Volume 51, Issue 2, March 2007
Index of content:
51(2007); http://dx.doi.org/10.1122/1.2433936View Description Hide Description
The vane rheometer has been used for more than two decades to characterize various complex materials. The objective of this work is to investigate for the first time the flow hydrodynamics of Newtonian, shear-thinning and yield stressfluids in one such rheometer by means of three-dimensional finite element simulation. The velocity field and stress distributions are predicted using finite element meshes that are much more refined than the two-dimensional meshes of previous studies. The validity of the no-slip boundary condition on the blade surfaces, which is commonly assumed in these previous studies, is assessed by comparing the calculated torque to experimental data in the case of Newtonian, shear-thinning and yield stressfluids. The effect of the power-law index and apparent yield stress on the stress profile near the blades and away from them is investigated and discussed. It is shown, in particular, that the uniform stress assumption at the vane ends is reasonable for power-law fluids with and yield stressfluids with large values of yield stress. It is also exposed how the computation of the torque contributions corresponding to the boundaries of the vane-in-cup geometry can lead to the determination of the corrected lengths associated with the end effects.
51(2007); http://dx.doi.org/10.1122/1.2437206View Description Hide Description
A simple rheological constitutive equation for immiscible blend is suggested in this work. The model is based on the ellipsoidal description of droplets.Droplet deformation is described by the ellipsoidal model, and droplet breakup∕coalescence is considered to affect the droplet size only. The discrete breakup∕coalescence process is approximated by a continuous dynamic equation. A simple rescaling method is suggested to integrate the volume-preserving ellipsoidal deformation model and the droplet size evolution dynamics. The predictions of the model are compared with experiments in start up of shear and shear rate sweep modes. The agreement is quite satisfactory on transient and steady rheological properties, as well as the morphology of droplets.
Elongation-induced crystallization of a high molecular weight isotactic polybutene-1 melt compared to shear-induced crystallization51(2007); http://dx.doi.org/10.1122/1.2426977View Description Hide Description
The extensional viscosity fixture (EVF) from TA Instruments was applied for elongation-induced crystallization (EIC) experiments on a high molecular weight isotactic polybutene-1 (PB-1). The results are compared to those previously obtained for shear-induced crystallization (SIC) on the same sample. After annealing the sample at a temperature well above the melting point to erase any memory effects, the sample is cooled to a temperature below the melting point . An increase of the transient elongational viscosity compared to the Trouton viscosity is used to define a crystallization onset time. Challenges caused by the chamber temperature control of the EVF in the ARES, like the delayed attainment of a stable sample temperature as well as the significant mismatch of the latter compared to the imposed nominal temperature, are addressed. By modifying the temperature program accordingly, it was finally possible to run shear and elongation experiments at a comparable sample temperature protocol to prove that quasi-quiescent crystallization occurs at the same time for both types of flow. At Hencky strain rates above , the onset time decreases rapidly with increasing strain rate. Compared to SIC, the decrease in EIC is much stronger (e.g., at the onset time in elongation is two powers of ten shorter than that in shear) and approaches a constant Hencky strain regime. A temperature change by had no significant effect on . Following a procedure introduced for SIC, a temperature-invariant plot of the onset time versus normalized flow rate was constructed containing both flow types.
A correlation between velocity profile and molecular weight distribution in sheared entangled polymer solutions51(2007); http://dx.doi.org/10.1122/1.2424947View Description Hide Description
In this work we attempt to answer several questions concerning the flow characteristics of entangled polymer solutions in a sliding plate shearing cell. We explore (a) how the molecular weight distribution affects the velocity profile in simple shear, (b) whether the observed shear banding is consistent with a nonmonotonic constitutive model, (c) whether the flow response and velocity profiles are different in simple shear depending on the different modes of shear. Our results provide a comparison with recent reports on a polydisperse polymer sample [Tapadia and Wang, Phys. Rev. Lett.96, 016001 (2006); Tapadia, et al., Phys. Rev. Lett.96, 196001 (2006)] that revealed the first evidence for inhomogeneous shear during startup in cone-plate flow geometry of a rotational rheometer. Using a highly monodisperse sample, we observed the sample to partition into two fractions with different local shear rates instead of possessing a smooth spatial variation of the local shear rate as seen for the polydisperse samples. In the stress plateau, the shear banding appears to involve various local shear rates instead of just two values.
51(2007); http://dx.doi.org/10.1122/1.2409737View Description Hide Description
In the present work we relate the microstructure of a distinct type of short fiber reinforced blends (SFRBs) to their melt rheology. These systems consist of polyamide-6 (PA6) and short glass fiber (GF) reinforcement dispersed in a polyethylene (PE) matrix. An appropriate choice of the components, their interfacial characteristics, and the processing route led to the formation of a continuous network, percolating throughout the matrix. The network consisted of fibers (GF) “welded” together by bridges of the dispersed phase material (PA6). A strong strain softening of the melts of the SFRBs was observed at strain amplitudes below 3%, which was absent for their binary equivalents with the same fiber loading and no second thermoplastic component. Based on the specific microstructure of the blends, a model is proposed that accounts for their strain softening behavior. The model considers the “capillary” forces acting between the fibers participating in the network. These capillary forces arise from the deformation of the PA6 bridges “welding” the fibers together. We propose that the macroscopic deformation process produces points of locally concentrated and magnified deformation. The deformation at these points is sufficient to produce the strain softening of the system. The three parameters that appear in the model have a clear physical meaning. The theoretical results are in good qualitative agreement with the experiments.
Drop shape dynamics of a Newtonian drop in a non-Newtonian matrix during transient and steady shear flow51(2007); http://dx.doi.org/10.1122/1.2426973View Description Hide Description
Transient and steady deformation of a single Newtonian drop immersed in a non-Newtonian matrix subjected to a homogeneous shear flow is investigated microscopically. Two model Boger fluids have been used as non-Newtonian matrices. The three-dimensional drop shape is completely determined as observations from both the velocity-vorticity and velocity-velocity gradient plane are available. Start-up and relaxation are investigated varying both the capillary number and the elasticity of the matrix fluid, while the viscosity ratio is kept constant. The extensive data set demonstrates that matrix elasticity reduces the steady dropdeformation and promotes droplet orientation, can induce a dropdeformation overshoot in the start-up experiments and slows down the relaxation phenomena. The experimental results have been compared with predictions of a phenomenological model [M. Minale, J. Non-Newtonian Fluid Mech.123, 151–160 (2004)], that is slightly modified in the present work. It shows good agreement with the experimental data up to moderate capillary numbers . For higher , the observed trends are still correctly predicted, although quantitative agreement is less satisfying. A systematic deviation is observed at the end of the relaxation process. This result, together with a systematic, quantitative discrepancy in the experimental data between the two Boger fluids, suggests that the underlying rheological model is probably too simplistic to allow a quantitative prediction of all effects caused by matrix elasticity.
51(2007); http://dx.doi.org/10.1122/1.2433701View Description Hide Description
The dynamic and steady state behavior of highly entangled polybutadiene solutions has been studied in shearing flow using high-spatial-resolution particle tracking velocimetry. In Couette geometry, the velocity profile is approximately linear upon the imposition of a step shear rate, but becomes significantly curved at later times, and attains the steady shape only after more than 100 strain units. In steady state, local shear rate varies smoothly across the gap and there is no shear banding. The constitutive relation is constructed from local stress and local shear rate extracted from velocity profiles. This procedure is not affected by the common problems of slip and edge instabilities as encountered with cone and plate geometry. The constructed constitutive curve is clearly monotonic for all tested solutions with entanglement numbers up to 47, notwithstanding a local power law index in the stress plateau as small as 0.01. The slope of the stress plateau is sensitive to both the chain polydispersity and entanglement number. Transient shear banding was observed under imposition of step shear in cone and plate geometry. This banding, however, disappears at longer times, which again confirms a monotonic steady state constitutive relation.
51(2007); http://dx.doi.org/10.1122/1.2433935View Description Hide Description
Dispersions of multiarm star polymers in an athermal solvent are studied as a model system to explore the effects of soft colloidal interactions on the dynamics of colloidalglasses.Linear viscoelasticmeasurements in the glassy state are congruent with Mode Coupling Theory predictions for hard sphere glasses at moderate frequencies, indicating similarities in the relaxation processes of hard and soft colloidalglasses near equilibrium. On the other hand, distinct features of the star relaxation (related to arm disengagement) are observed to affect the nonlinear behavior associated with shear melting of the glass, which exhibits transitions previously unreported for hard sphere systems. Whereas a single maximum in is evident under large amplitude oscillatory shear at frequencies near the beta relaxation, secondary transitions between yielding and the onset of macroscopic flow are observed at higher and lower frequencies. The latter are annealed out upon complete fluidization of the star polymersuspension. The overall nonlinear behavior of the soft glassysuspension is interpreted in terms of two primary mechanisms: (i) deformation and yielding of the cage-like glassy microstructure, as in the hard sphere case, and (ii) local stretching and disengagement of interpenetrating arms of the colloidal star polymers.