Volume 51, Issue 3, May 2007
Index of content:
51(2007); http://dx.doi.org/10.1122/1.2714640View Description Hide Description
We report the transformation from gels to viscous fluids within a time period of in mixtures of precipitated silica in silicone oil. Scaling behavior in elastic and viscousmoduli versus frequency, similar to that reported by Trappe and Weitz [Phys. Rev. Lett.85, 449–452 (2000)], has been observed for suspensions of different ages, silica volume fractions, and silicone oil molecular weights; this shows self-similarity in the stress-bearing silica network in these samples. The elastic plateau modulus, extracted from the scaling of the moduli for each suspension, decays exponentially with time, but the initial plateau modulus depends only on the silica volume fraction and is independent of the properties of the base fluids. The aging time constant is larger if thinner silicone oil or a higher silica volume fraction is used. The aging rate is slower for silica in methyl-terminated silicone oil than that in hydroxyl-terminated silicone oil by about an order of magnitude. We attribute the aging effects to the adsorption of poly(dimethylsiloxane) on silica and use a model based on the “poisons” model by Cohen-Addad and de Gennes [C. R. Ocad. Sci. Paris, Série II319, 25–30 (1994)] and on the simulation work by Tsige and co-workers [J. Chem. Phys.118, 5132–5142 (2003)] to explain our observations. No aging effect has been observed for silica-mineral oil mixtures.
51(2007); http://dx.doi.org/10.1122/1.2711962View Description Hide Description
Two-step reversing flows are severe tests of constitutive equations for polymer melts and have attracted interest for testing the response of branched polymers. Here we report on single-step and three types (types B, C, and D) of reversing torsional flows on branched and linear polyethylenes (PEs). The responses were analyzed using Kaye–Bernstein–Kearsley–Zapas (K-BKZ) theory. Comparisons between data and theory in type B and C flows for the two branched PEs (LD 103 and LD 146) and a linear HDPE are presented. For the LD 146 material and to strains higher than previously examined, the results agree with prior studies, i.e., the K-BKZ theory provides a good description of the response of branched polymers in reversing flows while the opposite is the case for the linear polymers. Surprisingly, it is found that the other branched polymer (LD 103) behaves more like a linear polymer and not a branched polymer in that it does not follow the K-BKZ theory. In the type D flow, one which was not previously investigated, the K-BKZ theory describes the normal force (NF) responses for branched LD 146 but not for the linear PE. Also, isochronal derivatives of the strain potential function ( and ) were calculated from torque () and NF responses in single-step tests. The damping function fits to a modified sigmoidal formula and is close to the BKZ limit as reported by Wagner (2004a) for branched polystyrenes. The normal stress difference ratio and damping function were extracted from and . Finally, the Cox–Merz rule was examined for one of the branched PEs.
51(2007); http://dx.doi.org/10.1122/1.2714641View Description Hide Description
We investigate drop breakup in dilute Newtonian emulsions in simple shear flow using high-speed video microscopy over a wide range of viscosity ratio , focusing on high capillary number ( up to , is the critical capillary number). The final drop size distribution of emulsions is found to be intimately linked to the drop breakup mechanism, which depends on and . Drop breakup is caused by end pinching at . For , breakup dynamics are strongly controlled by . For , capillary instability is the dominant drop breakup mechanism, and thread radius and wavelength at breakup are independent of the initial drop sizes. Fairly monodisperse emulsions are obtained, and the average drop size is inversely proportional to the shear rate. For , long wavelength capillary instability generates large satellite drops, resulting in emulsions with bimodal distribution. For , a new drop re-breaking mechanism is observed, producing polydisperse emulsions. The polydispersity increases with decreasing . The capillary number based on the thread radius at breakup is about and shows a minimum at . The measured agrees with slender body theory for . Drops deform pseudo affinely for , but deformation deviates from being pseudo affine otherwise.
Transient and steady-state solutions of 2D viscoelastic nonisothermal simulation model of film casting process via finite element method51(2007); http://dx.doi.org/10.1122/1.2714781View Description Hide Description
The various aspects of the nonlinear dynamics and stability of nonisothermal film casting process have been investigated solving a two-dimensional (2D) viscoelastic simulation model equipped with the Phan-Thien-Tanner (PTT) constitutive equation by employing a finite element method. This study represents an extension of the earlier report [Kim, Lee, Shin, Jung, and Hyun, J. Non-Newtonian Fluid Mech.132, 53–60 (2005)] in that two important points are additionally addressed here on the subject: the nonisothermal nature of the film casting, and the differentiation of extension-thickening (strain hardening) and extension-thinning (strain softening) fluids in their different behavior in the film casting process. The PTT model, known for its robustness in portraying dynamics in the extensional deformation processes which include the film casting of this study along with film blowing and fiber spinning as well, renders the transient and steady state solutions of the dynamics in the 2D, viscoelastic, nonisothermal, film casting capable of explaining the effects of various process and material parameters of the system on the filmdynamics of the process. Especially, the different behavior displayed by two polymer groups, i.e., the extension-thickening low density polyethylene (LDPE) type and the extension-thinning high density polyethylene (HDPE) type, in the film casting can be readily explained by the PTT equation-included simulation model. The three nonlinear phenomena commonly observed in film casting, i.e., draw resonance oscillation, edge bead, and neck-in, have been successfully delineated in this study using the simulation and experimental results.
51(2007); http://dx.doi.org/10.1122/1.2714643View Description Hide Description
The bulk viscosity of a suspension is defined analogous to that for a pure fluid as the constant of proportionality relating the deviation of the trace of the macroscopic stress from its equilibrium value to the average rate of expansion. In a suspension the equilibrium macroscopic stress is the sum of the thermodynamic pressure of the fluid and the osmotic pressure of the suspended particles. Rigid particles suspended in an expanding fluid cause a disturbance flow that contributes to the total mechanical pressure in the system, thereby changing the effective bulk viscosity. Expressions are derived to compute the effective bulk viscosity for all concentrations and all expansion rates for a system of rigid particles suspended in a uniformly expanding fluid. The expansion flow drives the suspension microstructure out of equilibrium and the thermal motion of the particles tries to restore the equilibrium. The Péclet number, defined as the expansion rate made dimensionless with the Brownian time scale, governs the departure of the microstructure from equilibrium. The contribution to bulk viscosity is determined to second order in volume fraction of particles for all compression rates (all negative Péclet numbers). A “compression thickening” of the suspension is observed at large compression rates.
51(2007); http://dx.doi.org/10.1122/1.2516399View Description Hide Description
Rheological measurements are used to compare clay nanocomposites prepared through melt mixing using two different polypropylene matrices. Steady state and transient nonlinear rheological experiments are employed to separate the contributions of flow induced orientation of the tactoids and particulate network build-up. The conditions under which the rheological properties are dominated by the aggregate network are subsequently identified. Under these conditions, the low frequency linear viscoelastic behavior is analyzed using scaling concepts for fractal networks to determine the degree of network formation by exfoliation. Moreover, the high frequency behavior of the moduli can be used to quickly assess the dispersion quality. The results from the analysis of the linear viscoelastic data are compared to structural features extracted from electron microscopy and small angle X-ray scattering data.
51(2007); http://dx.doi.org/10.1122/1.2716559View Description Hide Description
The stochastic chain model described by Xu et al. [J. Rheology50, 477–494 (2006)] provides fundamental insight into the mechanism of apparent wall slip in entangled polymer melts.Apparent slip is shown to be a consequence of a rapid decrease in the entanglement density for chains in the region of the wall. There is good agreement between model predictions and polydimethylsiloxaneexperiments of Durliat et al. [Europhys. Lett.38, 383–388 (1997)] in which a surface layer containing a known density of tethered chains contacts a bulk melt. The model cannot provide quantitative information about apparent slip when chains are simply adsorbed at the wall, since the surface density and effective chain length are required inputs to the calculation, but it does agree in broad terms with experiments of Mhetar and Archer [Macromolecules31, 8607–8616 (1998)] on 1,4-polybutadiene.
51(2007); http://dx.doi.org/10.1122/1.2714642View Description Hide Description
The flow behavior of particle stabilized oil-in-water emulsions with different dispersed volume fractions was analyzed in steady shear on a rotational rheometer employing a coaxial cylinder geometry. The dispersed phase of the emulsion was a mixture of equal volumes of a polar oil, isopropyl myristate, and a nonpolar oil, dodecane. The continuous phase was an aqueous suspension of hydrophilic colloidal silica particles of diameter with the pH adjusted to pH2 in order to stabilize the emulsion [Binks and Whitby, Colloids Surf., A253, 105–115 (1995)]. Droplet diameters were of the order of a few micrometers, and dropletsurfaces apparently show dense particle coverage. We show that the markedly different interfacial structure in particle stabilized emulsions when compared to surfactant stabilized emulsions is reflected in the rheological behavior. To illustrate these differences, the rheological behavior of a comparable surfactant stabilized emulsion with the particles in the aqueous phase replaced by Tween 20, was also investigated. The rheological characterization revealed a domain of shear thickening in the particle stabilized emulsions at high droplet phase volumes that is not observed for the classical surfactant stabilized emulsions.
51(2007); http://dx.doi.org/10.1122/1.2715392View Description Hide Description
A finite simple shear deformation of an elastic solid induces unequal normal stresses. This nonlinear phenomenon, known as the Poynting effect, is governed by a universal relation between shear strain and first normal stresses difference, valid for nondissipative isotropic elasticmaterials. We provide the first experimental evidence that an analog of the Poynting effect exists in aqueous foams where, besides the elastic stress, there are significant viscous or plastic stresses. These results are interpreted in the framework of a constitutive model, derived from a physical description of foam rheology.
51(2007); http://dx.doi.org/10.1122/1.2716448View Description Hide Description
This paper reports experimental and modeling work on biaxial extension of three highly filled microcrystalline cellulose suspensions employing squeeze flow with lubricated plates. The squeeze flow data explored both the initial compaction of the material before yield and subsequent flow rate dependant behavior. A Herschel-Bulkley model proved inadequate for describing the suspensions’ behavior across the range of strain rate studied as the consistency coefficient exhibited a strong shear-thinning characteristic depending on the strain rate at the onset of flow. This observation is interpreted here as a result of increasing fluid viscous forces, enhancing lubrication between the particles within the suspension as described by Coussot and Ancey [Phys. Rev.59, 4445–4457 (1999)]. A modified form of the Herschel-Bulkley model is proposed which incorporates this behavior. Pure biaxial extensional strain was only achieved at intermediate squeezing velocities: wall shear effects were observed at high deformation rates and liquid phase migration (LPM) was evident in slow flows; the onset and extent of LPM was also manifested as variations in the magnitude of extracted flow parameters. The threshold conditions for LPM did not agree with previously published models based on relative drainage and convection rates.
51(2007); http://dx.doi.org/10.1122/1.2716442View Description Hide Description
A recent, nonlinear viscoelastic theory for predicting the thermomechanical response of glassy polymers has been shown to predict behaviors from enthalpy relaxation to temperature-dependent mechanical yield in various modes of deformation quite well. The foundation of this theory rests on a “material clock” that depends on the potential energy of the system. The molecular basis for the clock and the Rational Mechanics framework for the constitutive equation are briefly reviewed. The theory is then used to predict and explain much more complicated behavior of glassy polymers: the change in compressive yield stress during physical aging at different temperatures, the peculiar enthalpic response of glassy polymers previously compressed to different strains, “volumetric implosion” on samples subjected to tensile strains, and the dependence of the shift factor on aging time and applied stress.
Investigating microstructure of concentrated suspensions of anisotropic particles under shear by small angle neutron scattering51(2007); http://dx.doi.org/10.1122/1.2716452View Description Hide Description
The microstructure of dense suspensions of anisotropic particles that are ordered at rest is explored under shear as a function of particle anisotropy and shear rate using small angle neutron scattering. For suspensions containing spheres and mildly anisotropic heteronuclear dicolloids, after preshearing, long-range order is present in the form of randomly stacked hexagonally close packed layers. As the shear rate is increased, long-range directional order is lost. At even higher shear rates, long-range order is re-established in the form of hexagonally packed layers sliding over one another. Suspensions of ordered homonuclear dicolloids are polycrystalline at rest, and as the shear rate is increased, sliding hexagonally packed layers develop while at larger shear rates long-range order is lost. These results demonstrate surprisingly large effects of shear on the microstructures of suspensions containing particles with small changes in anisotropy.
Ergodicity-breaking and the unraveling dynamics of a polymer in linear and nonlinear extensional flows51(2007); http://dx.doi.org/10.1122/1.2714820View Description Hide Description
We present both theory and simulations describing the unraveling dynamics of polymer chains in linear and nonlinear extensional flows. The nonlinearities associated with the hydrodynamic drag force lead to a conformational hysteresis. This hysteresis can be understood by reducing the problem to thermally activated transitions over an energy barrier described by a rate theory. Using this rate theory we show that in the limit of infinite chain size, molecules are kinetically trapped in a coiled or stretched state and ergodicity is broken at a fixed value of an appropriate Deborah number.