Index of content:
Volume 49, Issue 2, March 2005
49(2005); http://dx.doi.org/10.1122/1.1859793View Description Hide Description
The objective of this study was to investigate the effect of shear flow on the percolation concentration for solutions of fibrillar protein assemblies. Theoretical calculations were performed to obtain 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 to increase with increasing Peclet number. Results of flowbirefringencemeasurements were used to obtain the rotational diffusion coefficient at , which enables one to transform the theoretically obtained into a prediction for 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, , as function of shear rate can be predicted by combining theory and optical measurements.
49(2005); http://dx.doi.org/10.1122/1.1849188View Description Hide Description
The transient uniaxial elongational viscosity of BASF Lupolen 1840D and 3020D melts has been measured on a filament stretch rheometer up to Hencky strains of 6–7. The elongational viscosity of both melts was measured at 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, , decreases approximately as in both melts at high strain rates. The transient elongational viscosity,measured at a specific elongation rate at 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.
49(2005); http://dx.doi.org/10.1122/1.1849183View Description Hide Description
Zero-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 geltransitions. Increasing the strength of depletion attractions is found to drive suspensionviscosities 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.
49(2005); http://dx.doi.org/10.1122/1.1849187View Description Hide Description
In this paper, a dissipative particle dynamics (DPD) based approach for modeling suspensions is examined. A series of tests is 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 the application of DPD to model flow in another geometry, gravitational driven flow between parallel cylinders, which is of practical interest.
Rheological investigation of the melt state elastic and yield properties of a polyamide-12 layered silicate nanocomposite49(2005); http://dx.doi.org/10.1122/1.1859791View Description Hide Description
The dynamic and steady shear flow properties of a polyamide-12 melt layered silicate nanocomposite were studied as a function of the silicate volume fraction . In the dilute regime, the results were discussed in terms of intrinsic viscosity. Above a volume fraction threshold , and below a critical strain , the storage and loss moduli were shown to exhibit a low-frequency plateau, and , and the flow curve was shown to exhibit an apparent yield stress . The study of , and as a function of 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 showed the existence of a critical shear rate, separating a behavior governed by the networked domains from a behavior dominated by the polymer matrix.
49(2005); http://dx.doi.org/10.1122/1.1849181View Description Hide Description
This 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 [Donoho, D., Appl. Comput. Harmon. Anal.2, 101–126 (1995)]. 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 polymericgel and a granular suspension).
49(2005); http://dx.doi.org/10.1122/1.1853381View Description Hide Description
Two types of samples were prepared from the same epoxy precursor; one was cured with 4,-DDM to form a cross-linked 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 analyzed within the context of modified stress–optical rule to investigate the plastic deformation of epoxy glass. For the linear sample, rubbery stress increased after a yield point, while glassy stress remained almost constant. On the other hand, for the cross-linked 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.
49(2005); http://dx.doi.org/10.1122/1.1859792View Description Hide Description
Rheology is demonstrated to be a sensitive and quantitative probe of weak attractive forces acting in concentrated stable colloidaldispersions through comparison of rheology and small-angle neutron scatteringmeasurements 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 semiempirical 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.
Constriction flows of monodisperse linear entangled polymers: Multiscale modeling and flow visualization49(2005); http://dx.doi.org/10.1122/1.1849180View Description Hide Description
We explore both the rheology and complex flow behavior of monodisperse polymer melts. Adequate quantities of monodisperse polymer were synthesized in order that both the materials rheology and microprocessing behavior 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 microprocessing flows. The model is capable of matching both shearand extensional data with minimal parameter fitting. Experimental data for the processing behavior of monodisperse polymers are 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 behavior, and differentiate flow effects that arise separately from orientation and stretch.
Modeling the linear viscoelastic properties of metallocene-catalyzed high density polyethylenes with long-chain branching49(2005); http://dx.doi.org/10.1122/1.1853382View Description Hide Description
The hierarchical model modified by Park et al. (2005) is applied to single-site metallocene-catalyzed high density polyethylene (mHDPE) to predict the effect of long-chain branching (LCB) on the linear viscoelasticproperties. 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 viscoelasticproperties of mHDPE, and indicates the possibility of inferring branching levels from measurements of linear viscoelasticproperties of mHDPEs.
Investigation of shear-banding structure in wormlike micellar solution by point-wise flow-induced birefringence measurements49(2005); http://dx.doi.org/10.1122/1.1849179View Description Hide Description
Shear banding occurs in many wormlike surfactantsolutions subject to strong shear flow. We study this interesting phenomenon by pointwise flow-induced birefrigence (FIB) measurements in a well-known aqueous surfactantsystem, cetylpyridinium chloride/sodium salicylate. 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 point-wise FIB for the 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 bended structures, with either two bands or three bands across the gap, depending on the applied nominal shear rate.
49(2005); http://dx.doi.org/10.1122/1.1849185View Description Hide Description
Shear 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 nonlinear 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.