Showing papers in "Rheologica Acta in 2021"

Effective viscosity and Reynolds number of non-Newtonian fluids using Meter model

Abstract: The Meter model (a four-parameter model) captures shear viscosity–shear stress relationship (S-shaped type) of polymeric non-Newtonian fluids. We devise an analytical solution for radial velocity profile, average velocity, and volumetric flow rate of steady-state laminar flow of non-Newtonian Meter model fluids through a circular geometry. The analytical solution converts to the Hagen–Posseuille equation for the Newtonian fluid case. We also develop the formulations to determine effective viscosity, Reynolds number, and Darcy’s friction factor using measurable parameters as available rheological models do not correctly define these parameters for a given set of flow condition in circular geometry. The analytical solution and formulations are validated against experimental data. The results suggest that the effective Reynolds number and effective friction factor estimated using the proposed formulation help characterize non-Newtonian fluid flow through a circular geometry in laminar and turbulent flows.

Turning a yield-stress calcite suspension into a shear-thickening one by tuning inter-particle friction

Abstract: We show that a suspension of non-Brownian calcite particles in glycerol-water mixtures can be tuned continuously from being a yield-stress suspension to a shear-thickening suspension—without a measurable yield stress—by the addition of various surfactants. We interpret our results within a recent theoretical framework that models the rheological effects of stress-dependent constraints on inter-particle motion. Bare calcite particle suspensions are found to have finite yield stresses. In these suspensions, frictional contacts that constrain inter-particle sliding form at an infinitesimal applied stress and remain thereafter, while adhesive bonds that constrain inter-particle rotation are broken as the applied stress increases. Adding surfactants reduces the yield stress of such suspensions. We show that, contrary to the case of surfactant added to colloidal suspensions, this effect in non-Brownian suspensions is attributable to the emergence of a finite onset stress for the formation of frictional contacts. Our data suggest that the magnitude of this onset stress is set by the strength of surfactant adsorption to the particle surfaces, which therefore constitutes a new design principle for using surfactants to tune the rheology of formulations consisting of suspensions of adhesive non-Brownian particles.

Rheological response of soft flake-shaped carbonyl iron water-based MR fluid containing iron nanopowder with hydrophilic carbon shell

Abstract: In this work, the rheological response and sedimentation stability of micron-sized flake-shaped carbonyl iron (CI) water-based magnetorheological (MR) fluid were investigated using iron nanopowder with hydrophilic carbon shell as an additive. The rheological behaviors of MR fluid containing 1 wt% iron nanopowder were measured with and without a magnetic field under shear rate, shear strain, and shear stress sweep mode using a parallel plate rotational rheometer. The magnetic field strength was varied from 0 to 131 kA/m, and in the presence of magnetic field strength, micron flake-shaped CI and iron nanopowder particles lead to form a robust columnar structure. It was observed that using 1 wt% of iron nanopowder with hydrophilic carbon shell, the rheological responses and sedimentation stability of MR suspension were enhanced. We fitted the shear stress as a function of shear rate curves using the Bingham-plastic flow model, which shows a good agreement with the experimental results. We find that yield stress obtained in shear rate sweep mode was larger than that in the shear strain and shear stress sweep mode because, in shear strain and shear stress mode, the responding strain (or stress) may be unable to follow the stress (or strain) applied and a smaller value of strain (or stress) was observed. The amplitude sweep reveals a transition from viscoelastic-to-viscous behavior at the critical strainγcrt = 0.1%. The storage modulus (G') of the MR fluid showed a stable plateau region over the small strain area γ ≤ γcrt and G'independent of strain amplitude. The frequency sweep demonstrated that storage moduli (G') exhibit elastic response and stable plateau region over the entire frequency range, suggesting the distinguished solid-like behavior of the MR fluid.

Rheological properties and swelling behavior of nanocomposite preformed particle gels based on starch-graft-polyacrylamide loaded with nanosilica

Abstract: Gel treatment using preformed particle gels is a technique applied in mature reservoirs to control excess water production. In this study, a novel nanocomposite gel was proposed and optimized for better performance in high-salinity and high-temperature conditions of oilfields. Gels were prepared by grafting copolymerization of crosslinked polyacrylamide onto starch (starch-g-CPAM) and loaded with silica nanoparticles. Different tests were performed on nanocomposite prepared particle gels (NCPPGs) to investigate the effect of silica content (2–10 wt%), temperature, and brine concentration on network structure, swelling, and mechanical behavior of the gels. The NCPPG with 5 wt% showed a superior swelling ratio. Rheological studies depicted that nanosilica could increase the mechanical strength of the NCPPGS in high temperature and high salinity brines (90 °C and 225,000 ppm) up to 900 Pa. Finally, loading nanosilica up to 5 wt. % into the NCPPG resulted in capable mechanical stability and water uptake under harsh conditions.

Effect of particle size distribution and shear rate on relative viscosity of concentrated suspensions

Abstract: Suspension rheology is of widespread importance to industry and research. The rheology of spherical silica particle suspensions with different particle volume fractions and particle size distribution under different shear rates has been determined experimentally. Furthermore, the volume fraction at random close packing (φrcp) was introduced as the parameter of the initial relative viscosity model. In addition, suspension relative viscosity model including particle volume fraction, particle size distribution, and shear rate was obtained. The results showed that as the particle volume fraction increased, the shear thinning effect of the suspensions became more apparent. The relative viscosity decreased as the shear rate increased, and the decrease rate increased as the suspension particle volume fraction increased. As the range of particle size distribution increased, the relative viscosity of the suspensions decreased significantly, and the relative viscosity of the suspensions was independent of particle size in a certain range. The prediction of the suspension relative viscosity model proposed in this paper had a high degree of matching with the experimental data, effectively predicting the rheology of concentrated suspensions.

Rheological behavior of carboxymethylcellulose and cellulose nanocrystal aqueous dispersions

Abstract: This study investigated the rheological properties of cellulose nanocrystal (CNC) suspensions in carboxymethylcellulose (CMC) polymer solutions by studying steady shear viscosities, linear viscoelastic behaviors, applicability of the Cox–Merz rule, and time-dependent behavior. The rheological measurement showed that interactions between CNC and CMC resulted in liquid crystalline domains and a substantial increase in viscosity, shear-thinning, elasticity, and thixotropy. Depletion flocculation formed a percolated structure of liquid crystalline domains, resulting in a gel-like structure, which was evidenced by polarized optical microscopy. The strong effects on rheological properties by mixing small quantities of CMC and CNC suggest that this system may find applications where tunable rheological properties of aqueous polymeric systems are desirable.

Temperature effects and temperature-dependent constitutive model of magnetorheological fluids

Abstract: The knowledge of the temperature effect on magnetorheological fluid is critical for accurate control of magnetorheological devices, since the temperature rise during operation is unavoidable due to coil energization, wall slip, and inter-particle friction. Based on a typical commercial magnetorheological fluid, this work investigates the effect of temperature on magnetorheological properties and its mechanisms. It is found that temperature has a significant effect on the zero-field viscosity and shear stress of magnetorheological fluid. The Herschel-Bulkley model that has high accuracy at room temperature does not describe accurately the shear stress of magnetorheological fluids at high temperatures, as its relative error is even up to 21% at 70 °C. By analyzing the sources of shear stress in magnetorheological fluids, a novel constitutive model with temperature prediction is proposed by combining the Navier–Stokes equation and viscosity-temperature equation. The experimental results show that the error of the novel constitutive model decreases by 90% at different temperatures and magnetic field strengths, exhibiting an excellent accuracy. This temperature-dependent constitutive model allows the properties of an MR fluid to be widely characterized only in a few experiments.

Shear-induced crystallization of polyamide 11

Abstract: Shear-induced formation of crystal nuclei in polyamide 11 (PA 11) was studied using a conventional parallel-plate rheometer. Crystallization of PA 11 after shearing the melt at different rates for 60 s was followed by the evolution of the complex viscosity. The sheared samples showed in an optical microscope a gradient structure along the radius, due to the increasing shear rate from the center to the edge. The critical shear rate for shear-induced formation of nuclei was identified at the position where a distinct change of the semicrystalline superstructure is observed, being at around 1 to 2 s−1. Below this threshold, a space-filled spherulitic superstructure developed as in quiescent-melt crystallization. Above this value, after shearing at rates between 1 and 5 s−1, an increased number of point-like nuclei was detected, connected with formation of randomly oriented crystals. Shearing the melt at even higher rates led to a further increase of the nuclei number and growth of crystals oriented such that the chain axis is in parallel to the direction of flow. In addition, optical microscopy confirmed formation of long fibrillar structures after shearing at such condition. The critical specific work of flow of PA 11 was calculated to allow a comparison with that of polyamide 66 (PA 66). This comparison showed that in the case of PA 11 more work for shear-induced formation of nuclei is needed than in the case of PA 66, discussed in terms of the chemical structure of the repeat unit in the chains.

Enhanced magnetorheological effect of suspensions based on carbonyl iron particles coated with poly(amidoamine) dendrons

Abstract: Particle oxidation constitutes a serious ageing phenomenon in magnetorheological suspensions, bringing about deterioration in performance. This study describes commercial carbonyl iron particles that were successfully coated with poly(amidoamine) dendrons and then applied as an oxidation-resistant dispersed phase in magnetorheological suspensions. A synthesis method was adhered to whereby the particles were sequentially treated with ethylenediamine and methyl acrylate, leading to the formation of generation 2 and 2.5 dendrons; these had the capacity for composite particles with a nano-scale dendritic layer to be prepared on their surfaces. Success in applying the coating was confirmed by various techniques, including XPS, TEM, EDX, FTIR and Raman spectroscopy. The controlled approach adopted to coating the carbonyl iron particles resulted in them exhibiting sufficient oxidation stability, with only an ~ 4.5–4.7% decrease in saturation magnetization. Of interest was that their magnetorheological suspensions demonstrated ca 4.8% and 4% higher dynamic yield stress than a suspension based on non-modified particles at the highest intensity of magnetic field investigated, i.e. 438 kA m–1. Notably, sedimentation stability was evaluated by a unique method that involved the use of a tensiometer with a specific testing probe. The aforementioned coating process led to enhanced sedimentation stability of the magnetorheological suspensions based on coated particles possibly due to decrease in the overall density of the particles, enhanced dispersion stability and reduction in the size of their agglomerates in the silicone oil mixtures that were confirmed by optical microscopy. Modification of the particles as proposed has the potential to overcome one of the primary drawbacks of magnetorheological suspensions, this being oxidation instability (which leads to what is referred to as “in-use-thickening”), without negatively affecting their performance in the presence of a magnetic field.

Numerical analysis of a monotube mixed mode magnetorheological damper by using a new rheological approach in CFD

Abstract: In this study, the analytical Herschel-Bulkley fluid model of a monotube mixed mode MR damper was examined. The MR damper was modeled and simulated by using computational fluid dynamics (CFD) and magnetic finite elements analysis (FEA). The magnetic effects were modeled in a coupled manner with CFD. The actual rheological data was used in the CFD solver to find the cell-based viscosity by using a shear stress interpolation method. The MR damper was manufactured, and experiments were conducted to find the force-displacement and force-velocity correlations, and a good agreement was found between the numerical results and experimental data. The experimental results were also compared with analytical and numerical models under various current values. The CFD results are valuable in predicting the actual characteristics of the non-Newtonian flow inside an MR damper, and it can be used for various non-Newtonian fluid CFD models.

A semi-analytical approach to model drilling fluid leakage into fractured formation

Abstract: Loss of circulation while drilling is a challenging problem that may interrupt operations and contaminate the subsurface formation. Analytical modeling of fluid flow in fractures is a tool that can be quickly deployed to assess drilling mud leakage into fractures. A new semi-analytical solution is developed to model the flow of non-Newtonian drilling fluid in fractured formation. The model is applicable for various fluid types exhibiting yield-power law (Herschel-Bulkley). We use finite-element simulations to verify our solutions. We also generate type curves and compare them to others in the literature. We then demonstrate the applicability of the proposed model for two field cases encountering lost circulations. To address the subsurface uncertainty, we combine the semi-analytical solutions with Monte Carlo and generate probabilistic predictions. The solution method can estimate the range of fracture conductivity, parametrized by the fracture hydraulic aperture, and time-dependent fluid loss rate that can predict the cumulative volume of lost fluid.

Rheological variability of Pseudomonas aeruginosa biofilms

Abstract: A current trend in biofilm research uses viscoelasticity to evaluate biological mechanisms used to adapt to environmental changes. However, due to un-quantified variability in mechanical properties, it is difficult to make comparisons between different growth conditions, strains, or species. To interpret biofilm viscoelasticity requires understanding its statistical variance divorced from experimental technique. In this work, 23 P. aeruginosa biofilms at air/water interfaces are characterized using an interfacial rheometer. This provides the least disturbance to the biofilm, allowing a truer sense of biological variability. Experiments were conducted in 3 sets in 2 different labs. Moduli varied 1 order of magnitude within a single set. Statistical analysis confirms that under seemingly identical growth conditions, viscoelastic moduli were different based on experiment location, which is attributed to minor changes in ambient conditions. Understanding this variance will lead to a better evaluation and comparison of biofilm formation in future studies.

Elongational viscosity and brittle fracture of bidisperse blends of a high and several low molar mass polystyrenes

Abstract: Elongational viscosity data of four well-characterized blends consisting of 10% mass fraction of monodisperse polystyrene PS-820k (molar mass of 820 kg/mol) and 90% matrix polystyrenes with a molar mass of 8.8, 23, 34, and 73 kg/mol, respectively, as reported by Shahid et al. Macromolecules 52: 2521–2530, 2019 are analyzed by the extended interchain pressure (EIP) model including the effects of finite chain extensibility and filament rupture. Except for the linear-viscoelastic contribution of the matrix, the elongational viscosity of the blends is mainly determined by the high molar mass component PS-820k at elongation rates when no stretching of the lower molar mass matrix chains is expected. The stretching of the long chains is shown to be widely independent of the molar mass of the matrix reaching from non-entangled oligomeric styrene (8.8 kg/mol) to well-entangled polystyrene (73kg/mol). Quantitative agreement between data and model can be obtained when taking the interaction of the long chains of PS-820k with the shorter matrix chains of PS-23k, PS-34k, and PS-73k into account. The interaction of long and short chains leads to additional entanglements along the long chains of PS-820k, which slow down relaxation of the long chains, as clearly seen in the linear-viscoelastic behavior. According to the EIP model, an increased number of entanglements also lead to enhanced interchain pressure, which limits maximal stretch. The reduced maximal stretch of the long chains due to entanglements of long chains with shorter matrix chains is quantified by introducing an effective polymer fraction of the long chains, which increases with the increasing length of the matrix chains resulting in the excellent agreement of experimental data and model predictions.

Exploring the gelation of aqueous cellulose nanocrystals (CNCs)-hydroxyethyl cellulose (HEC) mixtures.

Abstract: We investigated the gelation and microstructure of cellulose nanocrystals (CNCs) in nonionic hydroxyethyl cellulose (HEC) solutions. Cellulose nanocrystals (CNCs) with a particle length of 90 nm and width of 8 nm currently produced by acid hydrolysis of wood pulp were used in this study. The microstructures of CNCs/polymer suspensions were investigated by performing linear small amplitude oscillatory shear (SAOS) and nonlinear large amplitude oscillatory shear (LAOS), in addition to constructing CNCs phase diagrams and measuring steady-state shear viscosities. Significant viscosity increases at low shear rates coupled with high shear thinning behaviors were observed in CNCs in HEC solutions above the overlapping concentration of HEC. The physical strength of CNCs/HEC solution gels increased with the increase in CNCs concentration and resembled the weakly crosslinked gels according to the scaling of linear dynamic mechanical experiments. According to LAOS analysis, CNCs/HEC mixtures showed type III behavior with intercycle stress softening, while the samples showed stress stiffening in single cycles.

Modeling elongational viscosity and brittle fracture of polystyrene solutions

Abstract: Elongational viscosity data of well-characterized solutions of 3–50% weight fraction of monodisperse polystyrene PS-820k (molar mass of 820,000 g/mol) dissolved in oligomeric styrene OS8.8 (molar mass of 8800 g/mol) as reported by Andre et al. (Macromolecules 54:2797–2810, 2021) are analyzed by the Extended Interchain Pressure (EIP) model including the effects of finite chain extensibility. Excellent agreement between experimental data and model predictions is obtained, based exclusively on the linear-viscoelastic characterization of the polymer solutions. The data were obtained by a filament stretching rheometer, and at high strain rates and lower polymer concentrations, the stretched filaments fail by rupture before reaching the steady-state elongational viscosity. Filament rupture is predicted by a criterion for brittle fracture of entangled polymer liquids, which assumes that fracture is caused by scission of primary C-C bonds of polymer chains when the strain energy reaches the bond-dissociation energy of the covalent bond (Wagner et al., J. Rheology 65:311–324, 2021).

Effect of carrier fluid and particle size distribution on the rheology of shear thickening suspensions

Abstract: We present a systematic rheological study on the fumed silica suspensions to understand the effect of particle and fluid parameters on the critical shear rate and shear thickening ratio. It was observed that removal of air bubbles and water contamination is crucial to prepare good shear thickening fluids. A careful sample preparation method was adopted to properly disperse the fumed silica particles in polyethylene glycol solution, and we achieved discontinuous shear thickening at low particle concentration. It was observed that the critical shear rate in shear thickening suspension is strongly influenced by both carrier fluid and particle concentration. However, the shear thickening ratio is mainly influenced by the particle parameters and the frictional forces between the particles. Increasing the amount of smaller particles in the suspension significantly decreases the maximum viscosity and shifts the onset of shear thickening to higher values of critical shear rates with much smaller shear thickening ratio. Further, a possible mechanism has been proposed based on the influence of carrier fluid and particle size distribution to explain the rheological behaviour of shear thickening suspension. Our study supports the theory of particle-particle frictional contacts as the main reason for the discontinuous shear thickening.

Quantification of shear viscosity and wall slip velocity of highly concentrated suspensions with non-Newtonian matrices in pressure driven flows

Abstract: The rheological characterization of concentrated suspensions is complicated by the heterogeneous nature of their flow. In this contribution, the shear viscosity and wall slip velocity are quantified for highly concentrated suspensions (solid volume fractions of 0.55–0.60, D4,3 ~ 5 µm). The shear viscosity was determined using a high-pressure capillary rheometer equipped with a 3D-printed die that has a grooved surface of the internal flow channel. The wall slip velocity was then calculated from the difference between the apparent shear rates through a rough and smooth die, at identical wall shear stress. The influence of liquid phase rheology on the wall slip velocity was investigated by using different thickeners, resulting in different degrees of shear rate dependency, i.e. the flow indices varied between 0.20 and 1.00. The wall slip velocity scaled with the flow index of the liquid phase at a solid volume fraction of 0.60 and showed increasingly large deviations with decreasing solid volume fraction. It is hypothesized that these deviations are related to shear-induced migration of solids and macromolecules due to the large shear stress and shear rate gradients.

Poroviscoelasticity and compression-softening of agarose hydrogels

Abstract: Agarose hydrogels are poroviscoelastic materials that exhibit a waterlogged-crosslinked microstructure. Despite an extensive use in biotechnologies and numerous studies of the elastic properties of agarose gels, little is known about the compressible behavior and the microstructural changes of such fibrillar hydrogels under compression. The present work investigates the mechanical response of centimeter-sized pre-molded agarose cylinders when applying a compressive strain ramp over an extended range of loading speed and polymer concentration. One of the original contributions is the simultaneous monitoring of the changes in the hydrogel volume to determine the Poisson’s ratio through a spatiotemporal method. The linear poroelastic response of agarose hydrogels shows a compressible behavior at strain rates less than 0.7 % s−1. The critical compressive strain of a few percent at the onset of the non-linear regime and the always positive Poisson’s ratio decrease when applying a slow compressive ramp. The mechanical response in the linear regime is typical of a deformation mode either dominated by the bending of semiflexible strands (enthalpic regime) or by the stretching of the network (entropic regime) at higher agarose concentration. Cyclic linear shear deformations superimposed to a compressive strain from 0.5 up to 40% further give evidence of a compression-softening of the network causing the transition to the non-linear regime without dependence upon the network topology and connectivity. Finally, the buckling-induced aging of the network under a weak compression and the poroviscoelasticity of the hydrogel are shown to impact the relaxation of the normal stress and the equilibrium stress.

The microstructural change causing the failure of the Cox-Merz rule in Newtonian suspensions: experiments and simulations

Abstract: Newtonian non-Brownian concentrated suspensions show a mismatch between the steady state and the complex viscosity, whatever the strain amplitude imposed in the oscillatory flow. This result is counterintuitive in the two extreme cases of vanishing strain amplitude and very large one. In the first case, the oscillatory flow should not be able to alter the steady microstructure, as well as in the other opposite limit for which the strain amplitude is so high that the oscillatory flow resembles a steady flow reversal. If the microstructure is not altered with respect to the steady one, similarly the complex viscosity should be equal to the steady one. We here investigate experimentally and numerically the origin of the viscosities mismatch at any imposed strain amplitude. We focus on the first two or three cycles of oscillations and different particle concentrations. Experimental and numerical results agree and allow to prove that for intermediate amplitudes, the oscillatory shear induces the breakage of particle clusters and the microstructure modifies so to minimise particle collisions. For very small strain amplitudes, the oscillatory shear only induces the rotation of few couples of touching particles and the complex viscosity results slightly smaller than the steady one, while for very large strains, the oscillatory flow reshuffles the particles inducing a microstructure as clustered as the steady state one but with a different angular distribution function. We show that the vast majority of the microstructure rearrangement takes place in the first half cycle of oscillation.

Elasticity recovery of crosslinked EPDM: influence of the chemistry and nanofillers

Abstract: The objective of this work was to study the elastic recovery of EPDM samples crosslinked either by a phenolic resin (resol) or by a radical peroxide (dicumyl peroxide, DCP). From compression set experiments, it was observed that radically crosslinked EPDMs have better elastic recovery properties. On the other hand, for the same crosslinking density, radically crosslinked EPDM shows better compression set than EPDM crosslinked with phenolic resins. The Chasset-Thirion equation was then used to successfully fit the experimental relaxation curves. As a notable result, the preferential statistics of a peroxide-crosslinked network over a phenolic resin (resol) showed that better elastic recovery properties were obtained. Finally, the influence of fillers (carbon black and silica) was also studied. Carbon black with DCP crosslinking was shown to improve elasticity recovery whereas silica fillers lead to worse properties. It was then assumed that the interaction between particle surface and a voir comme dans autre publi a modifier ce crosslinking agent induced crosslinking gradients in the inter-particle volume.

A two-fluid model for the formation of clusters close to a continuous or almost continuous transition

Abstract: Experiments have shown that spatial heterogeneities can arise when the glass transition in polymers as well as in a number of low molecular weight compounds is approached by lowering the temperature. This formation of “clusters” has been detected predominantly by small angle light scattering and ultrasmall angle x-ray scattering from the central peak on length scales up to about 200 nm and by mechanical measurements including, in particular, piezorheometry for length scales up to several microns. Here we use a macroscopic two-fluid model to study the formation of clusters observed by the various experimental techniques. As additional macroscopic variables, when compared to simple fluids, we use a transient strain field to incorporate transient positional order, along with the velocity difference and a relaxing concentration field for the two subsystems. We show that an external homogeneous shear, as it is applied in piezorheometry, can lead to the onset of spatial pattern formation. To address the issue of additional spectral weight under the central peak we investigate the coupling to all macroscopic variables. We find that there are additional static as well as dissipative contributions from both, transient positional order, as well as from concentration variations due to cluster formation, and additional reversible couplings from the velocity difference. We also briefly discuss the influence of transient orientational order. Finally, we point out that our description is more general, and could be applied above continuous or almost continuous transitions

Rheological properties of sulfonated polystyrene ionomers at high-ion contents

Abstract: Structure and dynamics are examined for unentangled randomly-sulfonated polystyrene ionomer samples with cesium as counterion, using the X-ray scattering, DSC, and linear viscoelasticity (LVE) measurements. The large size of cesium cation leads to low association energy, enabling the dynamics to be examined for the high-ion-content samples, i.e., samples having a fraction of ionized monomers p up to 19%. X-ray scattering data shows an ionic peak, indicating the formation of ion aggregates. DSC reveals the glass transition process that shifts to higher T and broadens significantly when the ion content p is higher than 10%. In accordance with this broadening, the glassy and rubbery regimes seen in the LVE merge into one broad process with a wide relaxation distribution. We propose that the transition occurs when the number of ionic groups per chain becomes comparable to that of Kuhn segments per chain so that the polymer strands between ionic groups become non-flexible.

Elongational viscosity scaling of polymer melts with different chemical constituents

Abstract: Morelly et al. (Macromolecules 52:915-922, 2019) reported transient and steady-state elongational viscosity data of monodisperse linear polymer melts obtained by filament-stretching rheometry with locally controlled strain and strain rate and found different power law scaling of the elongational viscosities of polystyrene, poly(tert-butylstyrene) and poly(methyl-methacrylate). Very good agreement is achieved between data and predictions of the extended interchain pressure (EIP) model (Narimissa et al. J. Rheol. 64, 95-110 (2020)), based solely on linear viscoelastic characterization and the Rouse time τR of the melts. The analysis reveals that both the normalized elongational viscosity and the normalized elongational stress are dependent on the number of entanglements (Z) and the ratio of entanglement molar mass Mem to critical molar mass Mcm of the melts in the linear viscoelastic regime through $$ {\eta}_E^0/\left({G}_N{\tau}_R\right)\propto {\left({M}_{\mathrm{em}}/{M}_{\mathrm{cm}}\right)}^{2.4}{Z}^{1.4} $$ and $$ {\sigma}_E^0/{G}_N\propto {\left({M}_{\mathrm{em}}/{M}_{\mathrm{cm}}\right)}^{2.4}{Z}^{1.4} Wi $$ , while in the limit of fast elongational flow with high Weissenberg number $$ Wi={\tau}_R\dot{\varepsilon} $$ , both viscosity and stress become independent of Z and Mem/Mcm, and approach a scaling which depends only on Wi, i.e. ηE/(GNτR) ∝ Wi−1/2 and σE/GN ∝ Wi1/2. When expressed by an effective power law, the broad transition from the linear viscoelastic to the high Wi regime leads to chemistry-dependent scaling at intermediate Wi depending on the number of entanglements and the ratio between entanglement molar mass and critical molar mass.

Yielding and rheopexy of aqueous xanthan gum solutions

Abstract: Xanthan gum (XG) is widely used in cosmetic and pharmaceutic products (creams, pastes) and in oil industry (drilling fluids) as a stabilizing and/or thickening agent. In literature, its rheological behavior is mainly presented as that of a shear-thinning or a yield stress fluid. Here, in order to clarify this rheological behavior, we study in detail the flow characteristics during continued flow under given conditions (i.e., controlled stress) for a mass concentration ranging from 0.2 to 5%. We are thus able to identify the apparent flow curve of the material after a short flow duration and the flow curve in steady state (i.e., after a long flow duration). The validity of this flow curve, determined from standard rheometry, is confirmed by magnetic resonance velocimetry. These materials start to exhibit a yield stress behavior beyond some critical xanthan or salt concentration. In that case, a significant increase (by a factor up to 5) of the apparent viscosity is observed during flow in some range of stresses, before reaching a steady state. This original rheopectic effect might be due, after some time of flow associated with deformation and reconfiguration of the XG molecules, to the progressive formation of intermolecular links such as hydrogen bonds and/or intermolecular association due to acetate residues.

Rheological characterization of flow inception of thixotropic yield stress fluids using vane and T-bar geometries

Abstract: In this work, two geometries are studied, the vane and the T-bar, which are best suited for assessing the start-up flow of thixotropic yield stress fluids because they minimize the sample disturbance. Based on step-shear measurements with the vane geometry at different angular velocities and on a wide range of products, mostly commercial toothpastes, we calculate the torque on the T-bar using computational fluid dynamics (CFD). The results are compared to the previously suggested approximate theory by Anderson and Meeten (AMT) and extensive original experiments. It turns out that the agreement between CFD, AMT, and the experimental data depends primarily on the shape of the flow curve which may be quantified by the fluid flow index, N, defined in the shear rate range which represents the flow around the rotating rod of the T-bar. While the CFD and AMT predictions agree well with each other (R2 = 0.98), they both underestimate the experimental data although the experimental-to-predicted ratio also correlates to N (R2 = 0.84) going up from 1 to around 2 as N increases from 0.1 to 0.5. This suggests that when using the T-bar for viscosity measurements, the user needs to take into account the flow index to which end a simple estimate of the effective shear rate is suggested also being a function of N.

Importance of viscoelasticity in the thixotropic behavior of human blood

Abstract: Recent work modeling the rheological behavior of thixo-elasto-viscoplastic (TEVP) materials such as human blood indicates that it has all of the hallmark features of a complex material, including shear-thinning, viscoelasticity, a yield stress, and thixotropy After decades of modeling steady-state human blood rheological data, and the development of simple steady-state models, like the Casson and Herschel–Bulkley, the advancement and evolution of TEVP modeling to transient flow conditions now has reinvigorated interest Using recently collected human blood rheological data, over a wide range of flow conditions from steady state to various oscillatory shear flows, we show and compare modeling efforts with the original and a viscoelasticity-enhanced version of the enhanced Apostolidis–Armstrong–Beris (EAAB) model The viscoelasticity enhancement is then justified by its ability to improve predictions of small and large amplitude oscillatory shear as well as uni-directional oscillatory shear flow The new viscoelastic parameter is then incorporated into a methodology to estimate the viscoelastic time scale of human blood Lastly, we compare our new TEVP modeling approach with another recently developed TEVP model, mHAWB, for context

Capillary flow of a suspension in the presence of discontinuous shear thickening

Abstract: The rheology of suspensions showing discontinuous shear thickening (DST) is well documented in conventional rheometer with rotating tools, but their study in capillary flow is still lacking. We present results obtained in a homemade capillary rheometer working in an imposed pressure regime. We show that the shape of the experimental curve giving the volume flow rate versus the wall stress in a capillary can be qualitatively reproduced from the curve $$\dot{\gamma }(\tau )$$ obtained in rotational geometry at imposed stress but instead of a sharp decrease of the volume flow rate observed at a critical stress, this transposition predicts a progressive decrease in flow rate. The Wyart-Cates theory is used to reproduce the stress-shear rate curve obtained in rotational geometry and then applied to predict the volume flow rate at imposed pressure. The theoretical curve predicts a total stop of the flow at high stress, whereas experimentally it remains constant. We propose a modification of the theory which, by taking into account the relaxation of the frictional contacts in the absence of shear rate, well predicts the high stress behavior. We also hypothesized that the DST transition propagates immediately inside the capillary, once the wall shear stress has reached its critical value: τR = τc, even if the internal shear stress τ(r < R) is below the critical one. In this way the whole experimental curve can be well reproduced by the modified W–C model.

Modelling inelastic non-colloidal suspensions

Abstract: We present an inelastic model of non-Brownian suspension behaviour based on the persistence of straining concept of Thompson and Souza Mendes (TSM model). It is used to compare with experiments and computations in steady shearing, uniaxial and planar elongation and shear reversal flows. Some remarks about small-strain motions are also given. The agreement is good in general, and the important influence of interparticle friction is shown. Friction has been incorporated in the model via a bootstrap concept.

A nonlinear viscoelastic model of mucociliary clearance

Abstract: In this study, we implemented a two-dimensional unsteady flow model to study mucociliary clearance. The model is defined in a two-layered geometry consisting of a Newtonian periciliary layer (PCL) and a nonlinear viscoelastic mucus layer that make up the airway surface liquid (ASL). The mucus property is modeled using a 5-mode nonlinear Giesekus constitutive equation. The 5-mode nonlinear Giesekus constitutive equation is known to produce results that are in very good agreement with the rheological properties of mucus. The major thrust of this study is modeling mucociliary clearance via a coupling of the mucus Giesekus model with active cilia motion. The projection finite difference method was used to obtain numerical solutions for the mucus Giesekus model on a staggered Eulerian grid. An immersed boundary method was used to describe the propulsive effect of cilia on the ASL. Numerical simulations were carried out to study the effect of cilia beat frequency (CBF), cilia length, mucus depth, and PCL depth on mucociliary clearance. Simulation results show that cilia length plays a significant role in mucociliary clearance and is independent of the CBF. Simulation results also indicate that variation in mucus depth at low values of CBF produces a negligible impact on mucociliary clearance.