Index of content:
Volume 53, Issue 3, May 2009
Soft colloidal matter: A phenomenological comparison of the aging and mechanical responses with those of molecular glasses53(2009); http://dx.doi.org/10.1122/1.3092933View Description Hide Description
Highly concentrated colloidal suspensions are often considered to exhibit behavior similar to that of glass-forming systems. While there is considerable rheological information in the literature concerning the flow behavior of such systems, there is little that has examined the mechanical response in a fashion that makes explicit comparisons with the relaxation behavior of molecular or polymericglasses. On the other hand there is a significant literature that looks at “shear melting” and subsequent aging of such glasslike or “pasty” liquids. Here we report results for a polymer latex particle system at different concentrations near to the glassy or pasty regimes. Stress relaxation experiments and aging after shear melting experiments were performed. Single step stress relaxation results presented as isochrones of stress vs strain show behavior similar to that of polymers at the lower concentrations (50% and 53%). That is, there is a linear regime of behavior (generally less than a deformation of 1%) followed by a nonlinear response and subsequent yield. At the higher concentration, while the linear behavior is retained, the “yieldlike” response is much more abrupt. Interestingly, we find an effect of loading sequence on the isochrones. Aging behavior subsequent to large sinusoidal shear melting histories was also investigated. Importantly, in all instances where “aging” occurred we found that there was no time-aging time superposition, which is contrary to the postulate that the shear melting has the effect of changing the effective temperature. This may be due to the overlap of the “” and “” relaxations.
53(2009); http://dx.doi.org/10.1122/1.3094911View Description Hide Description
Nonaqueous layered silicate suspensions have a complex rheological behavior due to the presence of a microstructure on multiple length scales, which is sensitive to flow and flow history. In the present work, the flow-induced orientation and anisotropy of the nonequilibrium metastable structures in nonaqueous layered silicate suspensions has been studied using a combination of light scattering,scatteringdichroism, and advanced rheometric measurements, including two dimensional small amplitude oscillatory shear (2D-SAOS) flow experiments. The nature of the structures during flow was mainly studied by means of small angle light scattering patterns. Linear dichroism measurements in the vorticity and velocity gradient directions were used to assess the microstructural anisotropy. The changes observed in the vorticity plane developed in the same range of shear rate as the shear-thinning behavior of the suspensions.Scatteringdichroism was used to demonstrate that the flow-induced anisotropy was locked in upon cessation of flow. To verify that this also leads to an anisotropy of the rheological properties, the linear viscoelastic moduli were measured using (2D-SAOS) experiments. This new technique proved to be particularly sensitive to the anisotropic nature of the metastable microstructure of organoclay suspensions. Both the flow-induced orientation and larger scale microstructural rearrangements are shown to contribute to the transient rheological response of the nonaqueous layered silicate suspensions.
53(2009); http://dx.doi.org/10.1122/1.3109574View Description Hide Description
The purpose of this note is to introduce an experimental technique to overcome ductile failure in uniaxial extensional flow and show a simple illustration of its effectiveness. Ductile failure prevents the rheological measurement of transient stress growth at higher strains for certain strain-hardening materials. This reduces the accuracy of nonlinear parameters for constitutive equations fit from transient stress growth data, as well as their effectiveness in modeling extensionally driven processes such as film casting. We propose an experimental technique to overcome ductile failure called encapsulation in which the material that undergoes ductile failure is surrounded by a resin that readily deforms homogeneously at higher strains. The materials are arranged in concentric cylinders and are deformed in parallel. A simple parallel model is shown to calculate the viscosity of the core material and is tested for its robustness in a linear low-density polyethylene/low-density polyethylene system in which the viscosities of each resin are known at all strains, strain rates, and temperatures investigated. Agreement is found at each strain rate investigated across all strains studied. The method is then extended to measure the viscosity of a sparsely branched high-density polyethylene (HDPE) system that undergoes ductile failure at higher strain rates. The technique shows good agreement with experimental results up to the onset of ductile failure in the pure resin and predicts viscosity growth of the HDPE resin well beyond the critical strain for the onset of failure seen in experimental measurements of the pure HDPE. Repeatability of the technique is also illustrated.
53(2009); http://dx.doi.org/10.1122/1.3093105View Description Hide Description
This paper reports recent experimental findings and rheological modeling on chemically treated single-walled carbon nanotubes(CNTs) suspended within an epoxy resin. When a CNTsuspension was subject to a steady shear flow, it exhibited a shear-thinning characteristic, which was subsequently modeled by a Fokker–Planck (FP) based orientation model. The model assumes that the shear flow aligns CNT in the flow direction, but there are events such as Brownian motion and tube–tube interaction trying to randomize the orientation. In the FP orientation model, randomizing events were modeled with an appropriate rotary diffusion coefficient and the shear-thinning behavior was explained in terms of progressive alignment of CNTs toward the shear direction. In terms of linear viscoelasticity (LVE), small-amplitude oscillatory measurements revealed mild elasticity for semidilute treated CNTsuspensions. The exact origin for this elasticity is not clear and both tube–tube interaction and bending/stretching of CNTs have been proposed by other authors as possible origins. It is, however, clear from the current modeling that the experimental evolution of storage modulus cannot be described using a single-mode Maxwell model or simple Brownian rod modeling. In this paper, experimental LVE data of the treated CNTsuspensions were fitted using the FP orientation model with an “effective diffusion coefficient” term and an empirical relation was subsequently identified for the effective diffusion term. Intuitively, chemical treatment has created a weakly interconnected network of CNT and it is believed that the mild elasticity originated from this weak network as well as other randomizing events (Brownian motion and tube–tube hydrodynamic interaction). Finally, step strain experiments confirmed the presence of a weak network at small strains, which at large strains was found to be destroyed. Incorporation of a strain softening factor allowed for the formulation of a self-consistent FP based orientation model describing both the steady shear and LVE responses of treated CNTsuspensions.
53(2009); http://dx.doi.org/10.1122/1.3088848View Description Hide Description
Prior work has demonstrated that colloidalsuspensions can be ordered with large amplitude oscillatory shear (LAOS), showing that the final type and degree of ordering depends on the frequency and amplitude of the applied oscillation. Here, we present LAOS results for the kinetics of ordering for colloidal particles at from a shear disordered state over a range of frequencies and shear amplitudes. A custom rheolight scattering (rheo-LS) device is used to quantify the type and degree of crystalline order observed either as a six spot Bragg pattern or a three streak twinning pattern. The dynamic shear stress and the alignment factor derived from the rheo-LS show the same time-dependent behavior indicative of a primary relaxation that is exponential in time. The rate of ordering to the r-hcp state is found to scale with the amplitude of oscillation and inversely with the frequency over strains from 100% to 300% and frequencies from 0.5 to 5 Hz. These results show that the rate of LAOS ordering does not simply reflect the rate of deformation, but rather a more complex mechanism pointing toward competition between shear ordering and shear-induced defect generation.
53(2009); http://dx.doi.org/10.1122/1.3089622View Description Hide Description
Dynamic (unsteady) defect structures arising in the hydrodynamics of sheared nematic polymers are investigated by numerical simulations and real-time diagnostics in two space dimensions. We simulate the Larson–Mead experiments on roll-cell formation and breakup, following recent numerical studies with a similar model [Klein et al., Phys. Fluids19, 023101 (2007a); Klein, D. H., Ph.D. thesis, University of California, Santa Barbara (2007b)]. The simulations are blindly monitored on the basis of tensorial defect metrics defined by eigenvalue degeneracies, which are local in space and time, and monitored cost free. The focus in the defect detection is shifted from topology to local conditions, yet the nonlocality of defect domains is recovered by graphics of metric level sets. These tools reveal the spawning of an array of oblate defect core domains, which then deform, propagate, collide, merge, and split in a dynamic process that numerically continues ad infinitum. Next, we paint topological features onto snapshots of the level set texture of the oblate metric, using the remainder of the tensor information: first, the full tensor morphology (triaxial ellipsoids per grid point), and then the principal axis (where identifiable) of each orientation ellipsoid. These enhanced textures yield the traditional topological defect metric based on nonlocal winding number of the principal axis and the regularization of each apparent half integer and integer degree singularity. The most compelling predictions of these simulations and diagnostics are persistence of interacting oblate defect domains, while topology is highly transient, and coincidence of topological transitions with oblate domain merger and splitting. Finally, the coupling between orientation features and the transient primary and secondary flow are amplified with additional graphics.
53(2009); http://dx.doi.org/10.1122/1.3086872View Description Hide Description
We have studied stress overshoot behavior in startup shear of four monodisperse polymer melts with a range of chain entanglement from to 160 entanglement points per chain. In the elastic deformation regime defined by , where is the Rouse relaxation time, (i) the peak shear stress scales with the time at the peak to −1/2 power, in contrast to an exponent of −1/4 in the viscoelastic regime (for ), (ii) changes linearly with the elapsed strain at the stress peak , which scales with the applied shear rate as , (iii) a supermaster curve collapses time-dependent shear stress growth curves up to the stress maximum at all shear rates for all the four styrene-butadiene rubber samples.
53(2009); http://dx.doi.org/10.1122/1.3103546View Description Hide Description
The rheological behavior of model suspensions consisting of a Newtonian silicone oil and fibers of different flexibilities have been investigated in steady and transient shear flows. Various fiber suspensions have been prepared to examine the effect of flexibility parameters (stiffness and aspect ratio) as well as the role of interactions in the semi-dilute and semi-concentrated regimes. The viscous and elastic properties of the fiber suspensions are shown to be strongly enhanced by fiber flexibility. With increasing flexibility, the suspensions also exhibit enhanced shear-thinning and the shear rate for the onset of shear-thinning decreases. In start-up flow and especially at low shear rate, the suspensions show large stress overshoots, and under reversal flow delayed overshoots. The magnitude of the overshoot increases as fiber flexibility gets larger. This last effect has been attributed to less orientation in the flow direction during the forward flow for the more flexible fibers. Finally, the effects are more pronounced in the semi-concentrated regime, and this is attributed to enhanced fiber-fiber interactions.
The effect of hydrophobic and hydrophilic fumed silica on the rheology of magnetorheological suspensions53(2009); http://dx.doi.org/10.1122/1.3086870View Description Hide Description
We investigate magnetorheological fluids (MRFs) prepared with carbonyl iron powder and different types of hydrophobic and hydrophilic fumed silica. The rheological properties of the MRF suspensions were investigated with and without an applied magnetic field. The MRF samples prepared with hydrophobicsilicas presented a more pronounced thixotropic effect and a higher recovery rate than those prepared with hydrophilicsilicas. The application of a magnetic field to all the MRFs samples investigated leads to an increase in the viscosity and the thixotropic effect. MRF prepared with hydrophobicsilicas presented smaller values of the viscosity than those prepared with hydrophilicsilicas. At low applied magnetic fields, the type of the silica used to prepare the MRF leads to noticeable differences in the shear stress. However, these differences disappear at high magnetic fields. The results obtained showed that MRF samples prepared with the hydrophobicsilica with the biggest particle diameter presented better characteristics for magnetorheological fluids, with higher values of yield stress, recovery rate, and elastic modulus.
53(2009); http://dx.doi.org/10.1122/1.3086871View Description Hide Description
Droplets containing a dilute polymer solution enter a T-shaped microfluidic junction and stretch as they pass through the stagnation point. Depending on the initial aspect ratio and speed, droplets may break into two segments. We characterize the breaking-nonbreaking behavior of these droplets and find that viscoelasticdroplets are less stable than Newtonian droplets of comparable shear viscosity. When droplets break, we observe that viscoelasticdroplet segments are connected by persistent filaments that first undergo stretching due to drag of the outer viscous liquid on the large connected segments. Later, filaments undergo iterated stretching and develop a series of beads along their length. Secondary filaments between beads no longer stretch with the outer flow, but rather they exhibit an exponentially decreasing diameter consistent with elastocapillary breakup. The relaxation time obtained from the filament diameter profiles is close to the estimated Zimm relaxation time for the polymer solution, but depends on the global flow parameters, including the initial size and speed of the droplet prior to entering the T-junction. Based on the microscale diameters of the filaments, we suggest that splitting of droplets at microfluidic T-junctions may be a useful way to characterize the extensional rheology of low viscosityelastic liquids.
53(2009); http://dx.doi.org/10.1122/1.3099314View Description Hide Description
Fiber suspension theory model parameters for use in the simulation of fiber orientation in complex flows are, in general, either calculated from theory or fit to experimentally determined fiber orientation generated in processing flows. Transient stress growth measurements in startup of shear flow and flow reversal in the shear rate range, , were performed on a commercially available short glass fiber-filled polybutylene terephthalate using a novel “donut-shaped” sample in a cone-and-plate geometry. Predictions using the Folgar–Tucker model for fiber orientation, with a “slip” factor, combined with the Lipscomb model for stress were fit to the transient stresses at the startup of shear flow.Model parameters determined by fitting at allowed for reasonable predictions of the transient stresses in flow reversal experiments at all the shear rates tested. Furthermore, fiber orientation model parameters determined by fitting the transient stresses were compared to the experimentally determined evolution of fiber orientation in startup of flow. The results suggested that fitting model predictions to the stress response in well-defined flows could lead to unambiguous model parameters provided the fiber orientation as a function of time or strain at some shear rate was known.
Viscoelasticity and shear flow of concentrated, noncrystallizing colloidal suspensions: Comparison with mode-coupling theory53(2009); http://dx.doi.org/10.1122/1.3093088View Description Hide Description
We present a comprehensive rheological study of a suspension of thermosensitive particles dispersed in water. The volume fraction of these particles can be adjusted by the temperature of the system in a continuous fashion. Due to the finite polydispersity of the particles (standard deviation: 17%), crystallization is suppressed and no fluid-crystal transition intervenes. Hence, the moduli and in the linear viscoelastic regime as well as the flow curves (shear stress as function of the shear rate) could be measured in the fluid region up to the vicinity of the glass transition. Moreover, flow curves could be obtained over a range of shear rates of 8 orders of magnitude, while and could be measured spanning over 9 orders of magnitude. Special emphasis has been laid on precise measurements down to the smallest shear rates/frequencies. It is demonstrated that mode-coupling theory generalized in the integration through transients framework provides a full description of the flow curves as well as the viscoelastic behavior of concentrated suspensions with a single set of well-defined parameters.
Rheology and spatially resolved structure of cetyltrimethylammonium bromide wormlike micelles through the shear banding transition53(2009); http://dx.doi.org/10.1122/1.3089579View Description Hide Description
We present the first combined study of spatially resolved structure and shear rheology for a modelshear banding fluid comprised of cetyltrimethylammonium bromide wormlike micelles. Combining conventional rheometry,velocimetry,flow birefringence, and flow-small angle neutron scattering (flow-SANS) in the 1–2 (flow-gradient) plane of shear completely characterizes shear banding in the system and enables comparison of local flowkinematics to local segmental orientation and alignment within the bands. The Giesekus constitutive equation with stress diffusion is shown to successfully model the viscoelasticity, steady shear viscosity, and shear bandingkinematics. Flow-SANS measurements in the 1–2 plane exhibit a critical alignment and orientation required for shear banding, followed by a first order orientational transition to a paranematic state in the high-shear band. Master curves of the segmental orientation and alignment are constructed by comparing the local structural features to the locally observed shear rate. The Giesekus-diffusion model successfully predicts the measured segmental orientation and alignment, connecting the microstructure to the macroscopic rheology and shear bandingkinematics. In doing so, a stress-SANS rule is developed, analogous to the stress-optic rule, that relates micellar flow alignment to the shear and normal stresses. The results confirm that shear banding is driven by a nonequilibrium shear-induced isotropic-nematic transition and suggest that the underlying phase behavior of the material is important in determining fluid microstructure and rheology during banding.