Volume 42, Issue 1, January 1998
Index of content:
42(1998); http://dx.doi.org/10.1122/1.550883View Description Hide Description
Human blood cells build a percolating physical gel all over the sample when at rest. This gel is progressively broken when it is continuously sheared in the bulk. It can slip at the wall, a phenomenon that depends on the roughness of the surface. Hence, smooth and rough walls were used to investigate the rheometrical shear properties of blood. A Couette-type rheometer with cylindrical walls allowed the shear rate to be varied in the range and the hematocrit in the range 0.53–0.95. Calibration was performed with standard silicon oil. The stress measured at low shear rates with rough walls seemed, indeed, to tend to a constant yield stress value. The value of the shear stress at the shear rate of was taken as a realistic approximation of the yield stress of blood. This yield stress was measured for different values of cell concentration. Data were fitted and showed to be proportional to the cube of the concentration over the range of hematocrit studied. However, variability between the donors was observed. This may have relevance in the case of complete cessation of flowin vivo. The higher the yield stress of the static blood, the higher the driving pressure needed to get the flow started again.
42(1998); http://dx.doi.org/10.1122/1.550886View Description Hide Description
The electrical conductivity of suspensions composed of electrically conducting polyaniline in nonconducting liquid (1,2,4-trichlorobenzene) was measured as a function of electric-field strength in dependence on the shear rate in Couette flow. The electrical properties are discussed in terms of the organization of conducting particles in electric fields and related to the Mason number, which is used to describe the electrorheological behavior of suspensions in the literature.
Rheology of bidisperse aqueous silica suspensions: A new scaling method for the bidisperse viscosity42(1998); http://dx.doi.org/10.1122/1.550935View Description Hide Description
The shear viscosity of electrostatically stabilized bidisperse aqueous silicasuspensions has been studied as a function of shear rate, particle size, total volume fraction of the particles, and volume ratio of small to large particles. The higher limiting relative viscosity (at ) of monodisperse suspensions of 0.6, 0.85, and 1.5 μm silica particles was found to be particle size dependent indicating deviation from hard sphere behavior. It is shown that the flow properties of bidisperse suspensions of colloidal size particles is not only affected by the volume (weight) ratio of the two sizes, but the absolute size of the particles and the particle size ratio also play an important role in determining the viscosity of the suspension. Binary mixtures of particles of smaller size ratios show higher viscosities than the suspensions containing larger size ratio particles. This indicates that the viscosity response of suspensions of colloidal size particles is controlled by both hydrodynamic interactions and colloidal forces. The Farris [Trans. Soc. Rheol. 12, 281 (1968)] model was found to be inadequate when applied to bidisperse suspension of colloidal size particles. An empirical method is suggested to estimate the viscosity of a bimodal suspension at a given volume fraction of the particles, from the viscosity of monodisperse systems and the viscosity of a bimodal suspension (at a fixed total volume fraction) as a function of the volume ratio of the two particles.
Relationships between rheology and morphology for immiscible molten blends of polypropylene and ethylene copolymers under shear flow42(1998); http://dx.doi.org/10.1122/1.550943View Description Hide Description
The linear and nonlinear viscoelastic properties of immiscible polymer blends polypropylene/ethylenevinylacetate–ethylenemethylacrylate [PP/(EVA–EMA)] have been investigated. The transient shear flow experiments reflect the structural changes of the blends during the flow. Overshoots in stress growth experiments are observed when the dispersed phase is deformable. In this case, a good description of these transient rheological data is obtained using modified versions of the Lee and Park [J. Rheol. 38, 1405 (1994)] and the Grmela and Ait-Kadi [J. Non-Newtonian Fluid Mech. 55, 191 (1994)] models. Predictions of the morphological evolution of the blend under transient shear flows were calculated from the modified models which are shown to describe the breakup and coalescence phenomena under moderately large deformationshear flow. When the dispersed phase is undeformable, these models, which are either based on the original Doi and Ohta [J. Chem. Phys. 95, 1242 (1991)] theory or derived to retrieve an extension of the Doi–Ohta theory [Lee and Park, J. Rheol. 38, 1405 (1994)], predict phase separation in contrast to the experimental evidence of stable emulsion. The Palierne emulsionmodel [Rheol. Acta 29, 201 (1990)] is used to characterize the linear viscoelastic properties of the blend before and after the transient shear experiments.
42(1998); http://dx.doi.org/10.1122/1.550890View Description Hide Description
We conduct a comprehensive experimental study of melt/wall interfacial slip behavior of various polymer melts in capillary flow.Polymers under study include two polystyrenes of different molecular weights ( and respectively), two low-density polyethylenes (LDPE), poly(ethylene vinyl acetate) (EVA), and polypropylene (PP). The experimental results reveal an important finding that has greatly extended our previous knowledge of the roles of melt/wall interfacial interactions and molecular entanglements in dictating capillary melt flow behavior. None of the six polymers exhibits an interfacial stick–slip transition or shows any sign of wall slip in bare aluminum dies. Yet, all are found to display a sizable wall slip when the strength of polymer/surface adsorption is reduced by lowering the die wall surface energy with a fluoropolymer coating. The degree of wall slip as characterized by the Navier–de Gennes extrapolation length is demonstrated to be explicitly proportional to the melt viscosity, such that the magnitude of wall slip in the coated die decreases with increasing shear thinning. Diminishing of a measurable wall slip at high stresses explains the well-known absence of spurt, flow oscillation, and sharkskin phenomena in capillary flow of commonly encountered polymers such as PS, LDPE, PP.
42(1998); http://dx.doi.org/10.1122/1.550933View Description Hide Description
Polymer melts with long-chain side branches and more than one junction point, such as commercial low density polyethylene (LDPE), have extensional rheology characterized by extreme strain hardening, while the shear rheology is very shear thinning, much like that of unbranched polymers. Working with the tube model for entangled polymer melts, we propose a molecular constitutive equation for an idealized polymer architecture, which, like LDPE, has multiple branch points per molecule. The idealized molecule, called a “pom-pom,” has a single backbone with multiple branches emerging from each end. Because these branches are entangled with the surrounding molecules, the backbone can readily be stretched in an extensional flow, producing strain hardening. In start-up of shear, however, the backbone stretches only temporarily, and eventually collapses as the molecule is aligned, producing strain softening. Here we develop a differential/integral constitutive equation for this architecture, and show that it predicts rheology in both shear and extension that is qualitatively like that of LDPE, much more so than is possible with, for example, the K-BKZ integral constitutive equation.
42(1998); http://dx.doi.org/10.1122/1.550884View Description Hide Description
In contrast to recent publications suggesting that particle cluster formation alone can play an important role in the shear thickeningflow behavior of concentrated colloidalsuspensions, we believe that there is little if any substantive evidence to prove it. To support this view, we use data from various studies, including data from studies concluding that layered flow is not involved. One reason for the confusion seems to center around the inability of various light-scattering and neutron-scattering techniques to show particle layering before shear thickening when the layers are not well defined. In this regard, one should understand that layered flow can occur without rigorous ordering of particles within the layers, and as the flowing suspension approaches the point of instability, the hydrodynamic forces driving for the instability will jostle the particles within the layers sufficiently to make it even harder to see the layering and any ordering, if it exists, within the layers. Having these views, we argue that the process described by Hoffman (1972, 1974) for shear thickening is still applicable with refinements. The major refinement is the idea that, after the hydrodynamic forces cause the instability which breaks up the layered flow, particle jamming probably involves cluster formation both with and without particle contact. Particle roughness and angularity will facilitate the contact. Finally, we agree with various authors who argue that the best chance of finding shear thickening in concentrated colloidalsuspensions without layer formation lies in Brownian hard-sphere suspensions, but the evidence given for it so far is not definitive.
The experimental observation and modeling of an “Ovaici” necklace and stick–spurt instability arising during the cold extrusion of chocolate42(1998); http://dx.doi.org/10.1122/1.550885View Description Hide Description
This paper reports experimental results and modeling relating to the ambient temperature, cold extrusion of chocolate. In particular, extrusion conditions are given where two different modes of instability are described and modeled. In the first case, at low flow rates, a stick–spurt instability is observed, which leads to large amplitude fluctuations in the extrusion pressure and extrudate velocity. This instability is modeled as a modified relaxation oscillation process in which the combined nonlinear plastic flow behavior of the chocolate and the compressibility of the material in the reservoir control the instability mechanism. The second mode of instability, observed for a specific thermal history, concerns the unusual extrusion of a “necklace” structure consisting of beads regularly spaced on a narrow extrudate core. This instability is explained in terms of the steady core–annular coextrusion of two Bingham-plastic chocolate components having different yield stresses.
The effects of adsorbed layers and solution polymer on the viscosity of dispersions containing associative polymers42(1998); http://dx.doi.org/10.1122/1.550934View Description Hide Description
Associative polymers adsorb on polymer lattices via hydrophobic interactions to generate dense layers that increase the hydrodynamic size of the particles but also reduce the polymer concentration in solution, thereby decreasing the continuum viscosity. The telechelic polymers employed in this work adsorb at low concentrations with both hydrophobes on the surface but at the concentrations of general interest have one hydrophobe free to couple with micelles in solution and adsorbed layers on other particles. Detailed measurements of adsorption and the steady shear viscosity of the dispersions provide consistent quantitative support for this scenario. The adsorbed amounts and layer thicknesses of hydrophobically modified poly(ethylene oxides) on poly(methylmethacrylate) particles suggest a dense layer of moderately stretched chains in very dilute dispersions. At higher concentrations, adsorption increases the intrinsic viscosity [η], by an amount consistent with the measured layer thickness, and the Huggins coefficient The former indicates strong coupling with the associated solution, equivalent to a no slip boundary condition. The value of exceeds that for hard spheres implying direct coupling between adsorbed layers of interacting particles. Thus the observed enhancement of the dispersion viscosity arises from the high viscosity of the polymer solution, the increased hydrodynamic volume of the particles, and direct interactions between adsorbed layers.
A closure approximation for liquid-crystalline polymer models based on parametric density estimation42(1998); http://dx.doi.org/10.1122/1.550887View Description Hide Description
A new rational closure approximation for the Doi model of liquid-crystallinepolymers(LCPs) is presented and compared with unapproximated results. The closure is based upon proposing a parametric form for the orientation distribution function, the Bingham distribution, which is appropriate for the spherical geometry of the configuration space. The closure enjoys the advantage over ad hoc moment closures of always yielding physical answers; moreover, it is designed to give the exact solution behavior in a number of limits. Its behavior for steady two-dimensional flows reveals a range of applicability that extends significantly beyond the intended limits. The main deficiency, a failure to predict a flow-aligning transition in simple shear, is shown to be due to the inability to predict skewing of the orientation distribution. Comparisons are also made for a center-gated disk geometry, addressing the performance in three-dimensional unsteady flow typically seen in processing of LCPs. The Bingham closure shows excellent quantitative agreement with unapproximated solutions, only underestimating the time scale of some transient behavior. The approximation is compared with other closures in the same vein, and suggestions are made for improvements.
42(1998); http://dx.doi.org/10.1122/1.550888View Description Hide Description
We present preliminary experimental evidence for the existence of Frank elasticity-induced particle–particle interactions in concentrated suspensions in a nematic carrier. Our model system consists of -(4-methoxybenzylidine)-4-butylaniline (MBBA; a low molecular weight, thermotropic liquid crystal) compounded with 31.5 vol % zeolite particles. Yield stress and oscillatory measurements on the nematic and isotropic samples (i.e., suspensions with nematic and isotropic carrier), display dramatically different signatures. The yield stress of the nematic system is considerably lower than that of the isotropic material, showing nearly a jump at the nematic to isotropic transition. Under small amplitude oscillations the nematic material exhibits viscoelastic response and memory effects, whereas the isotropic suspension behaves as expected for a Newtonian carrier. Theoretical considerations lend credence to the hypothesis of Frank elasticity-mediated particle–particle interactions.
42(1998); http://dx.doi.org/10.1122/1.550889View Description Hide Description
Dilute-solution data on amplitude ratio and phase difference of stress and strain in sinusoidal shear testing are normally converted first to dynamic viscosity and to where is mass concentration of solute, and then extrapolated as to obtain the intrinsic values and The major source of error is usually measurement of phase angle, while amplitude ratio is more accurately determined. Here, it is shown that can be obtained from amplitude ratio alone, and also from phase angle alone. Improved accuracy in obtaining is thereby possible with this new method of data analysis.
Letter to the Editor: Comment on “Full tensor optical rheometry of polymer fluids” [J. Rheol. 41, 343–364 (1997)]42(1998); http://dx.doi.org/10.1122/1.550882View Description Hide Description