Transient Rheological Behavior Research Articles

On the modelling of the shear thickening behavior in micellar solutions

The steady and transient nonlinear rheological behaviors of dilute rod-like micellar solutions are predicted here with a particular case of the generalized Bautista–Manero–Puig (BMP) model that consists of the upper-convected Maxwell constitutive equation and a dissipative power-dependent kinetic equation, which takes into account the formation and disruption of shear-induced structures (SISs). This model has been derived using the extended irreversible thermodynamic (EIT) formalism. In steady shear, the model predicts a Newtonian region at low shear rates and a characteristic shear rate (\( {\dot{\upgamma}}_{\mathrm{c}} \)) at which shear thickening develops. In the shear thickening region, the model predicts either a reentrant zone, which is caused by multi-valued shear stresses when the data are collected with a shear stress-controlled mode or a continuous increase in the shear stress-shear rate flow curve, when the data are collected with a shear rate-controlled mode; in this region, two coexisting phases are predicted. The coexisting phases at the same shear rate in the two-phase envelope and the spinodal-like region were evaluated from the extended Maxwell equal-area criterion, which was calculated from the equal values of the two minima of the plot of the extended Gibbs free energy versus shear rate. At higher shear rates, the model predicts a transition to shear thinning, and under transient flows, an induction time and a saturation time are obtained from the predicted and the experimental data as detailed in the text. The magnitudes of both, the induction and the saturation times, diminish as the shear rate departs from \( {\dot{\upgamma}}_{\mathrm{c}} \), but only the induction time decreases according to a power law with shear rate. The conditions under which these rheological responses arise are derived and justified in detail with the BMP model. The model predictions are compared with experimental data of two dilute micellar solutions. The model parameters were determined from a set of independent experiments without fitting.

Does Shear Thickening Occur in Semisolid Metals?

Abstract In the various forms of semisolid processing such as thixoforming and thixoforging, the entry into the die occurs in a fraction of a second so it is the transient rheological behavior which governs the initial stages of flow. In experiments in the literature, this rheological behavior is probed through applying rapid transitions in shear rate under isothermal conditions. There is contradictory evidence as to whether the behavior during these transitions is shear thinning or shear thickening, although it is clear that once in the die the material is thinning. Here the data in the literature are reanalyzed to obtain a rationalization of the contradictions which has not previously been available. It is argued that if a suspension is initially in a disagglomerated state (i.e., one which is initially sheared), the instantaneous behavior with a jump-up in shear rate is shear thickening (even if the long-term steady-state behavior is shear thinning) provided the fraction solid is greater than about 0.36 and the final shear rate at the end of the jump is greater than about 100 s−1. If the jump-up in shear rate is made from rest then yield masks the shear thickening.

Transient rheological behavior of natural polysaccharide xanthan gum solutions in start-up shear flow fields: An experimental study using a strain-controlled rheometer

The objective of the present study is to experimentally investigate the transient rheological behavior of concentrated xanthan gum solutions in start-up shear flow fields. Using a strain-controlled rheometer, a number of constant shear rates were suddenly imposed to aqueous xanthan gum solutions with different concentrations and the resultant shear stress responses were measured with time. The main findings obtained from this study can be summarized as follows: (1) For all shear rates imposed, however low it may be, the shear stress is rapidly increased with time (stress overshoot) upon inception of steady shear flow before passing through the maximum stress value and then gradually decreased with time (stress decay) until reaching a steady state flow. (2) As the imposed shear rate is increased, a more pronounced stress overshoot takes place and the maximum stress value becomes larger, whereas both times at which the maximum stress is observed and needed to reach a steady state flow are shortened. (3) The maximum shear stress is linearly increased with shear rate in a double logarithmic scale and becomes larger with increasing concentration at equal shear rates. In addition, the time at which the maximum stress occurs exhibits a linear relationship with the inverse of shear rate in a double logarithmic scale for all xanthan gum solutions, regardless of their concentrations. (4) The shear stress is sharply increased with an increase in strain until reaching the maximum stress at small range of deformations. The maximum stress is observed at similar strain values, irrespective of the imposed shear rates lower than 10 1/s. (5) The Bird-Leider model can be successfully used with regard to quantitatively predicting the transient behavior of concentrated xanthan gum solutions. However, this model has a fatal weakness in terms of describing a decrease in shear stress (stress decay).

정상전단유동의 급개시에 따른 폴리에틸렌옥사이드 수용액의 응력성장거동

The objective of this study is to systematically elucidate the transient rheological behavior of viscoelastic polymer solutions at start-up of steady shear flow. Using a strain-controlled rheometer [Advanced Rheometric Expansion System (ARES)], the stress overshoot behavior of concentrated aqueous poly(ethylene oxide) [PEO] solutions has been thoroughly investigated with applying a number of shear rates to these polymer solutions. In this article, the typical stress growth behavior of aqueous PEO solutions was firstly presented on the basis of experimentally obtained data and nextly the effects of shear rate, molecular weight, and concentration on this behavior were discussed in depth. In order to theoretically predict the stress growth behavior, the Wagner constitutive equation (a time-strain separable constitutive equation) and the Bird-Leider model were employed and the applicability of these models was examined in detail. The main findings obtained from this study can be summarized as follows: (1) At the inception of low shear rates, the stress overshoot behavior is not observed but the shear stress progressively increases with time until the steady state stress value is reached. As the shear rate is increased, however, aqueous PEO solutions exhibit a pronounced stress overshoot followed by a stress decay to reach the steady state flow condition. (2) The maximum shear stress increases linearly with an increase in shear rate in a double logarithmic scale and becomes larger with an increase in concentration. (3) The time at which the maximum shear stress occurs, <TEX>$t_{max}$</TEX>, shows a linear relationship with the inverse of shear rate in a double logarithmic scale, regardless of molecular weight and concentration. (4) When the Wagner constitutive equation is used, the Wagner damping function exhibits superior performance to the Soskey-Winter damping function in predicting the transient stress growth behavior of aqueous PEO solutions. (5) The Bird-Leider empirical model can be successfully used for predicting the transient stress growth behavior of aqueous PEO solutions. This model is especially effective for expressing the maximum shear stress as well as the time at which this maximum stress is observed.

Rheological Behavior of Semisolid AlSi6Mg2 Alloy at Transient State

In the present work, basing on the rheological model of Chen and Fan [1], the rheological behavior of AlSi6Mg2 alloy at transient state is investigated. It has been shown that the deagglomeration of particles is about two orders of magnitude faster than the agglomeration of them. The inequality of agglomeration rate and deagglomeration rate is thought as the origin of the thixotropy of SSMS. Subsequently, the similar trend of the variation of the viscosity and the average agglomerate size with shearing time and resting time shows that the microstructure of SSMS determines its rheological behavior, while the external flow conditions (such as shear rate, shearing time, etc) influence the viscosity by changing its microstructure. The present study predicts that the CF model can describe reliably the transient rheological behavior of AlSi6Mg2 alloy.

Can only rheology be used to determine the phase separation mechanism in dynamically asymmetric polymer blends (PS/PVME)?

We investigated theoretically and experimentally the correlation between the time evolution of different phase-separating morphologies and corresponding linear and transient rheological behaviors for the dynamically asymmetric PS/PVME (polystyrene/polyvinyl methyl ether) blend in which there is a large difference between the glass transition temperatures of the pure components (about 125 °C). The sensitivity of different rheological analyses was examined to distinguish different phase separation mechanisms from each other, including nucleation and growth (NG), spinodal decomposition (SD), and viscoelastic phase separation (VPS). We found that a combination of experimental and theoretical studies of the linear and nonlinear rheology could provide satisfactory criteria to distinguish effectively samples phase separating by different mechanisms. Furthermore, the variation of fractal behavior by phase separation time was investigated for interconnected and percolated network structures induced by SD and VPS, respectively, which suggested that both network structures are controlled by a diffusion-limited cluster aggregation (DLCA) mechanism.

Local transient rheological behavior of concentrated suspensions

This paper reports experiments on the shear transient response of concentrated non-Brownian suspensions. The shear viscosity of the suspensions is measured using a wide-gap Couette rheometer equipped with a Particle Image Velocimetry (PIV) device that allows measuring the velocity field. The suspensions made of PMMA particles (31$\mu$m in diameter) suspended in a Newtonian index- and density-matched liquid are transparent enough to allow an accurate measurement of the local velocity for particle concentrations as high as 50%. In the wide-gap Couette cell, the shear induced particle migration is evidenced by the measurement of the time evolution of the flow profile. A peculiar radial zone in the gap is identified where the viscosity remains constant. At this special location, the local particle volume fraction is taken to be the mean particle concentration. The local shear transient response of the suspensions when the shear flow is reversed is measured at this point where the particle volume fraction is well defined. The local rheological measurements presented here confirm the macroscopic measurements of Gadala-Maria and Acrivos (1980). After shear reversal, the viscosity undergoes a step-like reduction, decreases slower and passes through a minimum before increasing again to reach a plateau. Upon varying the particle concentration, we have been able to show that the minimum and the plateau viscosities do not obey the same scaling law with respect to the particle volume fraction. These experimental results are consistent with the scaling predicted by Mills and Snabre (2009) and with the results of numerical simulation performed on random suspensions [Sierou and Brady (2001)]. The minimum seems to be associated with the viscosity of an isotropic suspension, or at least of a suspension whose particles do not interact through non-hydrodynamic forces, while the plateau value would correspond to the viscosity of a suspension structured by the shear where the non-hydrodynamic forces play a crucial role.

Rheology and Morphology of Dynamically Asymmetric LCST Blends: Polystyrene/Poly(vinyl methyl ether)

We investigated the correlation between the time evolution of the different phase-separating morphologies and corresponding linear and transient rheological behaviors for the dynamically asymmetric PS/PVME blend in which there is a large difference between glass transitions of the pure components (about 125 °C). The phase diagram was obtained from dynamic temperature sweep experiments. Phase contrast optical microscopy was employed to investigate morphological evolution of PS/PVME blends at various regions of obtained phase diagram at a constant temperature of 105 °C. At this temperature depending on sample composition, the viscoelastic phase separation (VPS) was observed besides the usual phase separation mechanisms (nucleation and growth (NG) and spinodal decomposition (SD)), indicating the interplay between thermodynamics and viscoelasticity in phase-separation behavior of PS/PVME blends. The linear viscoelastic behavior for different phase separating mechanisms (NG, SD, and VPS) was measured to invest...

Thermal and shear flow effects on microstructure of a thermotropic liquid crystalline polymer

AbstractThe effects of annealing time and temperature on the texture and transient shear rheological behavior of a thermotropic liquid crystalline polymer were investigated. The texture evolution during annealing included two processes: the reduction of the defect density and the increase of the domain size but decrease of the domain number. We confirmed that the threaded texture caused the first shear stress peak and the shear stress minimum during shear flow startup. More importantly, using the wide‐angle X‐ray diffraction technique, we determined that the second shear stress peak during flow startup can be attributed to the evolution of the molecular orientation. POLYM. ENG. SCI., 46:1215–1222, 2006. © 2006 Society of Plastics Engineers

Influence of organic admixtures on the rheological behaviour of cement pastes

The rheological properties of cementitious pastes used to proportion Self-Consolidating Concretes (SCC) have been examined, in particular, the influence of High range water reducing admixture (HRWRA) with that of Viscosity Modifying Admixtures (VMAs) have been compared. HRWRAs are known to have dispersing effects on the cement particles through steric and/or electrostatic repulsion, while the effects of VMAs are expected to stabilise the paste by increasing the viscosity of the aqueous solution. Both transient and steady state rheological behaviour of the cementitious pastes proportioned with different dosages of HWRA and VMA were considered. Experimental results show that the influence of VMAs on rheological properties is actually minor compared to that of HRWRAs. These results are discussed in the framework of rheology of concentrated suspensions.

Two-phase flow in microchannels with surface modifications

Two-phase flows in microchannels with surface modifications are experimentally investigated. First, we investigate the shape of static and moving bubbles in microchannels with square cross-sections for different contact angles. Water and air are mixed on-chip in a cross-shaped mixing chamber. This mixing geometry allows for the production of monodisperse bubbles, the size of which can be controlled with the flow rates. We study the flow morphologies of mixtures made of pure water and air, and made of water with surfactant and air (aqueous foam), in both hydrophilic and hydrophobic microchannels. Second, we investigate the transient rheological behavior of polymer solutions when the length of the polymers is comparable to the height of the channel. The measured viscosity of the solution is several times larger than the expected value, and does not show the typical shear-thinning behavior. These experiments highlight the importance of wall properties for two-phase flows in microfluidic devices.

Transient rheological behavior of lyotropic (acetyl)(ethyl)cellulose/m-cresol solutions

The chiroptical properties and transient rheological behavior of (acetyl)(ethyl)cellulose (AEC) m-cresol liquid crystalline solutions have been investigated. Chiroptical properties were manipulated through (i) increasing degree of acetylation of ethyl cellulose (EC), and (ii) blending AEC with EC. At the same average degree of acetylation (DA), the chiroptical properties of pure AEC were different from those of the EC/AEC mixtures. However, at the same DA, the AEC and mixed AEC/EC solutions showed similar steady state flow and oscillatory behaviors, but the transient behaviors were different. At high flow rates the mixed AEC/EC solutions exhibited double recoil after cessation of steady-state flow, whereas the AEC solutions showed double recoil only in the high DA AEC solutions. All solutions, pure and mixed, had the same stress relaxation behavior. Both pitch and handedness affected the transient behavior. After cessation of high shear rate flow, the rate of modulus evolution decreased with increasing pitch, and was faster in right-handed mesophases than in left-handed ones at a similar pitch.

Steady state and transient rheological behavior of mesophase pitch, Part II: Theory

This paper presents a comprehensive nonlinear numerical analysis of the flow modeling of mesophase pitches performed using a previously formulated mesoscopic viscoelastic rheological theory (Singh and Rey (2000)) that takes into account short-range order elasticity, long range elasticity, and flow-induced texture transformations. A complete extra stress tensor equation is developed from first principles for liquid crystal materials under nonhomogeneous arbitrary flow. This mesoscopic viscoelastic model has been adapted to describe the rheology of flow-aligning thermotropic discotic nematic liquid crystals as models of mesophase pitches. Predictions for simple shear flow (under nonhomogeneous conditions) for the shear viscosity, first normal stress differences, and transient shear stress are presented. The accuracy of the numerical results is established by a thorough validation procedure based on [Cato et al. (2004)], which is the companion paper, and permit to validate this mesoscopic viscoelastic theory as model of liquid crystalline mesophase pitch. Very good qualitatively agreement between experiments and simulations is found for all rheological characterizations.

Steady state and transient rheological behavior of mesophase pitch, Part I: Experiment

The rheology of a liquid crystalline mesophase pitch is reported. Over the range of shear rates and temperatures investigated, the steady state shear viscosity and the first normal stress difference display region 1 and region 2 flow behavior typical of many liquid crystalline systems. A kink in the transition between regions 1 and 2 is also observed. The first normal stress difference is negative at low shear rates, but transitions to positive values in the shear rate region of the kink in the viscosity. These phenomena are explained by change in the molecular orientation of the sample. The evolution of the shear stress in transient startup and relaxation experiments is measured. There is a distinct overshoot in the startup of shearing, and a rapid decay of stress upon cessation of flow. As expected, the magnitude of the overshoot depends on the rest time between shearing. The estimated elastic constant is consistent with other work.

Structure evolution in polymer blending using microfabricated samples

The transient rheological behavior and morphology evolution of polymethyl methacrylate (PMMA)/polystyrene (PS) binary polymer blends with well-defined initial structure were measured in simple shear flow under isothermal conditions. The size and distribution of the dispersed phase and the composition of the blends were designed and fabricated by Computer Numerical Controlling (CNC) machining, photolithography, and micro-embossing. Compatibilizer can easily be placed at the interface of the two components during sample preparation. The effects of initial dispersed domain size, blend composition, and interfacial tension on rheological behavior and morphology evolution were investigated. It was found that the transient shear stress and first normal stress difference are very sensitive to these parameters. The transient rheological responses up to the breakup point are compared with those predicted by both Doi–Ohta and Vinckier–Moldenaers–Mewis models.

Experimental investigation of the transient and steady state rheological behaviour of Al–Si alloys in the mushy state

In order to properly model and control the semi-solid processing of metallic alloys, their thixotropic behaviour requires proper characterisation. In particular, the effects of shear rate, shear time, temperature and rest time on the rheology of such slurries needs to be understood and quantified. A purpose-built high temperature Searle rheometer was used to determine the rheological behaviour of aluminium alloy slurries at shear rates from 3.1 to 124.8 s −1, periods of shear of up to 60 min for each shear rate, and periods of rest (no stirring) of up to 60 min for Al–4wt.%Si and A356 alloys. Continuous cooling rheometry was used to determine the coherency point of the alloys. Isothermal fractions solid of 0.36 for Al–4%Si and 0.33 for A356 were investigated. Isothermal tests were used to follow the temporal evolution of viscosity, which was found to be significantly different for both alloys, particularly at low shear rates. Steady state viscosity values that were determined over a range of shear rates indicated severe pseudoplastic behaviour, as measured by a viscosity–shear rate power law index less than −1. This work confirms that this finding is an actual rheological feature and not an artefact of a particular measurement device. A study using shear rate jumps determined the isostructural behaviour of the alloys by discounting equipment inertial effects. It is shown that peak stress or apparent viscosity is a better indicator of slurry thixotropy than the hysteresis loop from shear ramping experiments, and the work also shows that the effect of agglomeration on fluidity is an important parameter to measure as it has consequences for thixoforming.

Prediction of the nonlinear transient and oscillatory rheological behavior of flour suspensions using a strain-separable integral constitutive equation

The evaluation and development of validated models for the nonlinear viscoelastic (VE) behavior of materials is an important area of research, which has impact on a number of industrial processes including those in the food industry. Various nonlinear VE models have been developed over the years and evaluated for petroleum-based polymers; however, our understanding of the nonlinear VE behavior of biopolymers of industrial import lags our understanding of synthetic polymers. In the work reported herein, the nonlinear VE behaviors of defatted oat flour, oat bran, barley flour, and oat flour suspensions were investigated. The rheological properties were measured using a Rheometrics Series IV controlled-strain rheometer equipped with a cone and plate fixture. The measurements were conducted at 23±0.5°C. The rheological data were interpreted using a strain-separable K-BKZ type (Wagner) model with a damping function evaluated from stress relaxation data. The Wagner model was found to provide an accurate description of the rheological behavior of the suspensions produced from defatted oat flour, barley flour, and oat bran. For suspensions produced from oat flour, the model predictions deviated from the experimental data to a greater degree than the other materials.

Transient stresses in immiscible model polymer blends during start-up flows

The transient rheological behaviour of immiscible polymer blends is studied during start-up flows. The stresses can be divided in a contribution from the component polymers and a contribution caused by the presence of an interface. By means of relaxation measurements, the component stresses after inception of flow can be determined. In this manner, the different contributions to the stress can be compared separately with model predictions. For the contribution of the components, one normally uses mixing rules that do not take into account the morphology of the blend. As expected, a slight morphology effect can be detected. The stress contributions from the interface are calculated combining the Doi–Ohta approach with different models that describe the droplet deformation. For flows at small capillary numbers, the Maffettone–Minale model leads to accurate stress predictions. With larger capillary numbers, the normal stress profiles become more complex. The initial part is rather well described by the model of Almusallam et al. [J. Reol. 44 (2000) 1055]. It is demonstrated that the morphological changes in the blend can be deduced in quite some detail from the measured stress profiles.

Microstructural lattice simulation and transient rheological behavior of a flow-aligning liquid crystalline polymer under low shear rates

A microstructural lattice simulation for textured liquid crystalline polymer is carried out to predict rheological behavior, especially the stress evolution after shear inception. It is based on a combination of two main concepts: (i) the director in each cell of a supramolecular lattice has an orientation described by the minimization of total energy of director map, and (ii) the torque balance of each director under shear flow and anisotropic relaxational shear moduli depends on the averaged orientation of the director map. By considering the interaction between the nearestneighbor directors, the spatial orientational correlation is introduced and the spatial heterogeneity, i.e., a polydomain texture, is generated simultaneously. For the start-up shear flow, the overshoot and the steady value of shear stress increase and the former shifts toward a shorter time as the applied shear rate increases. Also, the calculated stress evolution is compared with the experimental result of a thermotropic liquid crystalline poly(ester-imide).

Transient rheological behavior of tumbling side-chain liquid crystal polymers and determination of their λ parameters

The transient rheological behavior of several thermotropic nematic side-chain liquid crystal polysiloxanes is investigated. A homopolymer with a low-temperature smectic phase and two random copolymers, one consisting of smectogenic and non-smectogenic mesogens, the other one containing two spacers of different lengths, are studied. All systems are of tumbling type and exhibit oscillations in transient shear stress and first normal stress difference. Only the copolymer with mixed spacers shows a temperature-dependent transition from tumbling to flow-aligning within the nematic phase. The rheological response is slightly different for each polymer. The reactive parameter λ was calculated from the period of the oscillations and compared with rheo-NMR results. For all polymers, λ increases with increasing temperature. The sign of λ, obtained from NMR experiments, is positive for the copolymer with mixed spacers and negative for the other two systems, indicating a prolate shape of the first system and oblate shapes of the other two.