Volume 54, Issue 6, November 2010
Index of content:
54(2010); http://dx.doi.org/10.1122/1.3495981View Description Hide Description
Even though our appreciation of the importance and ubiquity of wall slip has grown substantially over the last decade or two, it is still common to find papers on disperse systems wherein scant details of the measurement methods are given and where no mention of the possibility of slip is made. It seems then that there is a need still to raise awareness and to promote the idea that the rheometry of disperse systems is not so straightfoward as it might seem. It takes experience, judgment, and skill to make meaningful measurements on disperse systems and it is suspected that the nature of the experimental challenge is under-estimated grossly by too many workers even now. Slip is not merely a rheometric complication; of course, it an intrinsic feature of the response of disperse systems and one deserving of more attention in its own right.
Characterization and prediction of long-chain branching in commercial polyethylenes by a combination of rheology and modeling methods54(2010); http://dx.doi.org/10.1122/1.3479044View Description Hide Description
By blending commercial long-chain branched polyethylenes, Dow AFFINITY™ (trademark of The Dow Chemical Company) PL 1840 Polyolefin Plastomer and Dow AFFINITY™ PL 1880 Polyolefin Plastomer, with the linear polyethylenes ExxonMobile Exact 3132 and Exact 3128 of very similar molecular weight and molecular weight distribution, we detect the effect of ultra-low levels of long-chain branching on linear rheological properties and validate a generalized “tube” model (the “hierarchical model”) for predicting this effect.
54(2010); http://dx.doi.org/10.1122/1.3479389View Description Hide Description
We studied polymer-polymer interfacial slip in bilayerfilms of highly immiscible (interaction parameter, ) polyethylene and fluoropolymer from medium to higher shear stresses (10–200 kPa) using both visualization and stress reduction. We found good agreement between results from the two methods as well as with previous studies using multilayers by Lee et al. [J. Rheol.53, 893–915 (2009)] and visualization of flow in a transparent capillary by Migler et al. [J. Rheol.45, 565–581 (2001)]. We observed two power-law regions: with a transition to at 50 kPa. This is in contrast to the theory of Brochard-Wyart and de Gennes [C. R. Acad. Sci., Ser. II: Mec., Phys., Chim., Sci. Terre Univers317, 13–17 (1993)], which predicts a transition from infinite slope to a slope of one at a critical stress.
Yielding processes in a colloidal glass of soft star-like micelles under large amplitude oscillatory shear (LAOS)54(2010); http://dx.doi.org/10.1122/1.3483610View Description Hide Description
The understanding of yielding and flow of a colloidal glass under large amplitude oscillatory shear (LAOS) represents a motivating challenge. Monitoring the higher harmonics in the stress signal by Fourier-transform (FT) rheology may provide useful insight on the progressive transition from linear to nonlinear viscoelastic response. However, the physical interpretation of FT-rheology data is still not obvious. Here we study the process of yielding in a colloidal glass formed by star-like block copolymer micelles with LAOS experiments and interrogate the nonlinear intracycle stress response by FT analysis and decomposition to an orthogonal set of Chebyshev polynomials [Ewoldt, R. H., et al.J. Rheol.52(6), 1427–1458 (2008)]. Such approach provides a robust framework enabling us to map out a rich phenomenology of intracylce nonlinearities that may relate to distinct physical mechanisms. We find that the nonlinearities during yielding are represented by intracylce shear thickening/thinning and strain hardening/softening of the viscous and elastic response of the system, respectively. We suggest that the underlying mechanisms are related to cage breaking and reformation as well as stress storing and relaxation within the period of oscillatory shear which are affected by an interplay between shear and Brownian motions and thus relate to Péclet number variation with strain and frequency.
First normal stress difference of entangled polymer solutions in large amplitude oscillatory shear flow54(2010); http://dx.doi.org/10.1122/1.3483611View Description Hide Description
The first normal stress difference in large amplitude oscillatory shear (LAOS) consists of two contributions: one from non-oscillating nonzero mean value and the other from oscillating even harmonics, while the nonlinear shear stress can be expressed as a sum of odd harmonics. In this study, the two nonlinear contributions of in entangled polymer solutions have been analyzed by systematical comparison with their shear stress counterparts (storage modulus and odd harmonics, respectively) as a function of strain amplitude and of Deborah number (De). The contributions of were greater than those of the shear stress in the LAOS region. The nonzero mean value of decreased with an increase in Deborah number, but conversely, the second harmonic of increased. The relative intensity (the second harmonic over the fourth harmonic of ) as an indicator of the nonlinearity of decreased with the Deborah number. We also compared experimental results to model predictions from the eight-mode Giesekus constitutive equation. For the model, we obtained two sets of nonlinear parameters from the best fit of model predictions with both steady shear viscosity and the first normal stress difference coefficient. The model could not quantitatively predict the first normal stress difference behavior in LAOS even with these nonlinear parameter sets, which fit the steady data very well. This study shows that the contribution of the first normal stress difference in LAOS is as vital as that of shear stress, and it can be used as a good estimate of the performance of the constitutive equation.
Numerical investigation of the reduction of wall-slip effects for yield stress fluids in a double concentric cylinder rheometer with slotted rotor54(2010); http://dx.doi.org/10.1122/1.3484955View Description Hide Description
Wall-slip effects in a rheometer with double concentric cylinder geometry may lead to significant errors in measurement of the apparent viscosity. Previously, we proposed to use a slotted rotor design to reduce these effects. In this paper, we conduct two as well as three-dimensional computational fluid dynamics(CFD) simulations to determine the differences in rheological measurements of yield stress fluids between the slotted and non-slotted rotor designs. The test fluid and the slip wall boundary of a rotor are characterized in our computational model by the constitutive equation of Zhu et al. [“Non-Newtonian fluids with a yield stress,” J. Non-Newtonian Fluid Mech.129, 177–181 (2005)] and the wall-slip length method, respectively. The model has been validated against the existing rheological data measured using a vane rheometer. The results of this study indicate that the rheometer equipped with a slotted rotor can measure the fluid properties with enhanced accuracy and less sensitivity to the wall-slip velocity than a rheometer with a non-slotted rotor. We also show that the wall-slip effects can be further reduced by either increasing the slot ratio or adding more slots to the rotor. This work illustrates that CFD analysis can be a powerful tool in rheometer design.
54(2010); http://dx.doi.org/10.1122/1.3490661View Description Hide Description
The dynamics of single droplets dispersed in a second, immiscible liquid undergoing a controlled mixture of shear and elongational flow has been studied using a home made eccentric cylinder device. The model system consists of poly(dimethyl siloxane) droplets in a poly(isobutylene) matrix, both Newtonian liquids at room temperature. In continuation of previous work [Boonen et al., Rheol. Acta48, 359–371 (2009)], the effect of changing the balance of shearing and elongational flow components and varying viscosity ratio on the deformation and orientation of the droplets has been systematically investigated under sub-critical flow conditions. The experimental results obtained from optical microscopy are compared with theoretical predictions of the phenomenological model by Maffettone and Minale [J. Non-Newtonian Fluid Mech.78, 227–241 (1998)], obtained using the transient form of the model and incorporating a flow type parameter that accounts for the relative amount of extension in the flow. Overall, a fair agreement was found between the model predictions and the experimental results for all sub-critical mixed flows applied and all viscosity ratios investigated here. This work provides an experimental reference data set which can be used to guide future modeling efforts.
54(2010); http://dx.doi.org/10.1122/1.3494134View Description Hide Description
Velocity profiles of highly entangled polymer solutions under shear have been measured using particle tracking velocimetry to ascertain whether steady-state shear banding occurs in these samples. Under controlled torque flow, the velocity profile becomes nonlinear and the shear rate starts to rise when the true shear strain reaches about 2.5, suggesting that the shear rate rise is initiated by chain alignment and/or disentanglement. At long times, the velocity profile can become either smoothly curved or sharply banded for the same sample in the same geometry, implying that shear banding is not a deterministic behavior as expected from a non-monotonic constitutive relation. It is suggested that the stable shear banding observed in the measured entangled polymer solution is not true steady-state shear banding but may be “trapped” transient banding.
Experimental evaluation of the pure configurational stress assumption in the flow dynamics of entangled polymer melts54(2010); http://dx.doi.org/10.1122/1.3496378View Description Hide Description
A filament stretching rheometer was used for measuring the startup of uni-axial elongational flow followed by reversed bi-axial flow, both with a constant elongational strain rate. A narrow molecular mass distribution linear polyisoprene with a molecular weight of 483 kg/mole was subjected to the flow in the non-linear flow regime. This has allowed highly elasticmeasurements within the limit of pure orientational stress, as the time of the flow was considerably smaller than the Rouse time. A Doi–Edwards [J. Chem. Soc., Faraday Trans. 274, 1818–1832 (1978)] type of constitutive model with the assumption of pure configurational stress was accurately able to predict the startup as well as the reversed flow behavior. This confirms that this commonly used theoretical picture for the flow of polymeric liquids is a correct physical principle to apply.
54(2010); http://dx.doi.org/10.1122/1.3479045View Description Hide Description
Magnetorheological(MR) properties were investigated for sphere, plate, and rod-like iron particles in suspension under the presence of magnetic fields to ascertain the effect of particle shape in MR performance. A novel two-step synthesis route for micrometer sized iron particles with different morphologies is described in detail. Small-amplitude dynamic oscillatory and steady shear flow measurements were carried out in the presence of external magnetic fields. Finite element method calculations were performed to explain the effect of particle shape in the magnetic field-induced yield stress. Compared to their sphere and plate counterparts, rod-like particle based MR fluids present a larger storage modulus and yield stress. The effect of particle shape is found to be negligible at large particle content and/or magnetic field strengths.
54(2010); http://dx.doi.org/10.1122/1.3494571View Description Hide Description
We study the rheology associated with gelation of hydrating cements used in oil well cementing and the use of accelerators to advance the rates of setting. Cement pastes made of API Grade H oil well cement with water/cement mass ratios of 0.25–0.40 are studied. Setting times are accelerated from 7 to 1 h by the addition of 5% wt/wt calcium chloride/cement. Complex viscosity follows an exponential time dependence and the shape of the gelation curve is essentially unaffected by the calcium addition levels. This leads to a “time-concentration” shifting law for the effect of the accelerator. A novel parallel plate geometry is introduced that eliminates slip and enables dynamic oscillatory measurements in the linear viscoelastic regime during setting. Another instrument based on the penetration of a sphere through the setting cement at rates of is presented. This instrument would enable high throughput evaluation of cement additives. The differences between small strain oscillatory measurements that measure a complex viscosity and continuous deformation measurements such as the penetrometer are discussed.