Volume 54, Issue 1, January 2010
Index of content:
Comparisons of modified effective medium theory with experimental data on shear thinning of concentrated latex dispersions54(2010); http://dx.doi.org/10.1122/1.3263700View Description Hide Description
Measuredviscosity vs shear rate relationships were analyzed for a wide variety of carboxylated latexes reported in the literature using a modified effective-medium theory proposed by one of the authors. The theory contains three main parameters, , , and , which are the maximum volume fraction, a particle interaction parameter, closely related to the secondary electroviscous effect, and the thickness of the stabilizing surface layer, respectively. It is assumed that sheared dispersions always approach close packing for high volume fractions, i.e., (for monodisperse suspensions). In general, increases with the magnitude of the secondary electroviscous effect and its value varies typically between 2.46 and 4.0 for low Péclet numbers, Pe, and approaches 1.7 for very high Pe. For dispersions of highly charged particles at low electrolyte concentrations, the electroviscous effects become especially significant and experiments show that the viscosity can increase by more than three orders of magnitude when the electrolyte concentration is reduced from to , an effect predicted by the modified effective medium theory. The viscosity of carboxylated latex is also influenced by a “hairy” surface layer, the thickness, , of which appears to be closely related to the amount and type of functional monomers, and the degree of dissociation of carboxylic acid groups on the latex surface. The theory also explains the shear thinning behavior of blends of latexes with different size.
Scaling relations in nonlinear viscoelastic behavior of aqueous PEO solutions under large amplitude oscillatory shear flow54(2010); http://dx.doi.org/10.1122/1.3258278View Description Hide Description
We have found empirical scaling relations in nonlinear viscoelasticity of poly(ethylene oxide) (PEO) solutions under large amplitude oscillatory shear flow. The scaling relations superpose dimensionless nonlinear viscoelastic functions, such as the normalized amplitudes of elastic and viscous stresses and normalized Fourier intensities, measured at different strain amplitudes and frequencies on a single curve irrespective of the molecular weight and the concentration of the polymer solutions. The scaling relations reveal that the nonlinear viscoelastic functions are functions of dimensionless variable , where is the phase lag of linear viscoelasticity. The validity of our superposition was checked for PEO aqueous solutions under the conditions that concentration:; molecular weight: ; . We suggest some material parameters, which are expected to indicate chain architecture as well as to measure the strength of nonlinearity because the parameters are independent of the test conditions and compositions of the polymer solutions.
54(2010); http://dx.doi.org/10.1122/1.3258076View Description Hide Description
We present a new method to measure the viscosity of a dense model suspension using an inclined plane rheometer. The suspension is made of mono-disperse, spherical, non-Brownian polystyrene beads immersed in a density matched silicon oil. We show that with this simple set-up, the viscosity can be directly measured up to volume fractions of and that particle migration can be neglected. The results are in excellent agreement with local viscosity measurements obtained by magnetic resonance imaging techniques by Ovarlez et al. [J. Rheol.50(3), 259–292 (2006)]. In the high density regime, we show that the viscosity is within the tested range of parameters, independent of the shear rate and the confinement pressure. Finally, we discuss deviations from the viscous behavior of the suspensions.
54(2010); http://dx.doi.org/10.1122/1.3248001View Description Hide Description
Many interesting industrial materials are highly viscous or paste-like, i.e., soft solids. Their complexity, proceeding from heterogeneous structures, often reveals interesting rheological properties. Their processing requires the determination of rheological parameters such as viscosity, modulus, and yield stress value. We compare three methods to measure the yield stress of one particular soft solid system, i.e., concentrated surfactant systems, models for bar soap. One method is based on orifice die extrusion and uses the Benbow–Bridgwater equation. Two methods used a rotational rheometer: in one, dynamic (small strain sinusoidal oscillation) experiments were performed as a function of increasing strain amplitude with serrated parallel plate geometry. The maximum in the elastic stress curve was used to estimate the yield stress. The other method using the rotational rheometer, which we call strand shearing, involves the use of a new fixture designed to grip these samples that were too stiff for serrated plates but too soft for traditional solids fixtures. In this method, the maximum of a plot of stress versus time at a constant shear rate is taken as the yield stress. The advantages and limitations of these techniques are discussed and applied to our particular soap model system.
54(2010); http://dx.doi.org/10.1122/1.3257724View Description Hide Description
The planar extensional flow resistance of a foaming-grade polystyrene, with and without a blowing agent, was characterized using two dies, each one consisting of a high-aspect-ratio straight rectangular channel followed by a hyperbolic convergent rectangular channel. With the hyperbolic geometry, the fluid near the centerline of the convergent channel was subjected to a constant rate of extension. The shear viscosity was found from the pressure drop along the straight channel, and the planar extensional viscosity was determined from the pressure drop in the convergent channel, taking into account the pressure drop due to shearing. Values of the planar extensional viscosity are compared with values of the uniaxial extensional viscositymeasured with a shear-free fixture. Tests with a polystyrene and with a solution of 5% carbon dioxide in the polystyrene revealed that the gas caused significant reductions in both the shear and extensional viscosities.
Analysis of the linear viscoelasticity of polyelectrolytes by magnetic microrheometry—Pulsed creep experiments and the one particle response54(2010); http://dx.doi.org/10.1122/1.3266946View Description Hide Description
We report experimental measurements on polyacrylamide (flexible polyelectrolytes), actin (semi-flexible polyelectrolytes), and self-assembled peptide (gelled semi-flexible polyelectrolytes) solutions. The measurements were obtained using a two-pole piece magnetic microrheometer based on an upright Olympus microscope with an oil immersion lens. Pulsed creep experiments produced high quality data over a wide time range with good agreement between passive particle tracking and magnetic microrheology results. This implies a commonality of the one probe particle response to its viscoelastic environment in both passive and active microrheology experiments.
54(2010); http://dx.doi.org/10.1122/1.3270524View Description Hide Description
Suspensions of multiarm star polymers are studied as models for soft colloidalglasses. Using an established pre-shearing protocol which ensures a reproducible initial state (the “rejuvenation” of the system), we report here the time evolution of the stress upon startup of simple shear flow for a range of shear rates. We show the existence of critical shear rates, which are functions of the concentration, . When the suspensions are sheared at rates below , the stress rises to a common value which is also a function of the concentration. The system thus develops a yield stress. This behavior manifests itself as an evolution from a monotonic slightly shear-thinning flow curve to a flow curve dominated by a stress plateau. We relate this bulk evolution to spatially resolved velocity profiles. Hence, yield stress is linked to shear banding in this class of soft colloids.
54(2010); http://dx.doi.org/10.1122/1.3246803View Description Hide Description
The effect of the mutual diffusion of two polymeric phases on the interaction and coalescence of two nearby drops in quiescent conditions is investigated for two partially miscible systems, differing in the miscibility of the components. Transient interfacial tension measurements show that the polybutene (PB)/polydimethylsiloxane (PDMS) system is highly diffusive in terms of diffusing low-molecular weight species, while the polybutadiene (PBD)/PDMS system is less miscible.Drops of the highly diffusive PB/PDMS system at distances closer than their equivalent radius attract each other and coalesce with a rate that, in the last stage of the coalescence process, is the same for all drop combinations. For the slightly diffusive PBD/PDMS system, no coalescence occurs, and, in contrast, repulsion between the drops is observed. These phenomena are qualitatively explained in terms of the overlap of diffuse layers formed at the dropsurfaces of two, close enough drops, yielding concentration gradients that cause gradients in the interfacial tension. These gradients yield Marangoni stresses that induce flow leading either to attraction or repulsion. To determine whether Marangoni stresses are strong enough to displace a drop in quiescent conditions, single drops of PB and PBD are placed in a PDMS matrix in the vicinity of a wall. A lateral drop motion toward the wall is observed for the highly diffusive PB/PDMS system only, while PBD drops do not move. The diffuse-interface model is considered as a good candidate to capture these phenomena described since it couples the mutual diffusion of the low-molecular weight component with both drop and matrix, while including hydrodynamic forces. The presented numerical simulations indeed show a diffusion-induced macroscopic motion that qualitatively reproduces the experimental phenomena observed and support our interpretations.