Showing papers in "Rheologica Acta in 2003"

Dynamic properties of shear thickening colloidal suspensions

Abstract: The transient shear rheology (i.e., frequency and strain dependence) is compared to the steady rheology for a model colloidal dispersion through the shear thickening transition. Reversible shear thickening is observed and the transition stress compares well to theoretical predictions. Steady and transient shear thickening are observed to occur at the same value of the average stress. The critical strain for shear thickening is found to depend inversely on the frequency at fixed applied stress for low frequencies (high strains), but is limited to an apparent minimum critical strain at higher frequencies. This minimum critical strain is shown to be an artifact of slip. Lissajous plots illustrate the transition in material properties through the shear thickening transition, and the energy dissipated by a shear thickening suspension is analyzed as a function of strain amplitude.

Flow as an effective promotor of nucleation in polymer melts: a quantitative evaluation

Abstract: An apparatus is presented which enables the application of defined portions of mechanical work to the polymer sample in its state of undercooled melt. For the purpose mainly intermittent shear creep is used. Results are presented for an industrial grade of polypropylene. A three-dimensional picture is presented, in which the resulting numbers of nuclei (per unit volume) are plotted against two responsible parameters: crystallization temperature and mechanical work. With decreasing temperature and with increasing mechanical work the number of nuclei increases by many decades. At sufficiently high mechanical loads a transition to thread-like precursors ("shishs") has been observed previously. Oriented structures (kind of "shish-kebabs") are formed in this way. The periods of shearing applied have always been extremely short compared with the time until crystallization becomes observable. In this way an accumulation of various processes could be avoided. The description of shear induced crystallization, as previously given, is modified in the light of the present results.

Effect of molecular weight on the flow-induced crystallization of isotactic poly(1-butene)

Abstract: Turbidity measurements were used to characterize the effect of shear flow on the crystallization kinetics of several isotactic poly(1-butene) samples of different molecular weight (MW). Polymer melts were rapidly cooled below the nominal crystallization temperature, and subjected to a shear flow of varying shear rate but constant total deformation. While the quiescent crystallization was found to be essentially MW-independent, a strong effect of MW on the flow-induced crystallization kinetics was observed. It was shown that such an effect could be cast in terms of a characteristic Weissenberg number, which measures the ability of flow to orient the polymer chains. The proposed scaling relation was found to predict correctly the dependence of flow-induced crystallization upon molecular weight, at least when samples of similar molecular weight distribution were considered. The molecular weight scaling was also found to explain qualitatively the observed transition from a low-shear rate isotropic morphology to a high-shear rate rodlike crystalline structure.

Monodomain response of finite-aspect-ratio macromolecules in shear and related linear flows

Abstract: Recent extensions of the Doi kinetic theory for monodisperse nematic liquids describe rigid, axisymmetric, ellipsoidal macromolecules with finite aspect ratio. Averaging and presumed linear flow fields provide tensor dynamical systems for mesoscopic, bulk orientation response, parameterized by molecular aspect ratio. In this paper we explore phenomena associated with finite vs infinite aspect ratios, which alter the most basic features of monodomain attractors: steady vs unsteady, in-plane vs out-of-plane, multiplicity of attracting states, and shear-induced transitions. For example, the Doi moment-closure model predicts a period-doubling cascade in simple shear to a chaotic monodomain attractor for aspect ratios around 3:1 or 1:3, similar to full kinetic simulations by Grosso et al. [Grosso M, Keunings R, Crescitelli S, Maffettone PL (2001), Prediction of chaotic dynamics in sheared liquid crystalline polymers. Preprint (2001) and lecture, Society of Rheology Annual Meeting, Hilton Head, SC, February 2001] for infinite aspect ratios. We develop symmetries of mesoscopic tensor models robust to closure approximations but specific to linear flow fields, and analytical methods to determine: Simulations highlight the degree to which scaling properties of Leslie-Ericksen theory are violated. By varying molecular aspect ratio, any shear-induced monodomain is reproducible among the well-known closure approximations, yet no single closure rule suffices to capture all known attractors and transition scenarios.

On the evaluation of some differential formulations for the pom-pom constitutive model

Abstract: In recent years, progress has been made in modelling long chain branched polymers by the introduction of the so-called pom-pom model. Initially developed by McLeish and Larson (1998), the model has undergone several improvements or alterations, leading to the development of new formulations. Some of these formulations however suffer from certain mathematical defects. The purpose of the present paper is to review some of the formulations of the pom-pom constitutive model, and to investigate their possible mathematical defects. Next, an alternative formulation is proposed, which does not appear to exhibit mathematical defects, and we explore its modelling performance by comparing the predictions with experiments in non-trivial rheometric flows of an LDPE melt. The selected rheometric flows are the double step strain, as well as the large amplitude oscillatory shear experiments. For LAOS experiments, the comparison involves the use of Fourier-transform analysis.

Pressure dependent viscosity and dissipative heating in capillary rheometry of polymer melts

Abstract: In high shear rate capillary rheometry the combined effect of pressure dependent viscosity and dissipative heating becomes significant. Analytical expressions are derived to treat curved Bagley plots and throttle experiments. End effects are taken into account by using an effective length over radius ratio. The non-adiabatic case is described using a lump heat transfer coefficient Λ following Hay et al. (1999). The latter enters into the dissipative heating coefficient % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBam % XvP5wqSXMqHnxAJn0BKvguHDwzZbqegm0B1jxALjhiov2DaeHbuLwB % Lnhiov2DGi1BTfMBaebbnrfifHhDYfgasaacH8srps0lbbf9q8WrFf % euY-Hhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9 % q8aq0-yq-He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqaba % WaaqaafaaakeaacqaH1oqzdaWgaaWcbaGaemiCaahabeaakiabg2da % 9iabeg8aYnaaCaaaleqabaGaeyOeI0IaeGymaedaaOWaaeWaaeaacq % WGJbWydaWgaaWcbaGaemiCaahabeaakiabgUcaRmaalyaabaGaeu4M % dWeabaGafmyBa0MbaiaaaaaacaGLOaGaayzkaaWaaWbaaSqabeaacq % GHsislcqaIXaqmaaaaaa!4D6C! $$ \varepsilon _p = \rho ^{ - 1} \left( {c_p + {\Lambda \mathord{\left/ {\vphantom {\Lambda {\dot m}}} \right. \kern- ulldelimiterspace} {\dot m}}} \right)^{ - 1} $$ (ρ density, c p heat capacity, % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBam % XvP5wqSXMqHnxAJn0BKvguHDwzZbqegm0B1jxALjhiov2DaeHbuLwB % Lnhiov2DGi1BTfMBaebbnrfifHhDYfgasaacH8srps0lbbf9q8WrFf % euY-Hhbbf9v8qqaqFr0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9 % q8aq0-yq-He9q8qqQ8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqaba % WaaqaafaaakeaacuWGTbqBgaGaaaaa!3C5E! $$ \dot m $$ mass flow rate). A rigorous treatment is possible for incompressible melts, assuming a flat radial temperature profile. For compressible melts, the downstream density variation reduces the effective temperature and pressure coefficients of viscosity. In addition, it causes less dissipative heating. The applicability of the treatment was carefully checked for a well characterised LDPE melt and consistent results from throttle experiments and the Bagley plot curvature are found. The pure dissipation effect was treated by a viscous FEM simulation. A fit of the expected analytical expression to the simulated axial pressure profile allows to extract Λ. Throttle experiments allow a reliable determination of the pressure coefficient of viscosity β η from a fit of the analytical prediction for the measured pressure loss Δp as function of the die inlet pressure P i , provided the dissipation coefficient from the FEM simulation is used. An analytical solution for the Bagley plot was derived for the pure dissipation or pressure effect, respectively. In the parabola approximation, however, the two contributions may be superimposed. Whereas dissipative heating increases the curvature of the axial pressure profile in a die in the same direction as the pressure effect, it operates in the opposite direction for the Bagley plot curvature. Pressure coefficients solely determined from Bagley plots are not reliable. The effect of variable melt density on the temperature and pressure coefficients of the LDPE remains below 5%.

Nonlinear viscoelastic properties of MR fluids under large-amplitude-oscillatory-shear

Abstract: Nonlinear viscoelastic properties of the MR fluid, MRF-132LD, under large-amplitude oscillatory shear were investigated. This was accomplished by carrying out the experiments under the amplitude sweep mode and the frequency sweep mode, using a rheometer with parallel-plate geometry. Investigations under the influence of various magnetic field strength and temperatures were also conducted. MR fluids behave as nonlinear viscoelastic or viscoplastic materials when they are subjected to large-amplitude shear, where the storage modulus decreases rapidly with increasing strain amplitude. Hence, MR fluid behaviour ranges from predominantly elastic at small strain amplitudes to viscous at high strain amplitudes. Large-amplitude oscillatory shear measurements with frequency sweep mode reveal that the storage modulus is independent of oscillation frequency and approaches plateau values at low frequencies. With increasing frequency, the storage modulus shows a decreasing trend before increasing again. This trend may be explained by micro-structural variation. In addition, the storage modulus increases gradually with increasing field strength but it shows a slightly decreasing trend with temperature.

Bagley correction: the effect of contraction angle and its prediction

Abstract: The excess pressure losses due to end effects (mainly entrance) in the capillary flow of a branched polypropylene melt were studied both experimentally and theoretically. These losses were first determined experimentally as a function of the contraction angle ranging from 10° to 150°. It was found that the excess pressure loss function decreases for the same apparent shear rate with increasing contraction angle from 10° to about 45°, and consequently slightly increases from 45° up to contraction angles of 150°. Numerical simulations using a multimode K-BKZ viscoelastic and a purely viscous (Cross) model were used to predict the end pressures. It was found that the numerical predictions do agree well with the experimental results for small contraction angles up to 30°. However, the numerical simulations under-predict the end pressure for larger contraction angles. The effects of viscoelasticity, shear, and elongation on the numerical predictions are also assessed in detail. Shear is the dominant factor controlling the overall pressure drop in flows through small contraction angles. Elongation becomes important at higher contraction angles (greater than 45°). It is demonstrated in abrupt contractions (angle of 180°) that both the entrance pressure loss and the vortex size are strongly dependent on the extensional viscosity for this branched polymer. It is suggested that such an experiment (visualisation of entrance flow) can be useful in evaluating the validity of constitutive equations and it can also be used to fitting parameters of rheological models that control the elongational viscosity.

Rheological behavior of sewage sludge and strain-induced dewatering

Abstract: In order to study the strain-induced water release in sewage sludge and its connection with rheological behavior, two types of rheological tests have been carried out. The rheology of sewage sludge samples stemming from a urban sewer was first characterized using a Couette cell system. In particular, the yielding behavior and the elastic modulus of the sludge has been considered under shear flow conditions. In these pure shear tests the reproducibility of the measurements was rather poor, limiting this study to low strains. Consequently, a second type of rheological tests, namely the squeeze test, which is more appropriate for these paste-like materials, has been considered. The rheological behavior along with the dewatering efficiency have been studied under the squeeze flow conditions. Surprisingly, it was found that, under certain conditions, the strain-induced water release mechanism became more effective when decreasing the squeeze speed. This was interpreted in terms of a competition between the paste flow and the water filtration through the porous media made up by the flocs.

Rheological studies of concentrated guar gum

Abstract: Polymers and surfactants are essential ingredients of the printing paste. Polysaccharides are used commercially to thicken, suspend or stabilise aqueous systems. Also they are used to produce gels and to act as flocculates, binders, lubricants, to serve as modifiers of film properties, and have a function as adjusters of rheological parameters. Surfactants, on the other hand, perform numerous functions acting as dispersants, wetting agents, emulsifiers and antifoaming agents. The rheological properties of polysaccharide thickeners (guar gums with different substitution levels and different producers) at different concentrations and temperatures and, second, the effects produced by the addition of nonionic surfactants (polyoxyethylene stearyl alcohols with different numbers of EO groups) have been studied under linear and nonlinear shear conditions. Experimental data have been correlated with the different models: flow curves with the Cross, Carreau and Meter-Bird model, and mechanical spectra with the generalized Maxwell model and Friedrich-Braun model. The surface tensions of aqueous systems containing polysaccharide and/or surfactants have been determined over extended concentration ranges in order to detect the CMC conditions and to provide a better understanding about the polysaccharide-surfactant interactions.

Characterisation of thixotropy revisited

Abstract: This paper re-examines the 'proof' of Cheng and Evans (1965) that the equilibrium flow curve of a thixotropic fluid has a positive slope, and shows that the slope can indeed be negative in parts. The implication of this on thixotropic behaviour in viscometers is described. Such thixotropic fluids can develop shear banding in wide-gap viscometers in common with many other materials, a brief survey of which is given. Additionally, the interrelationship of the Cross and Sisko fluids as models of thixotropy is examined, leading to the creation of a new class of thixotropic models.

A thermodynamic approach to the rheology of highly interactive filler-polymer mixtures. Part II. Comparison with polystyrene/nanosilica mixtures

Abstract: The dynamic properties as a function of frequency and strain amplitude, steady-state viscosity as a function of shear rate, and transient shear stresses at startup and cessation of shear flow of polystyrene (PS)/fumed silica mixtures of various concentrations were investigated. An abrupt change in the viscoelastic properties was noticed at a concentration above 1% by volume. Observations by means of scanning electron microscopy (SEM) indicate the presence of a three-dimensional network through the bridging of filler particles by the adsorbed polymer. The viscoelastic behavior is simulated utilizing a theory proposed in Part I (Havet and Isayev 2001) based on a double network created by the entangled polymer matrix and the adsorbed polymer with filler concentration taken into account through the bridging density of polymer-filler interactions and a hydrodynamic reinforcement. The steps taken for determining the model parameters required to carry out the simulation are described. The major features of the rheological behavior of highly interactive polymer-filler mixtures are captured qualitatively and in some cases, quantitatively predicted.

Rheological models with microstructural constraints

Abstract: Rheological models of complex fluids with a physically restricted microstructure are analyzed to obtain general classes of dynamical evolution equations for these materials. These classes insure that the appropriate mathematical constraints, associated with each type of physical restriction, are consistently incorporated into the corresponding model development. Describing the microstructure of the complex fluid with a second-rank tensor variable, a general class of dynamical evolution equations is derived for three physically meaningful constraints associated with constancy of the invariants of this microstructural tensor. The physical rationale for each of these constraints is discussed, and a corresponding set of constrained dynamical evolution equations is derived in general terms.

Rheology of carbon black suspensions. I. Three types of viscoelastic behavior

Abstract: Dynamic viscoelastic properties were investigated for carbon black (CB) suspensions in three suspending media, an alkyd resin (AR), a rosin-modified phenol resin (PR), and a polystyrene/dibutyl phthalate solution (PS/DBP). The affinity of the medium toward the CB particles decreased in this order, and different types of rheological behavior were observed accordingly. In AR having a high affinity, the CB particles were well dispersed to form no agglomerates. These particles exhibited a slow relaxation process attributable to their diffusion. In PR having a moderate affinity, the suspension showed a sol-gel transition with increasing CB concentration, and the critical gel behavior characterized with a power-law relationship between the modulus and frequency (ω), G′ = G″/tan(nπ/2)∝ωn with n=0.71, was observed at a critical concentration. This behavior suggested formation of a self-similar, fractal agglomerate of the CB particles in PR. These two suspensions in AR and PR exhibited a moderate nonlinearity. In contrast, in PS/DBP, the suspension exhibited a strong nonlinearity attributable to strain-induced disruption of a fully developed three-dimensional (3D) network structure of the CB particles therein. Thus, the structure and rheology of the CB particles changed with the affinity of the suspending medium. This result in turn posed a caution to a conventional structural interpretation that the CB particles in the actually used ink/paint (always) form the fully developed 3D agglomerates.

Suspensions of monodisperse spheres in polymer melts: particle size effects in extensional flow

Abstract: Suspensions in polymeric, viscoelastic liquids have been studied in uniaxial extensional flow. The fibre wind-up technique has been used for this purpose. The effects of particle size and particle volume fraction have been investigated, using monodisperse, spherical particles. The results have been compared with shear flow data on the same materials. The values of the relative extensional viscosities at low stretching rates are in agreement with the relative shear viscosities and relative moduli. This indicates that hydrodynamic forces are stronger than the particle interaction forces. At larger strain rates strain hardening occurs; it is suppressed when particles are added. Small aggregating particles reduce the strain hardening more strongly than larger particles; strain hardening can even be totally eliminated. When further increasing the stretching rate, hydrodynamic effects dominate again and the effect of particle size effect on strain hardening disappears.

Rheological evidence of modifications of polypropylene by β-irradiation

Abstract: Electron beam irradiation can be used to induce chemical changes in polymers. The resulting reactions lead to both degradation and crosslinking of polymer chains, depending on reaction conditions. In neat polypropylene, degradation dominates and results in a decrease of molecular weight and worsening of mechanical properties. Enhanced crosslinking can be achieved by utilising a polyfunctional monomer. Triallylisocyanurate (TAIC) serves this purpose and can be used to crosslink polypropylene effectively. The corresponding changes of the rheological properties can be observed using oscillatory and creep experiments when changing the amount of TAIC added as well as the absorbed radiation dose. Depending on these parameters, we attribute the rheological properties to chain degradation or formation of a network and gelation. The phase angle vs the dynamic modulus plot is a useful analytical tool for characterisation of the resultant structures. Some samples showed behaviour that leads us to assume broadening of the molecular weight distribution and long chain branching. Resulting from the parameter dependencies, a topological state diagram is proposed to map parameter values to corresponding polymer structures.

Instabilities of micellar systems under homogeneous and non-homogeneous flow conditions

Abstract: The rheological behavior of a cetylpyridinium chloride 100 mmol l–1/sodium salicylate 60 mmol l–1 aqueous solution was studied in this work under homogeneous (cone and plate) and non-homogeneous flow conditions (vane-bob and capillary rheometers), respectively. Instabilities consistent with non-monotonic flow curves were observed in all cases and the solution exhibited similar behavior under the different flow conditions. Hysteresis and the sigmoidal flow curve suggested as characteristic of systems that show constitutive instabilities were observed when running cycles of increasing and decreasing stress or shear rate, respectively. This information, together with a detailed determination of steady states at shear stresses close to the onset of the instabilities, allowed one to show unequivocally that "top and bottom jumping" are the mechanisms to trigger the instabilities in this micellar system. It is shown in addition that there is not a true plateau region in between the "top and bottom jumping". Finally, the flow behavior beyond the upturn seemed to be unstable and was found accompanied by an apparent violation of the no-slip boundary condition.

Effect of two gums on the development, rheological properties and stability of egg albumen foams

Abstract: Egg albumen foams (8%) with and without added propylene glycol alginate or xanthan gum (both at 02% concentration on wet weight basis) formed after 1–5 min whipping were tested for their consistency using squeezing flow viscometry and stability The apparent stress at two specimen heights (15 and 10 mm) and the residual stresses after 60 and 120 s relaxation, at 1 mm height, served as measures of the foams consistency and degree of solidity, respectively The drainage rate and changes in the air bubbles size distribution with time served as indicators of the foams' physical stability The two gums had a dramatic effect on both the foams consistency and degree of solidity and to a lesser extent on their stability In most cases though, the effects were clearly noticeable in foams beaten for 3 min or longer Since the two gums' effects on the foams properties could not be attributed to stoichiometric considerations, it is suggested that they were primarily due to strengthening the cells walls, an observation that is supported by the relatively small differences in the foams general structure

Shear rate dependence of the normal force of a magnetorheological suspension

Abstract: An experimental study was made of the normal force in a magnetorheological suspension under shear flow with the magnetic field applied normally to the shear planes. Under no deformation, the normal force was found to increase with magnetic flux density, and acted to push apart the rheometer plates. At the start-up of shearing under a constant shear rate, the normal force decreased with strain and reached a plateau value which became smaller as shear rate was raised. This behaviour can be interpreted in terms of the aggregate model of magnetorheological suspensions: the normal force arises from the elongated aggregates spanning the gap between the plates, and these are broken up into smaller particle clusters under shearing.

Interrelation of creep and relaxation for nonlinearly viscoelastic materials: application to ligament and metal

Abstract: Creep and stress relaxation are known to be interrelated in linearly viscoelastic materials by an exact analytical expression. In this article, analytical interrelations are derived for nonlinearly viscoelastic materials which obey a single integral nonlinear superposition constitutive equation. The kernel is not assumed to be separable as a product of strain and time dependent parts. Superposition is fully taken into account within the single integral formulation used. Specific formulations based on power law time dependence and truncated expansions are developed. These are appropriate for weak stress and strain dependence. The interrelated constitutive formulation is applied to ligaments, in which stiffness increases with strain, stress relaxation proceeds faster than creep, and rate of creep is a function of stress and rate of relaxation is a function of strain. An interrelation was also constructed for a commercial die-cast aluminum alloy currently used in small engine applications.

Field dependence of the response of a magnetorheological suspension under steady shear flow and squeezing flow

Abstract: A comparison was made of the behaviour of a magnetorheological suspension under steady shear flow and constant velocity squeezing flow. The strain rates and sample dimensions were chosen to be comparable in the two deformation modes, and the dependence of the mechanical properties on the magnetic flux density B was investigated. The measurements found that the mechanical response under squeezing flow scaled as B0.91, whereas the response under shearing scaled as B1.4, close to theoretical predictions. This difference of the field dependence between the shearing and squeezing flows was possibly due to the different microstructural rearrangement processes which occur in the two deformation modes.

Rheological properties of suspensions of TiO2 particles in polymer solutions. 1. Shear viscosity

Abstract: This paper presents results on the rheological behaviour of suspensions of two kinds of TiO2 particles in two different polymer solutions. The particles differ in their hydrophilic or hydrophobic properties. The dispersing media are a solution of high molecular weight polyisobutylene in decalin and a solution of a low molecular weight polybutene in decalin. The concentrations of polymer are adjusted in order to get the same zero shear viscosity. The shear viscosity measurements display an apparent yield stress in some cases. The existence and the values of the yield stress depend on the volume fraction of solid particles and on the type of particles. The evolution of the intrinsic viscosity and of the maximum packing fraction vs the shear rate is interpreted in terms of evolution of the size and of the shape of aggregates of particles under shear. The effect of temperature on the development of the yield stress is also discussed. The results are completed by microscopic observations.

Rheology of carbon black suspensions. II. Well dispersed system

Abstract: Linear viscoelastic properties were investigated for the suspensions of carbon black (CB) particles having covalently-fused aggregate structures of an average diameter a=120 nm. The suspending medium, an alkyd resin (AR), had a high affinity toward the CB particles, and the aggregates of these particles were well dispersed to form no higher-order agglomerates. Consequently, the suspensions obeyed the time-temperature superposition and their Arrhenius-type activation energy was identical to that of the medium. From comparison of the zero shear viscosity η0 for the CB suspensions and hard-sphere silica suspensions, an effective volume fraction φeff of the CB particles was found to be 2.7 times larger than the bare volume fraction of the particles. The CB particles exhibited a slow relaxation process, and the terminal relaxation time of this process was close to the Peclet time (Brownian diffusion time) evaluated from the aggregate size a and a high frequency viscosity. Furthermore, the terminal relaxation mode distribution of the CB suspensions was well scaled with an intensity factor Ht that was evaluated from the φeff in a way utilized for the hard-sphere silica suspensions. These results demonstrated that the slow relaxation in the CB suspensions was dominated by the Brownian diffusion of the CB aggregates, as similar to the situation for the silica suspensions.

Constrained Inversion of Rheological Data to Molecular Weight Distribution for polymer melts

Abstract: In this paper an inverse method for converting linear viscoelastic properties of polymer melts into molecular weight distribution (MWD) is presented. It relies on the use of the double reptation mixing rule and the Generalized Exponential function (GEX) describing the MWD; i.e. an a priori knowledge of the nature of the solution is postulated following previous attempts (Nobile et al. 1996b; Nobile and Cocchini 1999, 2001) where the Tuminello kernel was adopted to get analytical results. Here a MWD dependent kernel, recently proposed in the literature (Thimm et al. 1999), is applied, still obtaining analytical results but avoiding the consequences of the crude step-like approximation of the Tuminello kernel. In particular, the steady-state compliance and the elastic modulus at small frequencies do not suffer from the underestimation discussed in the cited papers. At high frequencies, the Rouse modes contribution has also been considered in the model. The present approach turns out to be simpler, more accurate and robust than the previous one. Moreover, the results can be more clearly stated in the frame of the theory. Careful and easy-to-use formulas for the zero-shear-rate viscosity, the steady-state compliance, the modulus and frequency at the G′-G′′ crossover, have been obtained as a function of the MW averages. As expected, the comparison among the results obtained through the two kernels explicitly confirms the minor effects of the kernel on the rheology of polydisperse polymers. Concerning the molecular weight averages values, the predictions obtained from the dynamic moduli measurements typically differ less than 10% from those obtained from the reference measurements obtained with other methods.

E-FiRST: Electric field responsive shear thickening fluids

Abstract: A novel electrorheological (ER) effect is presented where the application of an electric field, orthogonal to the vorticity-flow plane, increases the critical hydrodynamic stress required to shear thicken concentrated, colloidal dispersions (the E-FiRST effect). The shear thickening behavior of a Brownian charge stabilized dispersion (226 nm silica in 4-methylcyclohexanol at 53, 50. and 41 vol.%) is studied in the presence of an electric field as a function of the field strength and coupling parameter (β) where the latter is a function of a.c. field frequency due to diffusion limitations on the polarization of the particles' double layer. A mechanism is proposed whereby the applied electric field suppresses the formation of the self-organized hydrocluster microstructure responsible for shear thickening, thus delaying the onset of shear thickening to higher applied shear stresses. A Mason-number type scaling law is found to scale the effect, which supports the proposed mechanism.

Viscoelastic behaviour of partly hydrolysed polyacrylamide/chromium(III) gels

Abstract: This paper reports an experimental investigation of the crosslinking process of high molecular weight partly hydrolysed polyacrylamides (HPAAm) in aqueous brine solution by trivalent chromium ions (Cr(III)). Crosslinking took place in the presence of a retardant agent (sodium citrate). First, sol-gel phase diagrams (in the polymer and concentration space) were established using the tilting tube method. Then, for a fixed composition, the gelation process was monitored systematically using dynamic viscoelastic measurements, varying the main parameters (pH, time, temperature and retardant concentration). Network formation proceeds rather slowly and an equilibrium state was not reached within 12 h. The gel was formed only at pHs between 5 and 9 and thus two gel points (i.e. at two pHs) were determined with the Winter-Chambon method. This is in agreement with the chemistry of aqueous chromium and of acrylic acid groups along the polymer backbone. Kinetics of network formation depends strongly on retardant concentration. Temperature plays an important role: network formation proceeds much faster at high temperature, in agreement with chemical kinetics.

Gelatinization and gelation of corn starch followed by dynamic mechanical spectroscopy analysis

Abstract: The structural evolution of a maize flour was followed by means of oscillatory shear measurements during heating at a rate of 10 °C/min in presence of water. These measurements were performed in a special plate and plate vessel designed to prevent moisture loss. The phenomena of gelatinization and gelation were clearly identified and their connection with the moisture content also demonstrated. Moreover, the complex mechanisms involved in gelatinization and gelation in native starch were separated. Softening of the amylose zones, exchange of water and amylose within the starch granules followed by amylopectin melting leads to the gelation of starch.

Filament stretching rheometer: inertia compensation revisited

Abstract: The necessary inertia compensation used in the force balance for the filament stretching rheometer is derived for an arbitrary frame of reference. This enables the force balance to be used to extract correctly the extensional viscosity from measurements of the tensile force at either end of the elongating fluid column in any of the different experimental configurations that have been introduced to date. The present analysis eliminates a restriction inherent in the work of Szabo (Rheol Acta 36:277–284 (1997)).

A rheological approach to the stability of humic acid/clay colloidal suspensions

Abstract: Steady-state, oscillatory, and transient rheological determinations were used to assess the stability of homoionic sodium montmorillonite (NaMt) suspensions at constant ionic strength (10–2 mol/l NaCl) and different pH values, after adsorption of humic acid (HA) on the particles. The adsorption of the latter was first spectrophotometrically determined, at pH 3 and 9. While at pH 9 adsorption saturation was observed, at pH 3 the adsorption density continued to grow up to the maximum equilibrium HA concentration reached (∼200 mg/l). Considering the similarity between the structure of edge surfaces of NaMt particles and the surfaces of silica and alumina, the adsorption of HA was also investigated on the latter solids. The results suggest that at pH 3 humic acids adsorb preferentially on edge surfaces, mainly through electrostatic attraction with positively charged aluminol groups. This hypothesis is indirectly confirmed by zeta potential, ζ, values: while HA concentration has little effect on ζ for silica, the addition of HA yields the zeta potential of alumina increasingly negative for all pH values. Using shear stress vs shear rate plots, the yield stress of NaMt was determined as a function of particle concentration, C, for pH 3, 5, 7, and 9, with and without addition of 50 mg/l HA. The yield stress, σy, was fitted with a power law σy∝C n ; it was found that n values as high as 12 are characteristic of NaMt suspensions at pH 9 in the presence of HA. This indicates a strong stabilizing effect of humic acid. This stabilization was confirmed by oscillometric measurements, as the storage modulus G′ in the viscoelastic linear region also scales with C, displaying large n values at neutral and basic pHs in the presence of HA. The modulus (in the viscoelastic linear region, for a frequency ν=1 Hz) was found to increase with time, but G′ was lower at any time when HA was added, a consequence of the stabilization provided by HA. Similarly, creep-recovery experiments demonstrated that NaMt suspensions containing HA displayed a less elastic behavior, and a permanent deformation. Modeling the results as a Kelvin-Voigt model allowed one to establish a new scaling law of the reciprocal instantaneous deformation with C. As before, high values of n were found for suspensions at pH 9 in the presence of HA.

Drop deformation under small-amplitude oscillatory shear flow

Abstract: The deformation of an isolated drop in an immiscible liquid undergoing oscillatory shear flow is experimentally investigated as a function of frequency and up to moderate amplitudes. Oscillatory shear flow is generated by using a parallel plate apparatus. Drop shape is observed by video light microscopy along the vorticity direction of the shear flow. The two principal axes and the orientation of the drop in the plane of shear are measured by image analysis. In the small amplitude range, the time dependence of the axes is also harmonic, but not in phase with the applied strain, the phase difference being a decreasing function of the imposed frequency. The linear range (where the major axis is proportional to the amplitude) extends up to strains of 0.5. Good quantitative agreement was found with the Palierne linear viscoelastic model (Palierne, J. F., Linear rheology of viscoelastic emulsions with interfacial tension, Rheol. Acta, 29, 204–214, 1990), thus providing a further example of the good agreement between experiments and small deformation theory.