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Journal of RheologyVolume 49, Issue 2 (Mar-Apr 2005) |
Contents
Solving the Couette inverse problem using a wavelet-vaguelette decompositionChristophe Ancey École Polytechnique Fédérale
de Lausanne AbstractThis paper develops a new approach to computing the shear rate from the torque and rotational-velocity measurements in a Couette rheometer. It is based on wavelet-vaguelette decomposition (WVD) proposed by Donoho [Appl. Comput. Harmonic Anal. 2 (1995) 101-126]. This decomposition consists in expanding the shear rate into a truncated wavelet series, whose coefficients can be determined by computing the inner products of the wavelet functions with dual functions (vaguelette). Compared to other strategies used for recovering the shear rate such as Tikhonov regularization, the WVD method exhibits greater accuracy and faster convergence. Because of the spatial adaptivity of wavelets, it still performs well when the flow curve is irregular (yield stress, sudden behavior change, etc.) and thus no prior knowledge of the shear rate characteristics (e.g., existence of a yield stress, smoothness) is needed. Its efficiency is demonstrated by applying the method to two fluids (a polymeric gel and a granular suspension). Rheological investigation of the melt state elastic and yield properties of a polyamide-12 layered silicate nanocompositeThierry Aubrya), Tolotrahasina Razafinimaro, and Pascal Mederic Laboratoire de Rhéologie,
Université de Bretagne Occidentale AbstractThe dynamic and steady shear flow properties of a Polyamide-12 melt layered silicate nanocomposite were studied as a function of the silicate volume fraction f. In the dilute regime, the results were discussed in terms of intrinsic viscosity. Above a volume fraction threshold fp ~ 1.5%, below a critical strain gc, the storage and loss moduli were shown to exhibit a low-frequency plateau, G’0 and G”0, and the flow curve was shown to exhibit an apparent yield stress ty. The study of G’0, gc and ty as a function of f showed that the energy needed for removing connectivity on a mesoscale did not depend on the silicate loading. These original properties were attributed to the existence, in the quiescent state, of mesoscopic domains composed of correlated silicate layers. Moreover, the steady shear response of all samples at solid volume fractions above fp showed the existence of a critical shear rate ~ 1s-1, separating a behavior governed by the networked domains from a behavior dominated by the polymer matrix. a) Author to whom correspondence should be addressed. E-mail: thierry.aubry@univ-brest.fr Viscosity overshoot in the start-up of uni-axial elongation of LDPE meltsHenrik Koblitz Rasmussena), Jens Kromann Nielsenb), Anders Bachb), and Ole Hassagerb) The Danish Polymer Centre AbstractThe transient uni-axial elongational viscosity of BASF Lupolen 1840D and 3020D melts has been measured on a Filament Stretch Rheometer (FSR) up to Hencky strains of 6 to 7. The elongational viscosity of both melts was measured at 130ºC within a broad range of elongational rates. At high elongation rates, an overshoot or maximum in the transient elongational viscosity followed by a steady viscosity was observed. The steady elongation viscosity was about 40%-50% less than the maximum at high strain rates. The steady elongational viscosity as a function of the elongation rate, e-dot , decreases approximately as e-dot-0.6 in both melts at high strain rates. The transient elongational viscosity, measured at a specific elongation rate at 170ºC on the BASF Lupolen 3020D melt, did not follow the time temperature superposition principle based on linear viscoelasticity during the decrease in the transient elongational viscosity towards the steady state. Study on nonlinear deformation mechanism in epoxy glass using birefringenceHiroshi Kawakamia), Masatoshi Tomita, and Yukuo Nanzai Department of Mechanical and Physical Engineering, AbstractTwo types of samples were prepared from the same epoxy precursor; one was cured with 4,4'-DDM to form a crosslinked molecular structure and the other was polymerized by aniline to form a linear molecular structure. After it was shown that the modified stress-optical rule is valid for the birefringence data of the samples, birefringence change was monitored during uniaxial elongation in the glassy state as well as in the rubbery state. The results were analysed with in the context of modified stress-optical rule to investigate the plastic deformation of epoxy glass. For the linear sample, rubbery stress increased after an yield point, while glassy stress remained almost constant. On the other hand, for the crosslinked sample, rubbery stress increased cooperatively with glassy stress from the beginning of elongation, and such cooperative increase in stresses continued after the strain passing an upper yield point. a) E-mail: hkawakam@mech.eng.osaka-cu.ac.jp Modeling the linear viscoelastic properties of metallocene-catalyzed high density polyethylenes with long-chain branchingSeung Joon Parka) and Ronald G. Larsonb) Department of Chemical Engineering AbstractThe hierarchical model modified by Park et al. (2004) is applied to single-site metallocene-catalyzed high density polyethylene (mHDPE) to predict the effect of long-chain branching (LCB) on the linear viscoelastic properties. In this work we generate the distributions of molecular weight and LCB of mHDPE for the hierarchical model predictions using a Monte Carlo simulation of Costeux et al. (2002), where the simulation parameters are obtained from the average molecular weight and the degree of LCB. The parameters needed in the hierarchical model are determined from experimental data for linear polyethylenes. The model predicts the experimental data well, reveals the effect of LCB on the linear viscoelastic properties of mHDPE, and indicates the possibility of inferring branching levels from measurements of linear viscoelastic properties of mHDPE’s. a) Current address: Corporate Research &
Development, LG Chem, Ltd./ Research Park, 104-1, Moonji-dong, Yuseong-gu,
Daejeon, 305-380, Korea Study of a dissipative particle dynamics based approach for modeling suspensionsNicos S. Martys Materials and Structures Division AbstractIn this paper, a dissipative particle dynamics (DPD) based approach for modeling suspensions is examined. A series of tests are applied comparing simulation results to well established theoretical predictions. The model recovers the dilute limit intrinsic viscosity prediction of Einstein and provides reasonable estimates of the Huggins coefficient for semidilute suspensions. At higher volume fractions, it was necessary to explicitly include lubrication forces into the algorithm as the usual DPD interactions are too weak to prevent overlaps of the rigid bodies and account for other related effects due to lubrication forces. Results were then compared with previous studies of dense hard sphere suspensions using the Stokesian dynamics method and experimental data. Comparison of relative viscosity values determined from strain controlled shearing versus stress controlled shearing simulations are also given. The flow of spheroidal objects is studied. The rotation of a single spheroid under shear is consistent with the predictions of Jeffery. Simulations of sheared spheroids at higher volume fractions produce an apparent nematic phase. An example is given of application of DPD to model flow in another geometry, gravitational driven flow between parallel cylinders, which is of practical interest. Constriction flows of monodisperse linear entangled polymers: Multiscale modelling and flow visualisationM. W. Collisa), A. K. Lelea), M.
R. Mackleya), a) Department of Chemical Engineering,
University of Cambridge b) IRC in Polymer Science and Technology,
Department of Physics and Astronomy c) Department of Chemistry, University of Durham, Durham, UK d) Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK e) IRC in Polymer Science and Technology,
Department of Applied Mathematics f) Division of Chemical Engineering, University of Queensland, Queensland 4072, Australia AbstractWe explore both the rheology and complex flow behaviour of monodisperse polymer melts. Adequate quantities of monodisperse polymer were synthesised in order that both the materials rheology and micro processing behaviour could be established. In parallel, we employ a molecular theory for the polymer rheology that is suitable for comparison with experimental rheometric data and numerical simulation for micro processing flows. The model is capable of matching both shear and extensional data with minimal parameter fitting. Experimental data for the processing behaviour of monodisperse polymer is presented for the first time as flow birefringence and pressure difference data obtained using a Multipass Rheometer with an 11:1 constriction entry and exit flow. Matching of experimental processing data was obtained using the constitutive equation with the Lagrangian numerical solver, flowSolve. The results show the direct coupling between molecular constitutive response and macroscopic processing behaviour, and differentiate flow effects that arise separately from orientation and stretch. The effect of shear flow on the percolation concentration of fibrillar protein assembliesCecile Veerman, Leonard M. C. Sagis, Paul Venema, and Erik van der Lindena) Food Physics Group, Department of Agrotechnology and Food
Sciences AbstractThe objective of this study was to investigate the effect of shear flow on the percolation concentration (cp) for solutions of fibrillar protein assemblies. Theoretical calculations were performed to obtain cp versus Peclet number. They were based on a random contact model for rodlike particles, making use of a shear dependent excluded volume per fibril. We found cp to increase with increasing Peclet number. Results of flow birefringence measurements were used to obtain the rotational diffusion coefficient at cp, which enables to transform the theoretically obtained cp(Pe) into a prediction for cp versus shear rate. This prediction was used to fit viscosity measurements as a function of shear rate, near the percolation threshold. A satisfactory fit was found, indicating that the percolation threshold, cp, as function of shear rate can be predicted by combining theory and optical measurements. a) Corresponding author. Fax: + 31 317 483669; E-mail address: Erik.vanderLinden@wur.nl Shear thickening in filled Boger fluidsRossella Scirocco, Jan Vermant, and Jan Mewis Department of Chemical Engineering, K.U.Leuven AbstractShear thickening is studied in suspensions consisting of micrometer sized polystyrene spheres dispersed in Boger fluids. In comparison with available data for suspensions in Newtonian media, shear thickening occurs at much lower volume fractions. Suspensions in normal, non-linear viscoelastic media typically do not display shear thickening at all. In suspensions in Boger fluids, the onset of shear thickening is shown to be governed by a critical shear stress. The effect of particle size is consistent with a scaling with the particles radius to the third power, as for Brownian hard spheres. The critical stresses are, however, orders of magnitude higher than for similar suspensions in low viscosity, Newtonian media. The first normal stress coefficient is also affected by the presence of particles, but it displays shear thickening as well as shear thinning. No specific microstructural features have been observed during flow, neither alignment nor hydroclustering seem to occur. Shear thickening in Boger fluids is possibly caused by the enhanced hydrodynamic interparticle interactions, related to the specific viscoelastic properties of the suspending medium, i.e. the absence of shear thinning and pronounced extensional hardening. This seems to be confirmed by recent simulation results. The Influence of weak attractive forces on the microstructure and rheology of colloidal dispersionsLakshmi-narasimhan Krishnamurthy and Norman J. Wagnera) Center for Molecular and Engineering Thermodynamics AbstractRheology is demonstrated to be a sensitive and quantitative probe of weak attractive forces acting in concentrated, stable colloidal dispersions through comparison of rheology and small angle neutron scattering measurements on a model dispersion with added polyampholyte. Polyampholyte-stabilized dispersions are found to exhibit weak attractions in the form of depletion forces arising from free polyampholyte in the suspending medium. The depletion potential is modeled with the Asakura-Oosawa potential and mapped onto the sticky hard sphere potential to facilitate modeling. Independent validation of the interparticle potential is provided by quantitative prediction of the measured small angle neutron scattering spectra. A new semi-empirical, predictive model for the low shear viscosity of stable dispersions is proposed and validated against measurements on model dispersions over a range of compositions. This rheological constitutive relation provides an improved prediction of the low shear viscosity of stable mixtures of adsorbing polyampholyte and colloidal particles, and is anticipated to have broad applicability in modeling and predicting colloidal suspension viscosity. a) Corresponding author: wagner@che.udel.edu Investigation of shear-banding structure in wormlike micellar solution by pointwise flow-induced birefringence measurementsJ. Y. Leea), G. G. Fullerb), N. Hudsonc), and X-F.Yuana,d,e) a) Department of Mechanical Engineering, King’s
College London b) Department of Chemical Engineering, Stanford
University, Stanford c) Department of Pure and Applied Chemistry,
University of Strathclyde d) School of Chemical Engineering and
Analytical Science, the University of Manchester AbstractShear-banding occurs in many wormlike surfactant solutions subject to strong shear flow. We study this interesting phenomenon by pointwise flow-induced birefringence (FIB) measurements in a well-known aqueous surfactant system, cetylpyridinium chloride (CPyCl)/sodium salicylate (NaSal). The dynamic nature of the local rheo-optical properties such as birefringence and extinction angle across the gap of a Couette cell were investigated over a range of shear rates, in particular the stress plateau region where shear stress is nearly independent of shear rate. This is probably the first application of pointwise FIB for investigation of shear-banding. The variation of optical signals along the radial direction of the Couette cell indicates that the fluid becomes inhomogeneous and forms banded structures, with either two bands or three bands across the gap, depending on the applied nominal shear rate. e) The present contact address for the corresponding author: Xue-Feng Yuan, School of Chemical Engineering and Analytical Science, the University of Manchester, P.O.Box 88, Sackville Street, Manchester M60 1QD, UK. Email: xue-feng.yuan@manchester.ac.uk Cage melting and viscosity reduction in dense equilibrium suspensionsV. Gopalakrishnan, S. A. Shah, and C. F. Zukoskia) Department of Chemical and Biomolecular Engineering AbstractZero shear viscosities of dense colloidal suspensions are strongly correlated with the dynamics of particles in cages produced by nearest neighbors. At a given volume fraction, repulsive or attractive interparticle forces are expected to enhance cages, slow dynamics and increase zero shear rate viscosities. However recent studies have shown that hard sphere glasses melt when depletion attractions are introduced and this is correlated with the break up of local cages and enhancement of density fluctuation relaxation rates. Glass formation in hard sphere suspensions is attributed to the loss of free volume and the entropic localization of particles. Melting is associated with the ability of depletion attractions to increase the local free volume due to a decrease in nearest neighbor cages. Here we report evidence for the reduction in the zero shear viscosity of dense hard and near-hard sphere suspensions, as the strength of depletion attractions increases, at volume fractions well below the experimental glass and gel transitions. Increasing the strength of depletion attractions is found to drive suspension viscosities through a minimum. The magnitude of the drop in zero shear viscosity grows in magnitude as the volume fraction is increased. These results suggest that the localization and cage effects that characterize glass formation originate at volume fractions below those associated with glass formation. Experimental results are compared with models that incorporate the effects of microstructure on suspension dynamics. a) Author to whom correspondence should be addressed. Email: czukoski@uiuc.edu |
Please e-mail suggestions and comments to albertco@umche.maine.edu. Updated 06 March 2005 |