Steady-state Viscosity Research Articles

Effect of ultrasonic waves on the rheological features of secondary sludge

The increasing production of sewage sludge is a problem for wastewater treatment plants today. Research has sought to improve sludge processing efficiency and minimization. This study examined the changes in the rheological features of secondary sludge as a function of ultrasonic specific energy. It has been analyzed the dependency of steady state viscosity on shear rate and the hysteresis area. Steady state viscosity and the hysteresis area decreased significantly with increased ultrasonic specific energy. This may be due to the disruption of sludge flocks leading to a less rigid internal structure. The centrifugation of ultrasonicated sludge increased the total solid content by approximately 21%, leading to dewatered sludge. This sludge showed less elasticity, viscosity and thixotropy than untreated sludge.

PH effect on the synthesis, shear properties, and homogeneity of iron‐crosslinked hyaluronic acid‐based gel/adhesion barrier

Iron-crosslinked hyaluronic acid hydrogel (FeHA) has been used to reduce postsurgical adhesions in patients undergoing open, gynecological surgery. The performance of FeHA gel as an adhesion barrier device is influenced by many factors, including the physicochemical gel properties, which, in turn, depend on the chemistry and conditions of the device manufacturing. In this work, we demonstrate the effect of reaction pH on rheology and homogeneity of FeHA gels formulated in house and also compare the viscoelastic properties of FeHA gels with that of uncrosslinked HA solution of similar HA concentration and ionic strength. Dynamic mechanical analyses provide evidence that the reaction of HA with Fe(III) ions leads to the formation of "weak" gels. The viscoelastic properties and homogeneity of FeHA gels vary depending on the pH at which crosslinking was initiated. When solution pH, at the start of crosslinking, varied between 1.5 and 3, the low-shear rate viscosity of FeHA varied between 10,000 and 40,000 cPoise (10-40 Pa s). The highest steady-state shear viscosity and viscoelasticity were measured when pH was around 2.6, which is similar to the pH-dependent viscoelasticity of pure HA solution. Initiating HA crosslinking at pH ≤ 3 led to relatively homogenous solutions, while crosslinking higher pH > 3 caused instantaneous gel precipitation and inhomogeneities. Sensitivity of FeHA gel properties to small variations in reaction pH clearly supports the need for a tight manufacturing control during medical device fabrication.

Rheology and Thermotropic Gelation of Aqueous Sodium Alginate Solutions

In this work, we present the evaluation of the thermotropic gelling ability and steady-state rheology of aqueous sodium alginate (NaAlg) solutions. NaAlg is been considered due to its gelling capabilities and to determine its suitability as a film-forming agent with the potential capacity for immobilization of very potent, solid, and hydrophobic drugs. The existence of a thermally induced gelation of 1.0 to 2.5 wt.% sodium alginate solutions between 13°C to 22°C was demonstrated through three independent rheological tests: the constant-stress temperature-ramp viscosity, dynamic viscoelastic moduli, and thixotropy tests. Oscillatory dynamic tests also showed the thermoreversible nature of the NaAlg gels, but a large hysteresis between the gel formation and the gel melting was observed. The steady-state viscosity was experimentally determined as a function of shear rate for NaAlg concentrations between 1.0 to 2.5 wt.% and for temperatures from 20°C (or above Tgel) to 90°C. A viscosity master curve was constructed by applying time–temperature superposition and a semi-empirical shift factor for concentration. A power-law fluid model (\( \eta = k{{\dot{\gamma }}^n} \)) was adjusted to the data from 20°C to 60°C and found to provide good agreement with fitting parameters k = 4.81 and n = −0.449 ± 0.003.

Upper mantle rheology from GRACE and GPS postseismic deformation after the 2004 Sumatra‐Andaman earthquake

Mantle rheology is one of the essential, yet least understood, material properties of our planet, controlling the dynamic processes inside the Earth's mantle and the Earth's response to various forces. With the advent of GRACE satellite gravity, measurements of mass displacements associated with many processes are now available. In the case of mass displacements related to postseismic deformation, these data may provide new constraints on the mantle rheology. We consider the postseismic deformation due to the Mw = 9.2 Sumatra 26 December 2004 and Mw = 8.7 Nias 28 March 2005 earthquakes. Applying wavelet analyses to enhance those local signals in the GRACE time varying geoids up to September 2007, we detect a clear postseismic gravity signal. We supplement these gravity variations with GPS measurements of postseismic crustal displacements to constrain postseismic relaxation processes throughout the upper mantle. The observed GPS displacements and gravity variations are well explained by a model of viscoelastic relaxation plus a small amount of afterslip at the downdip extension of the coseismically ruptured fault planes. Our model uses a 60 km thick elastic layer above a viscoelastic asthenosphere with Burgers body rheology. The mantle below depth 220 km has a Maxwell rheology. Assuming a low transient viscosity in the 60–220 km depth range, the GRACE data are best explained by a constant steady state viscosity throughout the ductile portion of the upper mantle (e.g., 60–660 km). This suggests that the localization of relatively low viscosity in the asthenosphere is chiefly in the transient viscosity rather than the steady state viscosity. We find a 8.1018 Pa s mantle viscosity in the 220–660 km depth range. This may indicate a transient response of the upper mantle to the high amount of stress released by the earthquakes. To fit the remaining misfit to the GRACE data, larger at the smaller spatial scales, cumulative afterslip of about 75 cm at depth should be added over the period spanned by the GRACE models. It produces only small crustal displacements. Our results confirm that satellite gravity data are an essential complement to ground geodetic and geophysical networks in order to understand the seismic cycle and the Earth's inner structure.

Viscosity study of the optical tellurite glass: 75TeO 2–20ZnO–5Na 2O

The steady state viscosity of a tellurite glass with composition 75TeO 2–20ZnO–5Na 2O has been measured using parallel plate rheometry and found to decrease with shear rate and temperature. The viscosity is constant for lower shear rates but decreases rapidly above a critical rate γ̇ c, due to shear thinning. The shear rate dependence of the viscosity is found to be well approximated by the Cross model in the temperature range between 609 and 663 K while the temperature dependencies of both the viscosity and relaxation time are observed to follow an Arrhenius law in this temperature range. The activation energy obtained from the Arrhenius fit is used to determine the fragility of the tellurite glass, which is compared to that of other well-known glasses from the literature.

Stabilities and Rheological Properties of Coagulated Silica Sols Formed from Fumed Silica Suspensions in the Presence of Hexadecyltrimethylammonium Chloride

The stability and rheological response of coagulated silica sols formed from fumed silica suspensions were investigated in aqueous KOH solution at pH 11 upon the addition of hexadecyltrimethylammonium chloride (C(16)TAC) as functions of silica and surfactant concentration. The coagulated silica sols with negative charges are stable at given concentration ranges of silica and C(16)TAC, and the C(16)TAC molecules are completely adsorbed on the silica surface in these ranges. The average hydrodynamic diameters of the coagulated silica colloidal sols at C(16)TAC concentrations from 1.0 x 10(-4) to 5.0 x 10(-4) M are almost twice the diameter observed in the absence of C(16)TAC, independent of silica concentration. At C(16)TAC concentrations below 4.0 x 10(-4) M, the resulting coagulated colloidal sols showed Newtonian responses under hysteresis loop measurements, whereas at above 5.0 x 10(-4) M, the flow curve changes from a positive hysteresis loop to a crossover hysteresis loop with increasing concentration. This change under shear flow is due to a partial breakdown of the coagulated structure of the fumed silica suspensions as a result of electrical neutralization. Finally, the coagulated colloidal sols which gave a crossover hysteresis loop present a shear-thinning behavior and their transit shear stresses exhibit overshoots whatever the shear rate used. Plots of the resulting steady-state viscosities against shear rate indicate shear-thinning behavior.

Nonlinear Rheology of Unentangled Polymer Melts Reinforced with High Concentration of Rigid Nanoparticles.

A scaling model is presented to analyze the nonlinear rheology of unentangled polymer melts filled with high concentration of small spherical particles. Assuming the majority of chains to be reversibly adsorbed to the surface of the particles, we show that the emergence of nonlinearity in the viscoelastic response of the composite system subjected to a 2D shear flow results from stretching of the adsorbed chains and increasing desorption rate of the adsorbed segments due to the imposed deformation. The steady-state shear viscosity of the mixture in nonlinear shear thinning regime follows the power lawwhereis the applied shear rate. At large strain amplitude γ 0, the storage and loss moduli in strain sweep tests scale asandrespectively.

The interchain pressure effect in shear rheology

Recently, the tube diameter relaxation time in the evolution equation of the molecular stress function (MSF) model (Wagner et al., J Rheol 49: 1317–1327, 2005) with the interchain pressure effect (Marrucci and Ianniruberto, Macromolecules 37:3934–3942, 2004) included was shown to be equal to three times the Rouse time in the limit of small chain stretch. From this result, an advanced version of the MSF model was proposed, allowing modeling of the transient and steady-state elongational viscosity data of monodisperse polystyrene melts without using any nonlinear parameter, i.e., solely based on the linear viscoelastic characterization of the melts (Wagner and Rolon-Garrido 2009a, b). In this work, the same approach is extended to model experimental data in shear flow. The shear viscosity of two polybutadiene solutions (Ravindranath and Wang, J Rheol 52(3):681–695, 2008), of four styrene-butadiene random copolymer melts (Boukany et al., J Rheol 53(3):617–629, 2009), and of four polyisoprene melts (Auhl et al., J Rheol 52(3):801–835, 2008) as well as the shear viscosity and the first and second normal stress differences of a polystyrene melt (Schweizer et al., J Rheol 48(6):1345–1363, 2004), are analyzed. The capability of the MSF model with the interchain pressure effect included in the evolution equation of the chain stretch to model shear rheology on the basis of linear viscoelastic data alone is confirmed.

Tension thickening, molecular shape, and flow birefringence of an H-shaped polymer melt in steady shear and planar extension

Despite recent advances in the design of extensional rheometers optimized for strain and stress controlled operation in steady, dynamic, and transient modes, obtaining reliable steady-state elongational data for macromolecular systems is still a formidable task, limiting today's approach to trial-and-error efforts rather than based on a deep understanding of the deformation processes occurring under elongation. Guided, in particular, by the need to understand the special rheology of branched polymers, we studied a model, unentangled H-shaped polyethylene melt using nonequilibrium molecular dynamics simulations based on a recently developed rigorous statistical mechanics algorithm. The melt has been simulated under steady shear and steady planar extension, over a wide range of deformation rates. In shear, the steady-state shear viscosity is observed to decrease monotonically as the shear rate increases; furthermore, the degree of shear thinning of the viscosity and of the first- and second-normal stress coefficients is observed to be similar to that of a linear analog of the same total chain length. By contrast, in planar extension, the primary steady-state elongational viscosity eta(1) is observed to exhibit a tension-thickening behavior as the elongation rate epsilon increases, which we analyze here in terms of (a) perturbations in the instantaneous intrinsic chain shape and (b) differences in the stress distribution along chain contour. The maximum in the plot of eta(1) with epsilon occurs when the arm-stretching mode becomes active and is followed by a rather abrupt tension-thinning behavior. In contrast, the second elongational viscosity eta(2) shows only a tension-thinning behavior. As an interesting point, the simulations predict the same value for the stress optical coefficient in the two flows, revealing an important rheo-optical characteristic. In agreement with experimental indications on significantly longer systems, our results confirm the importance of chain branching on the unique rheological properties of polymer melts in extension.

Elongational Viscosities of Entangled Linear Polymers

Elongation studies are very important for validating the constitutive equations that describe the rheological behavior of fluids. The relaxation of a stretched polymer chain in the flow field has not been investigated in detail thus far. We extended the constitutive equations for predicting the start-up behavior and the power law relationship between the steady-state elongational viscosities and the strain rate. We also determined the steady-state elongational viscosity by fitting the calculated results with the measured values. The calculated results show the power law relationship between the steady-state elongational viscosities and the strain rate. Furthermore, we used these constitutive equations to predict the overshoot of the viscosities and stresses at initial stages of the steady elongational or shear flows by considering only one parameter, the stretch relaxation time.

Role of Different Pre-Treatments on Composition and Rheology of Synovial Fluids

Different pre-treatments applied to synovial fluids (SF) before their analyses are tested to characterize SF after storage under different conditions and to investigate their evolution along a viscosupplementation treatment. The main techniques proposed involve steric exclusion chromatography with triple detection (SEC) and viscometry; it is the first time that such a study is developed. SEC gives the molecular weight distribution and concentration of hyaluronan (HA) and proteins separately; the steady state viscosity is always non-Newtonian and not directly related to SF composition. Pre-treatment of SF (storage in cold, filtration, centrifugation) allows us to conclude that, in order to store SF, it is best to freeze it, even if in some cases, viscosity is modified but not the composition. All the data obtained (including protease pre-treatment) allow us to conclude that a small fraction of HA-protein complex forms a loose 3D-network and controls the rheology.

Extensional viscosity in uniaxial extension and contraction flow—Comparison of experimental methods and application of the molecular stress function model

Extensional viscosity of a low-density polyethylene was measured at three temperatures in uniaxial extension by Sentmanat Extension Rheometer, and in contraction flow using the Cogswell analysis. The molecular stress function model was applied to describe the experimental results. The achieved maximum values from uniaxial transient tests were in accordance with the ones obtained by Cogswell method at similar strain level, and the molecular stress function model was able to describe the experimental transient uniaxial extensional data. The steady-state extensional viscosity was not reached in the experiments.

Correlation of dynamic and quasistatic relaxations: The Cox–Merz rule for metallic glass

The correlation of quasistatic and dynamic relaxations was discussed in a typical strong Zr55Al10Ni5Cu30 metallic glass from room temperature to Tg. The quasistatic relaxation behavior, investigated by high temperature compressive testing at a constant strain rate, was compared with dynamic tensile relaxation behavior. A correlation equation of the dynamic frequency and quasistatic strain rate was successfully deduced, and then its validity was experimentally confirmed in a fragile metallic glass. Using this correlation, the Cox–Merz rule, derived for correlating the steady-state and dynamic viscosities of the polymers, is found to be applicable to metallic glasses.

Measurements and model predictions of transient elongational rheology of polymeric nanocomposites

Transient elongational rheology of two commercial-grade polypropylene (PP) and the organoclay thermoplastic nanocomposites is investigated. A specifically designed fixture consisting of two drums (SER Universal Testing Platform) mounted on a TA Instruments ARES rotational rheometer was used to measure the transient uniaxial extensional viscosity of both polypropylene and nanoclay/PP melts. The Hencky strain rate was varied from 0.001 to 2 s − 1, and the temperature was fixed at 180°C. The measurements show that the steady-state elongational viscosity was reached at the measured Hencky strains for the polymer and for the nanocomposites. The addition of nanoclay particles to the polymer melt was found to increase the elongation viscosity principally at low strain rates. For example, at a deformation rate of 0.3 s − 1, the steady-state elongation viscosity for polypropylene was 1.4 × 104 Pa s which was raised to 2.8 × 104 and 4.5 × 104 Pa s after addition of 0.5 and 1.5 vol.% nanoclay, respectively. A mesoscopic rheological model originally developed to predict the motion of ellipsoid particles in viscoelastic media was modified based on the recent developments by Eslami and Grmela (Rheol Acta 47:399–415, 2008) to take into account the polymer chain reptation. We show that the orientation states of the particles and the rheological behavior of the layered particles/thermoplastic hybrids can be quantitatively explained by the proposed model.

Transient shear rheology of carbon nanofiber/polystyrene melt composites

We characterize the transient shear rheology of polystyrene/carbon nanofiber composites. Our experimental measurements of the composites show increasing stress overshoot responses to transient shear as the carbon nanofiber concentration increases. We also find the steady state viscosity reached at long times during application of a constant shear rate increases with increasing carbon nanofiber concentration. Flow reversal experiments show the effects of nanofiber orientation and structural evolution on the composite's rheological response. We present a microstructurally based constitutive model where all but two parameters are determined by rheological characterization of the pure polymer and the shape and concentration of the nanoparticles. The Folgar-Tucker constant, C I , is treated as a fitting parameter, while several definitions for the shape factors A, B, C and F are evaluated. We make note of the effects each parameter has on the model's predictions. We find that the constitutive model is in agreement with our experimentally measured transient shear rheology of the PS/CNF melt composites for the CNF concentrations and shear rates presented.

Effects of Perfluorosulfonic Acid Adsorption on the Stability of Carbon Black Suspensions

Carbon black particles were stabilized by the adsorption of perfluorosulfonic acid (PFSA) onto their surfaces in a mixture of water and isopropyl alcohol. The resulting carbon black suspensions were characterized by measurements of the adsorbed amounts of PFSA, the hydrodynamic diameter of carbon black particles with adsorbed PFSA, and by steady state shear viscosity measurements as functions of the PFSA and carbon black particle concentrations. An equilibrium adsorption of PFSA on carbon black particles was established within 1 day, and the resulting adsorption isotherm was dependent on the carbon black particle concentration; the amounts of adsorbed PFSA decreased with increased carbon black particle concentration. The hydrodynamic diameter of the carbon black particles in suspension increased with the amount of adsorbed PFSA, resulting in stabilization of the carbon black suspensions. Thus, the apparent steady-state shear viscosities of the carbon black suspensions at the first and second shear thinning regions decreased with an increase in the amount of adsorbed PFSA.

Modelling elongational and shear rheology of two LDPE melts

Experimental data of two low-density polyethylene (LDPE) melts at 200°C for both shear flow (transient and steady shear viscosity as well as transient and steady first normal stress coefficient) and elongational flow (transient and steady-state elongational viscosity) as published by Pivokonsky et al. (J Non-Newtonian Fluid Mech 135:58–67, 2006) were analysed using the molecular stress function model for broadly distributed, randomly branched molecular structures. For quantitative modelling of melt rheology in both types of flow and in a very wide range of deformation rates, only three nonlinear viscoelastic material parameters are needed: Whilst the rotational parameter, a 2, and the structural parameter, β, are found to be equal for the two melts considered, the melts differ in the parameter $f_{\rm max}^2 $ describing maximum stretch of the polymer chains.

Time−Temperature and Time−Concentration Superposition of Nanofilled Elastomers: A Molecular Dynamics Study

We systematically investigated the structural and dynamic properties of nanofilled elastomer on the basis of the idealized model of elastomer and nanoparticle. The simulated results indicate that the introduced nanoparticles induce more efficient chain packing. Meanwhile, a mobility gradient of polymer chains is found to exist approaching the nanoparticles. For the dynamic properties, we find for the first time that both the time−concentration and time−temperature superposition principles (TCSP and TTSP) are applicable at the chain length scale, while both break down at the segmental length scale for the filled system. However, the TTSP still holds at the segmental length scale for the pure system. Furthermore, the time−temperature−concentration superposition principle (TTCSP) applies well for the terminal relaxation of polymer nanocomposites. This scaling behavior has the underlying implication that the rheological properties (i.e., steady-state shear viscosity) of polymer nanocomposites possess thermorh...

Rheology of dense-phase vibrated powders and molecular analogies

By means of a stress imposed rheometer coupled with a “vibrating cell,” generating a Brownian motion at a macroscopic scale into the samples, we have shown that dense-phase vibrated powders exhibit rheological behaviors archetypal of non-Newtonian viscoelastic fluids. These behaviors have been accurately described through a free volume structural model based on simple “stick-slip” granular interactions. As a result, the evolution of the steady-state viscosity has been accurately expressed as a function of the shear rate, the frictional stress, the granular pressure, the mass of the samples, the vibration frequency, the vibration energy, the intergranular contact network mean life, and the free volume distribution. The model is consistent with Hookean, Coulombian, and Newtonian limits and is not only descriptive but also explicative and predictive of the encountered phenomena. In particular, a “time-granular temperature superposition principle,” theoretically predicted by the model, has been experimentally verified, the “granular temperature” being controlled through the vibration energy and frequency. Moreover, this superposition principle has been precisely described by a “Vogel-Fulcher-Tammann” law, leading to very close analogies with molecular systems near their glass transition point.

Nonlinear, time-dependent shear flow behaviour, and shear-induced effects in wheat flour dough rheology

Retardation test, step-shear rate experiments, low-amplitude and large-amplitude dynamic measurements have been combined to study the nonlinear and time-dependent viscosity of dough and shear-induced effects of flow on dough structure. Despite large quantitative differences in linear viscoelastic constants, doughs from different flours or with different water contents display the same type of flow behaviour. Shear-induced structural changes cause flow to shift from a high viscosity steady-state regime to a low viscosity one. The process, irreversible, is responsible for the time-dependent character of dough viscosity and seems to be controlled by the mechanical energy absorbed. Nevertheless, the two steady-state viscosities follow the same shear-thinning flow curve, fitted by a Cross equation with an exponent close to 1; the Newtonian plateau is approached at very low shear rate values. Viscosity data obtained on different doughs yield a unique flow master curve in reduced coordinates. Shear-induced structural changes cause also the linear viscoelastic plateau modulus of dough to decrease; this progressive weakening of the network structure is irreversible and seems governed by the accumulated strain. These characteristics of dough rheology are discussed with reference to the behaviour of concentrated suspensions.