung, and Zeng (1997a) for an ion-mediated polymer gelation mechanism, determined the energy of cross-linking at C ∗ to be only slightly less than reported for self-supporting gellan gels. This model also indicated that the number of Ca 2+ ions involved in cross-linking two adjacent double helical gellan strands was three, which suggested that the minimum size of a junction zone was equivalent to a single pitch of the helix. In addition, the apparent viscosity of 0.1 and 0.2% (w/w) aqueous gellan dispersions was examined over a range of Ca 2+ concentrations and shear rates. Results indicated an increase in apparent viscosity to a maximum, which corresponded to a 0.5:1 ratio of Ca 2+ ions to gellan carboxyl groups. Upon further Ca 2+ addition, apparent viscosity declined, indicating a weakening or breakdown of pre-gel structure.

Contribution of the isotropic phase to the rheology of partially anisotropic coal-tar pitches

A commercial impregnating coal-tar pitch was thermally treated at 430°C for 2–5 h in nitrogen. Resultant pitches, which contained mesophase ranging from 10 to 46 vol.%, were filtered to separate the isotropic phases, in order to study their contribution to the rheology of the whole pitches. The parent pitch, thermally treated pitches and the individual isotropic phases were characterized by measuring their elemental composition, softening point, carbon yield, solubility in toluene and in N-methyl-2-pyrrolidone, mesophase content (by optical microscopy) and X-ray diffraction parameters. The rheological behavior of the pitches and the isotropic phases was studied using steady shear rheometry. Results showed that not only had the whole pitches, which were partially anisotropic, a non-Newtonian behavior, but also the single isotropic phases.