Volume 51, Issue 5, September 2007
Index of content:
51(2007); http://dx.doi.org/10.1122/1.2748782View Description Hide Description
The aim of this work was to investigate the influence of morphology evolution in immiscible blends of Polyisobutene/(Polydimethylsiloxane) on their rheological properties both in steady and transient states under linear and nonlinear flow regimes. The focus was made on the region below the critical shear rate of drops breakup. Such critical shear rate was determined experimentally through a new route using small angle light scattering technique. Interfacial shear stress, first normal stress difference and shear viscosity of the blend were measured in steady shear regime below the critical shear rate and the obtained results were compared to the predictions of the model of Yu et al. [Yu, W., M. Bousmina, M. Grmela, and C. Zhou, “Modeling of oscillatory shear flow of emulsions under small and large deformation field,” J. Rheol.46, 1401–1418 (2002a) and Yu, W., M. Bousmina, M. Grmela, J. F. Palierne, and C. Zhou, “Quantitative relationship between rheology and morphology in emulsions,” J. Rheol.46, 1381–1399 (2002b)]. Finally, the influence of the initial morphology on the rheological material functions of the blend was investigated during shear rate sweep-up and shear rate sweep-down tests. The obtained results showed a nice hysteresis that was inferred to morphology development during flow.
51(2007); http://dx.doi.org/10.1122/1.2745991View Description Hide Description
The effect of hydrodynamic interactions on the adsorption of a polyelectrolyte molecule onto a wall in shear flow is investigated using a bead-spring dumbbell model. Bead-bead and bead-wall electrostatic interactions are taken into account using screened Coulombic interactions, and the hydrodynamic interactions are incorporated using the approach proposed by Ma and Graham [Phys. Fluids17, 083103 (2005)]. An analytical expression for the concentration profile of the polyelectrolyte is derived which predicts a competition between bead-wall hydrodynamic interactions and bead-wall electrostatic attraction. The behavior of the concentration profile is explored as a function of the Weissenberg number, surface (wall) charge density, charge on the beads, and screening length. The charge on the beads assists migration of the dumbbell away from an uncharged wall, whereas for an oppositely charged wall it increases the probability of finding the dumbbell close to the wall. In some cases, the concentration profile shows a very sharp peak near the wall whose distance from the wall increases with dumbbell size, indicating the possibility of size-based separation.
51(2007); http://dx.doi.org/10.1122/1.2750657View Description Hide Description
Shear stress versus shear rate and, hence, viscosity measurements for industrial materials are often difficult to measure using conventional rheometer geometries due the effects of particle size, particle migration, settling, and slip. It is demonstrated here that a four bladed vane is not only useful for yield stressmeasurement but is also a convenient device for the determination of the shear stress versus shear rate characteristics of industrial materials. The application considered is that of the vane in an infinite medium (a bucket) where the shear rate is precisely determined. Data for an alumina sample at two different states of dispersion and three industrial nickel laterite suspensions are presented to confirm the conclusion that a vane rotated in a bucket of fluid is a practical industrial rheometer.
51(2007); http://dx.doi.org/10.1122/1.2751384View Description Hide Description
In this study, we devised an extension-induced mixer to disperse carbon nanotube(CNT) clumps into individual tubes by imposing extensional stress continuously and periodically. The rheological behavior of the CNT-dispersed suspension was investigated to examine whether the hydrodynamic dragging induces the dispersion of individual CNT from strongly entangled CNT by van der Waals force between adjacent tubes. When CNT clumps were subjected to a continuous extensional flow for a prolonged time, the optical microscopic and cryo-transmission electron microscopy studies showed that the CNT agglomerates were effectively dispersed and some of the individual nanotubes were disentangled from the CNT clump. Shear viscosity of the dispersion increased when the CNT-Boger fluid suspension was subjected to the continuous and periodic extensional flow for a prolonged time, indicating the dispersion of the individual tubes by the extensional flow. Under shear flows, however, aggregated CNT clumps formed larger, weakly bound agglomerates with diameters up to millimeters. The newly suggested mixing technique can be used effectively for mixing two different kinds of polymers with a large difference in viscosity.
51(2007); http://dx.doi.org/10.1122/1.2751385View Description Hide Description
Particle tracking microrheology is used to study the effect of a constant applied shear during gelation of aqueous gellan gum with a monovalent salt. Shear modifies the gellan gum hydrogelmicrostructure and the bulk rheological properties of the system, depending on whether shear is applied during gelation or afterwards. The microstructure determines the linear elastic response of the gel, as well as the critical strain and stress above which the response becomes nonlinear. Bulk oscillatory rheology is used to study microstructured gellan gum hydrogels at different polymer and salt concentrations. The similarity between our system and concentrated microgel particle suspensions can be explained by considering the microstructured gellan system to be composed of microgel particles whose size is set by the applied shear stress magnitude during gelation. Polymer concentration and ionic strength control the individual microgel particles’ elastic properties. We also find the gellan system exhibits an isoenergetic transition from the jammed to un-jammed state when sheared, similar to jammed colloidal systems [C. G. Robertson and X. R. Wang, “Isoenergetic jamming transition in particle-filled systems,” Phys. Rev. Lett.95, 075703 (2005)].
51(2007); http://dx.doi.org/10.1122/1.2749701View Description Hide Description
The relation between composition, rheology, and morphology in phase separated pullulan-sodium dodecyl sulphate systems containing sodium chloride has been investigated using rheo-optical methods. The rheological measurements showed that the apparent viscosity of these aqueous two-phase systems depends on the chemical composition of each phase, their volumetric composition, and the viscosity ratio of the separated phases. Optical observations revealed a droplet like morphology over a wide range of shear rates at low to moderate volume fractions of pullulan-rich phase. In some mixtures, string phases were observed at higher volume fractions of pullulan-rich phase and shear rates. Simultaneous analysis of the rheological data and observed structures at different shear rates/compositions indicates a close link between rheology and morphology of aqueous two-phase systems and provides a simple tool for predicting morphology on the basis of rheological data.
51(2007); http://dx.doi.org/10.1122/1.2764089View Description Hide Description
The electric field strength and shear rate dependence of the apparent shear viscosity of electrorheological (ER) suspensions can often be represented by a function of only the Mason number. A Mason number defined for magnetorheological (MR)suspensions by direct substitution of magnetostatic variables for electrostatic variables does not produce a similar collapse of shear viscosity data for MRsuspensions. We show that a Mason number defined in terms of the suspension magnetization can be employed to produce a collapse of experimental data at various magnetic field strengths and shear rates. As for ER suspensions, this Mason number can be calculated from experimentally measured quantities.
Use of high-temperature, high-torque rheometry to study the viscoelastic properties of coal during carbonization51(2007); http://dx.doi.org/10.1122/1.2754317View Description Hide Description
When coal is heated in the absence of oxygen it softens at approximately C, becomes viscoelastic, and volatiles are driven off. With further heating, the viscous mass reaches a minimum viscosity in the range of – and then begins to resolidify. A high-torque, high-temperature, controlled-strain rheometer with parallel plates has been used to study the rheology during this process. Under shear, the viscosity of the softening mass decreases with increasing shear rate. During resolidification, the viscosity increases as bond formation and physical interactions gives rise to an aromatic network, but, under shear, the network breaks apart and flows. This is viewed as a yielding of the structure. The higher the shear rate, the earlier the yielding occurs, such that if the shear rate is low enough, the structure is able to build. Also, further into resolidification lower shear rates are able to break the structure. It is proposed that resolidification occurs through the formation of aromatic clusters that grow and become cross-linked by non-covalent interactions. As the clusters grow, the amount of liquid surrounding them decreases and it is thought that the non-covalent interactions between clusters and liquid could decrease and the ability of growing clusters to move past each other increases, which would explain the weakening of the structure under shear. This work is part of a program of work aimed at attaining a greater understanding of microstructural changes taking place during carbonization for different coals, in order to understand the mechanisms that give rise to good quality cokes and coke oven problems such as excessive wall pressure.
Moderately concentrated solutions and gels of atactic polystyrene in solvent mixtures of carbon disulfide and tricresyl phosphate51(2007); http://dx.doi.org/10.1122/1.2750666View Description Hide Description
Viscoelastic properties are followed during the formation of gels by moderately concentrated solutions of atactic polystyrene in a mixed solvent of carbon disulfide and tri-m-tolyl phosphate (TCP). This mixture is a thermodynamically “good” solvent for and with solution glass transition temperatures far below the temperatures used in the study. Gel formation is driven by the component, at temperatures and polymer concentrations similar to those observed in solutions of in pure . The TCP adjusts the viscoelastic response to a convenient range. The results show appreciable hysteresis in the melting and formation temperatures of gels, with the gel formation being a slow process, permitting dynamic mechanical or creep and creep recovery studies as a function of ageing during gel formation, as well as on the solutions prior to permit gel formation. These data reveal systematic behavior analyzed using the linear viscoelastic constitutive relation to obtain the viscosity and a measure of the steady-state recoverable compliance prior to gel formation and the equilibrium compliance after gel formation. This behavior is discussed in terms of models in which segments of the backbone possess a helical conformation stabilized by interactions with and gel crosslink loci formed via these sequences.
51(2007); http://dx.doi.org/10.1122/1.2754293View Description Hide Description
We describe the dynamics of an isolated, flexible polymer molecule in an extension-dominated planar mixed flow. We find that as vorticity is added to planar extensional flow, the conformationalfluctuations of the molecule increase and the coil-stretch hysteresis [e.g., Schroeder et al., Macromolecules37(24), 9242 (2004); Science301, 1515 (2003)] vanishes for a fixed Deborah number and Hencky strain. We present Brownian dynamics simulation results and a kinetic theory, valid in the hysteretic regime, which allows us to determine both the rate at which a coiled molecule unravels and the rate at which a stretched molecule relaxes to a coil when subjected to planar mixed flow. We demonstrate that for all values of the added vorticity, the coil-stretch/stretch-coil transition rates increase with increasing vorticity as a consequence of the conformationalfluctuations, which can be explained by the molecule sampling a shear flow through Brownian fluctuations off the extensional axis. We demonstrate that if , where is the radius of gyration, is the molecular contour length, and is the so-called “mixedness” parameter, the transition rate increase is explained in terms of an effective conformational diffusivity, which can be evaluated using a convective dispersion analysis.
51(2007); http://dx.doi.org/10.1122/1.2750654View Description Hide Description
Micellar casein particles (submicelles) were formed by removing calcium from native casein through chelation with sodium polyphosphate. The submicelles aggregated with time and eventually formed a gel if the casein concentration is sufficiently high. The frequency dependent shear modulus was measured during the gelation process as a function of temperature, concentration, and . Gelation occurred via a percolation process at all conditions, but increasing the temperature or the concentration accelerated the process. The concentration and temperature dependence of the gel time were independent and could be described by a simple expression valid over a wide range of and . The shear modulus of the gels increased with , while varying the temperature had only a very weak effect.
51(2007); http://dx.doi.org/10.1122/1.2771175View Description Hide Description
Parallel superposition experiments are used here for the linear and nonlinear rheological characterization of a commercial polydisperse polymer melt in the entangled state. In the limit of small amplitude of the oscillations, we focus (to our knowledge for the first time) on the time-temperature superposition principle. With increasing oscillation amplitudes, the storage and loss moduli become amplitude-dependent at amplitudes which vary with the strength of the background flow. Experimental results are compared with predictions of a recent molecularly-based constitutive equation in order to validate the equation itself as well as to interpret the data in terms of the molecular mechanisms accounted for in the constitutive equation.
Viscoelasticity and crystallization of poly(ethylene oxide) star polymers of varying arm number and size51(2007); http://dx.doi.org/10.1122/1.2751076View Description Hide Description
We investigated the linear melt viscoelasticity and the crystallization kinetics of a series of model poly(ethylene oxide) stars with different functionalities ( arms) and moderately entangled arms (their molecular masses ranging from ). The limited data in the homogeneous state indicated that the zero-shear viscosity was adequately described by the Milner–McLeish model for functionalities , where the core effect is insignificant; a similar behavior was observed for the recoverable compliance which depended on both the molecular weight and the number of the arms. Below the melting point, the isothermal crystallization was measured with differential scanning calorimetry and rheology, and analyzed in terms of the Avrami theory, expanding over a wide range of temperatures. The results were supported by additional polarizing optical microscopy data and indicated a slower crystallization kinetics of the stars compared to their linear analogues. They showed a strong dependence of the crystallization rate on the arm molecular weight, whereas the available experimental evidence is suggestive of some functionality dependence as well.
51(2007); http://dx.doi.org/10.1122/1.2750668View Description Hide Description
We have investigated the shear flow behavior of a dynamically symmetric polymeric bicontinuous microemulsion using rheology and in situ small angle x-ray scattering. The microemulsion consists of a ternary blend of poly (ethylene-alt-propylene) (PEP), poly (butylene oxide) (PBO), and a PEP-PBO diblock copolymer. Steady shear experiments reveal an unusual shear thickening behavior at the onset of the non-Newtonian regime, which is consistent with the strain hardening and frequency thickening (at large strains) under oscillatory shear. Scattering experiments indicate development of anisotropy in the bicontinuous structure within the thickening regime. Subsequent shear thinning is observed at intermediate shear rates. Shear-induced bulk phase separation is detected at very high rates. This work complements previous studies on a dynamically extremely asymmetric bicontinuous microemulsion, and thereby establishes the universal rheological properties of polymericmicroemulsions. Possible underlying molecular mechanisms for these rich rheological phenomena are discussed.
51(2007); http://dx.doi.org/10.1122/1.2750665View Description Hide Description
Wormlike micellesolutions are submitted to small-amplitude oscillatory shear superimposed to steady shear in the shear banding regime. By imposing a shear oscillation, the interface between high- and low-shear regions oscillates in time. A two-fluid semiphenomenological model is proposed for superposition rheology in the shear banding regime, which allows us to extract a characteristic velocity for the interface dynamics from experiments involving only a standard rheometer. Estimates of the stress diffusion coefficient can also be inferred from such superposition experiments. The validity of our model is confirmed by directly recording the interface displacement using ultrasonicvelocimetry.
51(2007); http://dx.doi.org/10.1122/1.2754309View Description Hide Description
It is well known that when a suspension of noncolloidal, neutrally buoyant rigid particles is drawn into an empty tube, the inward shear-induced migration of the particles results in a continuously growing packed layer of particles at the advancing meniscus. For non-neutrally buoyant suspensions, however, gravity perpendicular to the flow direction modifies the fully developed concentration profile and can lead to depletion of particles at the meniscus. This accumulation/depletion phenomenon was studied experimentally as a function of the buoyancy number , the ratio of gravitational to viscous forces. The rate of accumulation or depletion was used to determine the area average concentration as a function of and the feed concentration. These results are in good agreement with the profiles calculated theoretically using the suspension balance model of Nott and Brady [J. Fluid Mech.275, 157 (1994)] coupled with the anisotropicconstitutive equations of Zarraga et al. [J. Rheol.44, 185 (2000)].