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Journal of RheologyVolume 44, Issue 2 (March-April 2000) |
Contents
Ageing and rheology in soft materialsS. M. Fielding P. Sollich M. E. Cates AbstractWe study theoretically the role of ageing in the rheology of soft materials. We define several generalized rheological response functions suited to ageing samples (in which time translation invariance is lost). These are then used to study ageing effects within a simple scalar model (the “soft glassy rheology” or SGR model) whose constitutive equations relate shear stress to shear strain among a set of elastic elements, with distributed yield thresholds, undergoing activated dynamics governed by a "noise temperature", x. (Between yields, each element follows affinely the applied shear.) For 1 < x < 2 there is a power-law fluid regime in which transients occur, but no ageing. For x < 1, the model has a macroscopic yield stress. So long as this yield stress is not exceeded, ageing occurs, with a sample's apparent relaxation time being of order its own age. The (age-dependent) linear viscoelastic loss modulus G"(ω, t) rises as frequency is lowered, but falls with age t, so as to always remain less than G'(ω, t) (which is nearly constant). Significant ageing is also predicted for the stress overshoot in nonlinear shear startup and for the creep compliance. Though obviously oversimplified, the SGR model may provide a valuable paradigm for the experimental and theoretical study of rheological ageing phenomena in soft solids. The characterization of the total stress of concentrated suspensions of noncolloidal spheres in Newtonian fluidsIsidro E. Zarraga, Davide A. Hill, and David T.
Leighton, Jr.* *Corresponding Author AbstractThe total stress of a concentrated suspension of noncolloidal spheres in a Newtonian fluid was characterized by independent measurements in viscometric flows. Using a suspension balance formulation, the normal stress in the vorticity direction (Σ33) for a suspension undergoing simple shear was extracted from Acrivos, et al.'s [Int. J. Multiphase Flow 19, 797 (1993)] resuspension data in a Couette device. Employing a new correlation for the relative viscosity μr which obeys the Einstein relation in the dilute limit while diverging at random close packing, it was found that Σ33/τ (where τ is the magnitude of the shear stress) was a strong function of the solid volume fraction φ, scaling as φ 3e2.34φ. The relative viscosity, measured in a parallel plate viscometer, was in good agreement with the proposed correlation, while the normal stress differences Nl and N2 for concentrated suspensions (φ = 0.30 to 0.55) were characterized using parallel plate and cone-and-plate geometries, as well as laser profilometry measurements of the suspension surface deflection in a rotating rod geometry. The normal stresses were proportional to the shear stress τ, and with β º N1/τ and δ º N2/τ , the parameter combinations resulting from the three experimental geometries, β - δ, β, and δ + 1/2 β, were all seen to increase with φ according to the derived scaling φ 3e2.34φ. Furthermore, the best-fit Nl and N2 values consistent with the set of experiments were both negative, with ½N2½ > ½Nl½ at any given concentration and shear rate. Taken together, the results obtained allow a complete determination of the total stress of a sheared suspension and in particular enabled us to compute the shear-induced particle-phase pressure II, as defined in Jeffrey, et al. [Phys. Fluids A 5, 2317 (1993)]. Brownian dynamics simulations of dilute polystyrene solutionsLei Li and Ronald G. Larson Tam Sridhar AbstractBrownian dynamics simulations of a bead-spring chain are used to predict the shear and extensional flow properties of dilute polystyrene "Boger fluids" in the filament-stretching device. Using an accurate representation of the inverse Langevin function for the elastic "spring" force of a freely jointed chain, and parameter values extracted from Zimm molecular theory combined with the molecular properties of the polystyrene solutions, quantitatively accurate predictions of the steady-state shear viscosity and first normal stress difference are obtained. The growth of the uniaxial extensional viscosity after start-up of steady extension is also reasonably accurately predicted for solutions of 1.95 million and 3.9 million molecular weight polystyrene, but for higher polystyrene molecular weights there is a growing overprediction of the high-strain plateau viscosity and a growing lag in the predicted stress growth relative to the measured one. Introduction of "deformation-dependent drag" into the simulations only worsens the predictions. In deformation histories in which there is a sudden step up in extension rate or a re-start of extensional flow after a pause, the stress rises very rapidly in a viscous-like manner. The simulations show that this behavior occurs because the conformations of the sub-population of molecules that contributes most to the stress contains long fully-extended portions. Using the Toms effect for rheokinetic study of the initial stage of polymerizationA. Ya. Malkin G. V. Nesyn and A. V. Ilyusnikov V. N. Manzhai AbstractTraditionally, the Toms effect was widely used in its direct application for drag reduction in channel flow or for the movement of solids through a liquid medium. In this paper, however, we have considered a possibility for using this effect as a method allowing one to monitor the very beginning of the polymerization process. Our approach was based on the high sensitivity of the Toms effect to minor changes in polymer concentration in a low-molecular weight solvent. We have centered our discussion on chemical reactions that occurred in very dilute solutions. As a typical example, we have examined ionic polymerization of hexene. Application of the Toms effect gave us a unique chance to investigate chemical processes happening when the polymer concentration is on the order of tens of ppm. This concentration range was much lower that the ranges studied by traditional chemical or physical methods. Droplet vorticity alignment in model polymer blendsK. B. Migler AbstractWe utilize stroboscopic optical microscopy to monitor the shear induced deformation of polymeric droplets in an immiscible polymeric matrix and find that under conditions of high droplet elasticity, the droplets can align in the vorticity direction. We consider the case where the viscosity ratio of the two phases is near unity but the elasticity ratio of the droplet to the matrix is greater than 100. This is achieved by using a matrix phase of polydimethylsiloxane and a droplet phase of a polyisobutylene based "Boger" fluid. In the limit of weak shear and small droplets, the droplet alignment is along the shear direction, whereas for strong shear and large droplets, the alignment is along the vorticity direction. There is a range of conditions for which alignment can be along either axis. For droplets aligned along the vorticity axis, the distribution of aspect ratios is broad. The kinetic transformation from droplet flow alignment to vorticity alignment upon increase of shear flow has been observed, as well as the relaxation back to a spherical shape upon cessation of shear. We deduce from the kinetic experiments that the thickness in the gradient direction is less than that in the flow and vorticity directions. Influence of the fragmentation of multiple globules on the rheological properties of W/O/W multiple emulsionsV. Muguet M. Seiller G. Barratt N. Hebel J.P. Marty J.L. Grossiord, Corresponding Author AbstractThe purpose of our work was to study the behavior of W/O/W multiple emulsions submitted to a shear flow by applying the theoretical framework developed by Taylor to describe the deformation and the bursting of simple oily globules under shear. The formulation parameters taken into consideration were the mass fraction of dispersed phase and the viscosity of the continuous phase. The conductometric analysis carried out to study the release under shear of the electrolyte entrapped inside the internal aqueous phase of the multiple emulsions, together with microscopic and granulometric analyses, showed that Taylor's framework could be applied to multiple emulsions. Rheological analyses were also carried out and confirmed the bursting of the globules in parallel with the granulometric studies. This phenomenon could be visualized by rheograms showing a more or less large hysteresis loop. The mechanisms which occur during the bursting are complex and depend on the shear time. Stick and slip phenomena during extrusion of polyethylene melts as investigated by laser-Doppler velocimetryH. Münstedt, M. Schmidt, and E. Wassner* *Now at BASF AG, Polymer Research Division AbstractThe flow behaviour of polyethylene melts in a slit die was investigated using laser-Doppler velocimetry (LDV). Two polyethylenes, a linear and a long chain branched sample, were compared in order to get an insight into the influence of branching on stick and slip phenomena. For the long-chain branched polyethylene velocity profiles were obtained which do not give any indication to wall slip in the range of shear rates applied. For the linear polyethylene the velocity distributions in the three regions, commonly distinguished by different pressure-output relations in the literature, were measured. Pronounced wall slip velocities are detected at low apparent shear rates. The importance of this finding for the determination of viscosity functions is discussed. At higher output rates the well-known pressure oscillations are accompanied by velocity fluctuations of the same frequency but totally different shapes of amplitudes. Significant periodic features of the time dependence of the velocity are tried to be interpreted by a hypothesis of entanglement and disentanglement of the molecules close to the wall. Exceeding an upper critical output value a flow region is reached which is distinguished by a constant pressure reading as a function of time again. Within this region a nearly ideal plug flow is observed, i.e. there is an indication for strong slip at the wall. The Johnson-Segalman model with a diffusion term in cylindrical Couette flowP. D. Olmsted, O. Radulescu1 and C.-Y. D. Lu2 1Department of Physics and Astronomy 2Department of Physics, National Central
University AbstractWe study the Johnson-Segalman (JS) model as a paradigm for some complex fluids which are observed to phase separate, or "shear-band" in flow. We analyze the behavior of this model in cylindrical Couette flow and demonstrate the history dependence inherent in the local JS model. We add a simple gradient term to the stress dynamics and demonstrate how this term breaks the degeneracy of the local model and prescribes a much smaller (discrete, rather than continuous) set of banded steady state solutions. We investigate some of the effects of the curvature of Couette flow on the observable steady state behavior and kinetics, and discuss some of the implications for metastability. Rheology and microstructures of electrorheological fluids containing both dispersed particles and liquid drops in a continuous phaseByung Doo Chin and O Ok Park* *Corresponding Author AbstractThe effects of dispersed drops in the electrorheological (ER) behavior of a polyaniline particle suspension were considered. Oil-in-oil emulsions, which differ in the electrical conductivity and dielectric constant, were employed for the liquid bi-phase. The yield stress behavior of ER suspension under steady shear and electric field was examined. Only when the dispersed drops with a higher conductivity formed the dispersed liquid phases in the presence of dispersed polyaniline particles was a synergistic effect in the yield stress observed, giving a better ER performance and reduced current density. A direct microscopic observation demonstrated that such a synergistic effect is due to the unique microstructures in a complex composed of particles and deformed emulsion drops. Not only the magnitude but also the dependence of the yield stresses on the electric field strength was strongly affected by the change in relative composition of particulate and liquid drop phases. The associated mechanism of ER response in particle-drop bi-dispersion was explained on the basis of the relationship between the microscopic change of structures and macroscopic rheological properties. Modeling the effect of plasticizer on the viscoelastic response of crosslinked polymers using the tube-junction modelPhilip P. Simon and Harry J. Ploehn* *Corresponding Author AbstractPlasticizers modify the mechanical properties of polymeric materials. The effects of plasticizers on glass transition temperatures can be most clearly observed in isochronal temperature sweep profiles of viscoelastic dynamic moduli. However, no simple mathematical models of plasticization are available to those who wish to design and employ plasticized materials in specific applications. We extend a phenomenological, molecular-level model (known as the Tube-Junction model) for crosslinked polymers to describe the effect of plasticizers on dynamic moduli. We show that the increase in free volume fraction due to the presence of the plasticizer can account for the shift in the glass transition in dynamic moduli. We also show that the secondary effects of plasticizers on the shape of the temperature sweep profiles can be explained in terms of increased width of the distribution of activation energies associated with intermolecular frictional forces. Orientational order in concentrated dispersions of plate-like kaolinite particles under shearA. B. D. Brown S. M. Clarke P. Convert A. R. Rennie AbstractThe orientational order in concentrated dispersions of plate-like, kaolinite particles subjected to shear has been studied using neutron diffraction. The relationship between the direction of orientation and the extent of alignment, calculated as an order parameter, is similar to that predicted for dilute dispersions. The plate-like particles align with their normals in the compressional direction. As the shear rate is increased the degree of order increases and the particles align with normals closer to the gradient direction. However the degree of order observed at a particular shear rate was found to vary with concentration in a complex manner. At intermediate concentrations, the extent of order is enhanced by the particle interactions. In contrast, at high concentrations the extent of order is reduced. An explanation for this effect is suggested in terms of an effective temperature arising from particle interactions and collisions. On the Rouse spectrum and the determination of the molecular weight distribution from rheological dataWolfgang Thimm1, Christian Friedrich1*,
Michael Marth1, and Josef Honerkamp1,2 1Freiburger Materialforschungszentrum,
Stefan-Meier-Straße 21 2Universität Freiburg, Fakultät für Physik,
Hermann-Herder-Straße 3 AbstractMaier et al. (1998) examined the reconstruction of binary molecular weight distributions from rheological data for a series of polystyrene mixtures using a recently introduced generalized mixing rule (Anderssen and Mead (1998)). They found an unexpected high value for β (β = 3.84 ± 0.1), the mixing parameter, which is one for the reptation and two for the double reptation or entanglement models. This result can be understood when the relaxation time spectrum is decomposed into a Rouse and an entanglement part and only the latter is used for the determination of the molecular weight distribution. Applying a procedure which separates the Rouse processes from the spectrum determined from measured dynamic shear moduli, β-values are found which are in accordance with the double reptation theory and which give very good agreement between molecular weight distributions determined by size-exclusion chromatography (SEC) and by rheological data evaluation. Rheological responses of oil-in-oil emulsions in an electric fieldJong-Wook Ha and Seung-Man Yang* *Corresponding Author. E-mail: smyang@kaist.ac.kr AbstractIn the present article, the rheological responses of oil-in-oil emulsions in a dc electric field were investigated experimentally. Specifically, the dispersed phase of the emulsions considered in this work was less conducting than the continuous phase. Depending on the relative strength between the shear flow and electric fields, three distinctive responses of the emulsions were observed in steady and dynamic oscillatory shear tests. First, the apparent viscosity enhancement (positive electrorheological effect) was produced when the electric field was predominant. Secondly, when the shear flow was strong and dominant, the electric field played a trivial role on the rheological behavior of emulsions. Finally, the viscosity reduction (negative electrorheological effect) was generated when the shear flow and electric fields were competitive. The viscosity enhancement was induced by the formation of chain-like microstructures of the dispersed droplets as in a typical electric-field responsive particle suspension. Meanwhile, the viscosity reduction was closely associated with the electric-field-induced rotation of the dispersed droplets, which was confirmed by the electrohydrodynamics of a single conducting drop in a more conducting ambient fluid. Finally, the rheological responses of the model emulsions to a dynamic small-amplitude oscillatory shearing were considered in conjunction with the morphology evolution under the action of the electric and flow fields. The results showed that the interfacial contribution to the rheological response appeared quite differently depending on the conductivity ratio of the two contiguous fluids. |
Please e-mail suggestions and comments to albertco@umche.maine.edu. Updated 25 January 2004 |