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Journal of RheologyVolume 45, Issue 2 (March-April 2001) |
Contents
SALS analysis of morphology development of a model immiscible polymer blend in transient slit-contraction flowsBrian E. Priore and Lynn M.
Walker* AbstractA novel experimental system is developed to study the morphological development of a model immiscible polymer blend system (polyisobutene- polydimethylsiloxane) in complex, transient flows. A simplified analysis technique using small angle light scattering (SALS) probes the microstructure of the blend in-situ and provides information concerning the overall deformation state of the blend. Depending on the viscosity ratio of the blend. the morphological evolution is more strongly influenced by either the transient shear or elongational component of the flow. Steady state modeling is also applied to describe these transient flows. The models are able to predict the general trends seen in the experimental data including the effect of changing blend viscosity ratio and initial morphology. * Author to whom correspondence should be addressed. E-mail: lwalker@andrew.cmu.edu Corroboration of Princen's theory to cosmetic concentrated w/o emulsionsA. Pontona), P. Clémentb), J. L. Grossiordc) a) Laboratoire de
Biorhéologie et d'Hydrodynamique Physico-chimique (L.B.H.P.) b) Centre international de Recherche Bio-Végétale
Yves Rocher, c) Laboratoire de Physique Pharmaceutique, Université Paris XI, 5 rue Jean Baptiste Clément, 92296 Châtenay - Malabry, France AbstractThe rheological behaviour of cosmetic concentrated water in oil (w/o) emulsions has been investigated in relation with their microstructure. Oscillatory measurements and droplet size distribution have been performed as a function of the dispersed phase percentage. The elastic shear modulus is shown to exhibit a universal dependence on the volume fraction Fv according to Princen's equation i.e. increasing as Fv1/3(Fv - Fc) where Fc, the critical volume fraction, is equal to » 0.714, a value close to the one obtained by Princen (0.712). Nonlinear rheology of telechelic polymer networksJ.-F. Berret*, Y. Séréro, B. Winkelman and D. Calvet, A. Collet, M. Viguier AbstractWe report on the nonlinear rheology of aqueous solutions of telechelic associating polymers in the network regime. Different telechelic polymers with a poly(ethylene oxide) middle chain and semi perfluorinated end-caps C8F17(CH2)11 were synthesized and characterized with respect to their functionalisation. At a telechelic concentration c = 4 %, the aqueous solutions studied are highly viscoelastic and close to Maxwellian fluids. Due to the strong hydrophobicity of the end-caps, the network relaxation time can be very long compared to that of polymers with fully hydrogenated hydrophobes. We have exploited these long relaxation times to explore the time-resolved stress responses obtained in start-up experiments. The steady shear stress versus shear rate curves exhibit two main and stable branches separated by a discontinuity .The first branch of the flow curve is Newtonian and shear-thickening, whereas the second branch is shear-thinning. At the onset of shear-thickening, we show that the viscosity increase is related to the hardening of the network, i.e. to an increase of the effective elastic modulus. In the shear-thinning branch, extremely slow transients are observed and the steady state is reached for deformations as large as several hundreds of strain units. It is suggested that these slow relaxations, as well as the discontinuity in the shear stress are the signatures of the onset of an inhomogeneous flow resulting from the breakdown or rupture of the network. * berret@gdpc.univ-montp2.fr The dynamics of dilute and semi-dilute DNA solutions in the start-up of shear flowJoe S. Hur, Eric S.G. Shaqfeh Hazen P. Babcock, Douglas E. Smith and Steven Chu AbstractWe have investigated the dynamics of dilute(10-5C*) and semi-dilute(£ 6C*) DNA solutions both in steady and in the start-up of shear flow by combining fluorescence microscopy, bulk rheological measurements and Brownian dynamics simulations. First the microscopic states, i.e. the conformational dynamics of single DNA molecules in solution during the start-up of shear flow were examined by fluorescence microscopy. To investigate the macroscopic response resulting from the changes in the microscopic state, the bulk shear viscosity of the same DNA solutions was also measured. While the transient dynamics of individual molecules is highly variable, an overshoot in the ensemble-averaged molecular extension is observed above a critical Wi following an overshoot in shear viscosity for both dilute and semi-dilute DNA solutions. These two overshoots are further analyzed and explained on a physical basis from our simulation findings. Based on the physical picture, we have derived a simple scaling to predict the strain at which an overshoot in shear viscosity occurs. Next, to study the effect of intermolecular interactions on the dynamics at steady-state, the microscopic states of dilute and semi-dilute DNA solutions in steady shear flow were experimentally examined. We find that, for both the steady and the start-up of shear flow, when time is scaled with the longest polymer relaxation time, i.e. when we compare the chain dynamics at the same Wi, no measurable change in the character of the individual chain dynamics is observed in DNA solutions up to six times the overlap concentration (C*). Rheology of polyampholyte (gelatin) stabilized colloidal dispersions: The tertiary electroviscous effectK. Abraham Vaynberga) and Norman J. Wagnerb) AbstractThe shear rheology of gelatin stabilized acrylic and polystyrene latices is investigated as a function of particle concentration, electrolyte concentration, and pH. Independent measurements of the adsorbed polyampholyte amount and layer structure are used to predict the rheology within the context of hard-sphere mappings. Dynamic light scattering measurements of the hydrodynamic particle diameter of gelatin coated colloids yield the best predictions of the zero shear viscosity and can qualitatively predict the observed tertiary electroviscous effect. Comparisons between dispersions demonstrate the effect of hydrophobic anchoring of the adsorbed gelatin on dispersion rheology. a) Current address:
Hercules Inc., 500 Hercules Road, Wilmington, DE 19808 Step strain flow: Wall slip effects and other error sourcesHalil Gevgilili and Dilhan M. Kalyon* AbstractThe traditional technique for the experimental characterization of the shear stress relaxation modulus by applying a step shear strain was investigated using flow visualization. A high-speed camera was employed in conjunction with cone-and-plate and parallel-disk fixtures of a Rheometric Scientific ARES rheometer. In the nonlinear region the true shear strain imposed on a polyethylene melt deviates considerably from the targeted strain. The main source of the deviation is the wall slip of the polymer melt. The presence of wall slip reduces significantly the range of strains for which the strain- dependent relaxation modulus can be determined for the linear polyethylene melt. Errors associated with the control of the motion of the tool which introduces the shear strain are also documented. * For correspondence: dkalyon@stevens-tech.edu Novel viscosity equations for emulsions of two immiscible liquidsRajinder
Pal AbstractStarting from the Taylor equation (Taylor, G. I., Proc. R. Soc. London A, 138, 41-48 (1932)) for the viscosity of very dilute emulsions, new viscosity equations are developed for concentrated emulsions using the effective medium approach. .According to the equations derived in the paper, the viscosity of concentrated emulsions increases with the increases in the volume fraction of the dispersed phase and the viscosity ratio (K) of dispersed-phase to continuous-phase. In the limit of K ® ¥, the equations reduce to the well-known formulae for solids-in-liquid suspensions. The proposed equations are evaluated in light of a large body of experimental data for concentrated emulsions, covering a broad range of dispersed-phase to continuous-phase viscosity ratio (3.87´10-4 £ K £ 3.25´105). Rheology of non-Brownian rigid fiber suspensions with adhesive contactsMohend Chaouche Donald L. Koch AbstractAn experimental investigation is undertaken into the shear-thinning behavior of suspensions of non-Brownian rigid fibers in Newtonian fluids. In particular, the influence of the shear stress and the fiber concentration is investigated. The shear stress is adjusted by varying both the shear rate and the solvent viscosity. In the semi-dilute concentration regime where direct mechanical contacts between fibers are rare, the suspension is found to be nearly Newtonian over the stress range investigated. In the concentrated regime, the suspension becomes shear thinning below a certain shear rate. The shear thinning increases with concentration and decreases with solvent viscosity. Although shear-thinning behavior of fiber suspensions has often been reported in the literature, its physical origins are not well understood. Our experiments are interpreted in terms of the formation and breakage of fiber flocs due to the competition between hydrodynamic and colloidal forces. Our interpretation is confirmed by measurements of the adhesive forces between two individual fibers. Ligament creep behaviour can be predicted from stress relaxation by incorporating fibre recruitmentG. M.
Thornton, Post-Doctoral Fellow McCaig
Centre for Joint Injury and Arthritis Research AbstractStructural modelling and morphological crimp analysis were used to investigate if collagen fibre recruitment could account for our previous finding that ligament creep behaviour cannot be predicted using inverse stress relaxation behaviour directly. Ligament creep behaviour was accurately predicted using our simple nonlinear structural model that incorporated collagen fibre creep and collagen fibre recruitment. Collagen fibre creep was modeled using the inverse stress relaxation function and estimated fibre modulus. Collagen fibre recruitment was modeled with a linear variation in crimp over an idealized rectangular ligament cross-section. Concomitantly, significant differences in collagen crimp patterns were observed as a result of creep testing; however; no significant changes were observed as a result of relaxation testing. This morphological evidence supported the model assumptions that fibre recruitment occurs during creep and that stress relaxation behaviour results from the viscoelastic response of an unchanging group of fibres. Not only was the prediction improved compared to the inverse stress relaxation behaviour alone, the model demonstrated that fibre recruitment increased the load-bearing area of the ligament over time and correspondingly stress was redistributed, reducing stress on the fibres initially loaded. These findings may have important implications to both models and experiments on ligament structure and function at low loads. * Corresponding author. E-mail: shrive@ucalgary.ca Magnetorheology and magnetostriction of isolated chains of nonlinearly magnetizable spheresY. M. Shkel
and D. J. Klingenberg AbstractThe magnetic, magnetorheological, and magnetostrictive behavior of nonlinearly magne- tizable particles in a nonmagnetic continuum is considered. The magnetization of isolated spheres in a uniform magnetic field is solved exactly for the case where the field-dependent sphere permeability is given by the Frohlich-Kennelly equation. This solution is incorporated into a point-dipole model for suspensions composed of aligned, isolated chains of spheres. The suspension magnetization and shear storage modulus reflect the nonlinear magnetization, saturating at large field strengths, consistent with some experimental observations and in agreement with previous calculations. Analysis of the magnetostrictive behavior reveals that additional magnetostriction parameters are required compared to materials whose magnetization is linear in the applied field. Yield stress for particle suspensions within a clay dispersionChristophe Ancey and Hélenè Jorrot AbstractThis article focuses on suspensions of coarse particles within a clay dispersion. The behavior of such suspensions is generally dictated by the colloidal fine fraction, notably its yield stress. The dependence of this yield stress on the solid concentration (in coarse particles) is examined. It has been experimentally shown that adding coarse particles usually induced an increasingly marked enhancement of yield stress. However, in some cases, adding a small amount of coarse particles led to a decrease in bulk yield stress. We propose two mechanisms responsible for variations in bulk yield stress. First. at low concentrations. depletion of clay particles may be sufficient to induce an increase in the bulk yield stress. Two values for the depletion layer thickness have been found depending on the coarse particle type. At large concentrations. the substantial increase in bulk yield stress has been ascribed to the development of a coarse particle network within the dispersion. In this case. yielding results from the breakdown of indirect (lubricated) contacts between particles. An experimental and simulation study of dilute polymer solutions in exponential shear flow: Comparison to uniaxial and planar extensional flowsThomas C. B. Kwana), Nathanael J. Woob), and Eric S.G. Shaqfeha,c) a) Department of Chemical Engineering b) Scientific Computing/Computational
Mathematics Program AbstractThe rheological properties of dilute polymer solutions in exponential shear , uniaxial and planar extensional flows are compared using Brownian dynamics simulations of both freely-draining, flexible bead-rod and bead-spring chains. We introduce a novel stress function for exponential shear which uses the extinction angle, c, to take into account the orientation of the chain as it aligns in the flow. Comparing this new stress function during startup and relaxation in exponential shear with t11 - t22 in planar extensional flow and t11 - ( t22 + t33)/2 in uniaxial extensional flow, we find that for both models there is a quantitative agreement among the three different flows over a large range of Wi, strain, and chain length. Furthermore, the distributions of maximum extension show a microstructural equivalence between ensembles of chains in all three flows up to strains of 3 or 4 at all values of Wi simulated. Finally, we show three comparisons between experiment and simulation of the various flows: a) simulations and exponential shear experiments of a polyisobutylene/polybutene Boger fluid, b) simulations and uniaxial extension data from literature, and c) exponential shear and planar extensional data of a low-density polyethylene melt from literature. c) Author to whom correspondence should be addressed. E-mail: eric@chemeng.stanford.edu A striking hydrodynamic phenomenon: Split of a polymer melt in capillary flowM. Fernandez and A. Santamaria* A. Muñoz-Escalona and L. Méndez AbstractWe present experimental results, acquired during capillary flow of very elastic polymer melts, showing a double extrudate at the exit of the die. The initial extrusion conditions give rise to an extrudate with very severe sharkskin instability , but when a certain critical shear stress is exceeded a second extrudate emerges from a scission of the first one. When the process is stabilized both branches become identical, with a sharkskin characterized by a double wavelength than the initial extrudate, but with the ridges out of phase. A tentative simple model, based on stick-slip hydrodynamic effect, is presented. * Author to whom correspondence should be addresed. The role of surface tension on the elastic decohesion of polymeric filamentsHenrik Koblitz Rasmussen Ole Hassager AbstractWe simulate the rapid extension of polymeric filaments between parallel plates with special attention to the role of surface tension in the symmetry breaking azimuthal instability that may occur near the end plates. The instability is viewed as a precursor to the break up into separate fibrils and the eventual elastic decohesion of the filament from the plate. It is demonstrated that high Deborah numbers are needed to initiate the instability and that surface tension provides a wavenumber selection. Moreover, the surface tension has a stabilising effect on the end plate instability. Microscopic theory of convective constraint releaseS. T. Milner T. C. B. McLeish and A. E. Likhtman AbstractWe develop a microscopic description of the contribution to stress relaxation in entangled polymer melts of convective constraint release, which is the release of entanglement constraints due to the effects of convective flow on chains surrounding a given chain. Our theory resolves three of the main shortcomings of the Doi-Edwards model in nonlinear rheology, in that it predicts 1) a monotonically increasing shear stress as a function of shear rate, 2) shear stress independent of molecular weight at sufficiently high shear rates, and 3) only modest anisotropies in the single chain scattering function, in agreement with experiment. In addition, our approach predicts that a stress maximum and resulting shear-banding instability would occur for living micelle solutions, as observed. Visualizing the elimination of sharkskin through fluoropolymer additives: Coating and polymer-polymer slippageK. B. Migler C. Lavallee M. P. Dillon, S. S. Woods C. L. Gettinger AbstractWe developed a capillary rheo-optics technique to visualize how fluoropolymer polymer processing additives (PPA) eliminate a surface distortion called "sharkskin" in the extrudate of linear low density polyethylene (LLDPE). The measurements were carried out in a transparent sapphire tube located at the exit of a twin-screw extruder. Depth resolved optical microscopy was used to measure both the polymer velocity profiles and to image the coating process of the (PPA) onto the capillary wall. In the absence of PPA, no slippage occurs between the capillary wall and the polyethylene; sharkskin was observed at all flow rates. Upon addition of the PPA to the LLDPE, the PPA migrates to the capillary wall where it sticks and induces slippage between itself and the LLDPE, concomitant with the elimination of sharkskin. The interface between the PPA and LLDPE is characterized by long stripes in the flow direction. Large values of the polymer-polymer slippage parameter were found which indicate that the fluoropolymer and LLDPE are fully disentangled at their interface. The PPA acts by dramatically reducing the extensional deformation of the LLDPE at the exit surface. Interrupted shear flow of unentangled polystyrene meltsP .G. Santangelo and C. M. Roland AbstractLow molecular weight polystyrene melts were subjected to shearing flow which was periodically halted. Weak maxima in both the viscosity and normal stress were observed upon startup of the flow, notwithstanding the absence of entanglements. The strain associated with the viscosity maxima was independent of shear rate, and consistent with the overshoot strains for entangled polymers. The stress overshoots disappeared or were of weaker intensity when the flow was resumed after brief interruption. For sufficiently long rest periods between flow, the original behavior was reproduced. The time scale for recovery of the maxima was about the same as that for relaxation of the stress from steady state flow. This differs from the behavior of entangled polymers. Simulation of melt spinning including flow-induced crystallization. Part III. Quantitative comparisons with PET spinline dataAntonios K. Doufasa) and Anthony J. McHughb) AbstractThe mathematical model for melt spinning of Doufas et al. (1999, 2000a,b) coupling polymer microstructure (molecular orientation, chain extension, and crystallinity) with the macroscopic velocity/stress and temperature fields is tested against low- and high-speed spinline experimental data of PET melts. The model includes the combined effects of flow-induced crystallization (FIC), viscoelasticity , filament cooling, air drag, inertia, surface tension and gravity and simulates melt spinning from the spinneret down to the take-up roll device (below the freeze point). As is the case with nylon systems, model fits and predictions are shown to be in very good quantitative agreement with spinline data for the fiber velocity, diameter, and temperature fields at both low- and high-speed conditions, and, with flow birefringence data available for high speeds. Our model captures the necking phenomenon for PET quantitatively and the associated extensional softening which is shown to be related to non-linear viscoelastic effects and not the release of latent heat of crystallization. Although crystallization is quite slow under low-speed spinning conditions, the model captures the occurrence of the freeze point naturally, and is thus a significant improvement over existing melt spinning models that enforce the freeze point at the glass transition temperature. This paper demonstrates robustness of our microstructural FIC model to melt spinning of quite slow crystallizers in the quiescent state, while robustness for faster crystallizers was shown previously [Doufas et al. (2000a,b)]. a) Current address: The Dow Chemical Companyt
M.E. Pruitt Research Centert 1702 Buildingt Midland MI 48674 |
Please e-mail suggestions and comments to albertco@umche.maine.edu. Updated 25 January 2004 |