Volume 56, Issue 2, March 2012
Index of content:
56(2012); http://dx.doi.org/10.1122/1.3676001View Description Hide Description
The emergence of turbidity in startup flows of aqueous solutions of cetyltrimethylammonium bromide (CTAB)/sodium salicylate (NaSal) between parallel discs was investigated. An induction time and a critical shear rate required for the emergence of turbidity were measured for solutions of several molar concentration ratios of NaSal to CTAB and at several temperatures. The critical shear rate depends on the relaxation time of CTAB/NaSal solutions. Furthermore, the dependency was classified according to the concentration ratio into three groups. These results indicate that the emergence of turbidity depends on the networkstructure of wormlike micelles that varies with the concentration of counter ions. In addition, it was found that a critical Weissenberg number defined by the product of the relaxation time and the critical shear rate was constant for each group.
56(2012); http://dx.doi.org/10.1122/1.3676741View Description Hide Description
Recent experiments performed on a variety of soft glassy materials have demonstrated that any imposed shear flow serves to simultaneously fluidize these systems in all spatial directions [Ovarlez et al., Nature Mater. 9, 115–119 (2010)]. When probed with a second shear flow, the viscous response of the experimental system is determined by the rate of the primary, fluidizing flow. Motivated by these findings, we employ a recently developed schematic mode-coupling theory [Brader et al., Proc. Natl. Acad. Sci. U.S.A. 106, 15186–15191 (2009)] to investigate the three-dimensional flow of a colloidal glass, subject to a combination of simple shear and uniaxial compression. Despite differences in the specific choice of superposed flow, the flow curves obtained show a good qualitative agreement with the experimental findings and recover the observed power-law describing the decay of the scaled viscosity as a function of the dominant rate. We, then, proceed to perform a more formal analysis of our constitutive equation for different kind of “mixed” flows consisting of a dominant primary flow subject to a weaker perturbing flow. Our study provides further evidence that the theory of Brader et al., Proc. Natl. Acad. Sci. U.S.A. 106, 15186–15191 (2009) reliably describes the dynamic arrest and mechanical fluidization of dense particulate suspensions.
56(2012); http://dx.doi.org/10.1122/1.3679469View Description Hide Description
The idea of incorporating multiple models of linear rheology into a superensemble, to forge a consensus forecast from the individual model predictions, is investigated. The relative importance of the individual models in the so-called multimodel superensemble (MMSE) was inferred by evaluating their performance on a set of experimental training data, via nonlinear regression. The predictive ability of the MMSE model was tested by comparing its predictions on test data that were similar (in-sample) and dissimilar (out-of-sample) to the training data used in the calibration. For the in-sample forecasts, we found that the MMSE model easily outperformed the best constituent model. The presence of good individual models greatly enhanced the MMSE forecast, while the presence of some bad models in the superensemble also improved the MMSE forecast modestly. While the performance of the MMSE model on the out-of-sample training data was not as spectacular, it demonstrated the robustness of this approach.
56(2012); http://dx.doi.org/10.1122/1.3679461View Description Hide Description
Using Brownian dynamics simulations, without excluded volume and hydrodynamic interactions, on single polymer molecules represented by bead-spring models with stiff Fraenkel springs mimicking a single Kuhn step, we find multiple nonlinear regimes of deformation in shear flows that are controlled by the Peclet number (Pe), which is the shear rate times the relaxation time of a Kuhn step. We observe that, for all chain lengths investigated, the average stretch in the flow direction initially increases with increasing Pe, followed by saturation in chain stretch, as observed in previous studies. At even higher Pe, the stretch begins to decrease with increasing shear rate, in accordance with similar simulations of Sendner and Netz, Eur. Phys. J. E30, 75–81 (2009). At these rates, the trajectories reveal “premature” recoiling of the chain before attaining a fully extended state during the phase of the “tumbling orbit” in which the chain is stretching. An increasing stretch at even higher Pe is characterized by peculiar orientation “locking” effects that may be sensitive to modeling details. We also show that the stretch predictions of coarse-grained springs agree well with those of the fine-grained chains until high shear rates are reached, where the flow perturbs the individual springs from equilibrium.
Large amplitude oscillatory shear (LAOS) measurements to obtain constitutive equation model parameters: Giesekus model of banding and nonbanding wormlike micelles56(2012); http://dx.doi.org/10.1122/1.3684751View Description Hide Description
Large amplitude oscillatory shear (LAOS) is an increasingly popular nonlinear rheological test method, and the interpretation of the measurements is still an active area of research. Here, we demonstrate a new method whereby the nonlinear parameters of a popular constitutive equation used to model polymeric and other viscoelastic systems, the Giesekus model, may be determined directly from LAOS measurements without complicated, nonlinear model fitting. We define the stress response of the sample as an expansion in deformation strain and oscillation frequency. To leading order in strain, we derive the explicit analytic expressions for the first three harmonics of the stress response during LAOS for the Giesekus constitutive model. This allows for rapid determination of the Giesekus model parameters, including the nonlinear coupling parameter, directly from plots of the response obtained from modern rheological instruments. We demonstrate the validity, utility, and limits of this new method on a well-studied wormlike micelle (WLM) surfactant solution, cetyltrimethylammonium bromide (CTAB), and water. This robust and simple approach is especially advantageous for systems where steady shearing in the nonlinear regime is problematic and can provide rapid determination of parameters in rheological constitutive equations.
Performance of mesoscale modeling methods for predicting rheological properties of charged polystyrene/water suspensions56(2012); http://dx.doi.org/10.1122/1.3690105View Description Hide Description
We examine the accuracy and performance of several leading discrete-element-modeling approaches to predicting equilibrium and dynamic rheological properties of an aqueous, polystyrene suspension. What distinguishes each approach presented is the methodology of handling the solventhydrodynamics. Specifically, we compare stochastic rotation dynamics (SRD), fast lubrication dynamics (FLD), and dissipative particle dynamics (DPD) methods of including solventhydrodynamics against each other and against experimental data. Quantities examined are equilibrium structure properties (e.g., pair-distribution function), equilibrium dynamic properties (e.g., long-time diffusivities), and dynamic response (e.g., steady shear). In all approaches, we deploy the Derjaguin-Landau-Verwey-Overbeek (DLVO) potential for colloid–colloid interactions. Comparisons are made over a range of volume fractions and salt concentrations. Long-time diffusivities are especially difficult to compute and exhibit clear discrepancies across methods. Significant effort is devoted in this paper to explain the reasons for the observed inconsistencies. Shear viscosities are predicted to within experimental and numerical error estimates with both SRD and FLD methods, while traditional DPD proves to be too inefficient to be useful in this regard.
56(2012); http://dx.doi.org/10.1122/1.3690178View Description Hide Description
We refer to the elongational testing of bread dough using a constant strain rate and specimens of varying initial diameter. Previously we found that 5 mm diameter specimens showed a much larger tensile stress for a given strain and strain rate than larger (20 mm diameter) specimens [Tanner et al., in Proceedings of the 5th International Symposium on Food Rheology and Structure, edited by P. Fischer, M. Pollard, and E. J. Windhab (ETH, Zurich, 2009), pp. 348–351]. It is important to understand this result because if it is true it is not possible to write a simple constitutive equation for the dough. In this paper, we examine the roles of ambient humidity and method of specimen preparation. The latter factor was seen to be crucial to obtaining consistent results in elongational testing of dough with different size specimens; it seems essential to limit pretest deformation to a Hencky strain of about 0.5 (extension ratio of about 1.6). Ambient humidity variations also affected the results, but the apparent shift with diameter was much smaller.
56(2012); http://dx.doi.org/10.1122/1.3687300View Description Hide Description
The effect of copolymer compatibilizer on the rheology of polymer blends, as well as four-roll mill studies of the effect of compatibilizer on coalescence of a pair of drops is reported, with a focus on so-called symmetric systems with equal bulk fluid viscosities and a copolymer with equal block molecular weights. The primary experimental system consists of polybutadiene (PBd) and polydimethylsiloxane(PDMS) as the bulk polymers, with a “symmetric” copolymer with equal molecular weights on each block as compatibilizer. Motivated by earlier work of Martin and Velankar, we performed experiments with PBd as the drop phase and PDMS as the matrix phase and also experiments with the roles of these two materials reversed. The rheological properties for the inverted blends are distinct, as also shown by Martin and Velkankar, and the symmetric copolymer is also shown to have an asymmetric effect in the suppression of drop coalescence when we reverse the materials of drop phase and matrix phase. Two mechanisms can lead to the suppression of drop coalescence: immobilization of the interface due to Marangoni stress leading to slow drainage of the thin film between colliding drops, and steric hindrance of film thinning. At the same copolymer concentrations, we find that the Marangoni effect is the same independent of which fluid is the drop. However, the steric effect arises from the adsorbed copolymer forming an extended polymer brush configuration on the outside of the drop, and for our present system this contribution depends on whether PDMS or PBd is the continuous phase. In general, the conformation and thickness of a polymer brush depends not only on the copolymer concentration on the interface but also the MW of the copolymer block and the molecular weight of the corresponding bulk homopolymer. With PBd drops and PDMS as the continuous phase, the experimental evidence suggests that the brush layer formed by the PDMS block of the copolymer is too thin even at very high copolymer concentrations to produce a steric effect on coalescence. On the other hand, with PDMSdrops and PBd as the continuous phase, the brush thickness external to the drop is sufficiently large at high copolymer concentrations (Γ > Γc), to totally prohibit drop coalescence in the four-roll mill. These studies suggest that it is the presence or absence of an extended brush, along with the effect on average drop size that is responsible for the differences in the rheological properties of the inverted blends.
56(2012); http://dx.doi.org/10.1122/1.3693272View Description Hide Description
In this work, the rheological properties of suspensions of micron-sized gypsum particles dispersed in water were studied in the presence of different fluidizer molecules. The yield stress and the shear moduli were measured versus the volume fraction in the presence of these molecules. Using the same polyelectrolyte with different molecular weights, the dependence of yield stress versus the gyration radius of the polymer was investigated; also different sizes of the gypsum particles allowed to check the size dependence of the yield stress. A particular attention was brought to the change of the thickness of the polymer layer with the volume fraction. From a model, which relates the steric interaction between the two polymer layers to the yield stress and shear modulus, an important compression of the polymer layer with the volume fraction was found. At higher volume fractions a dynamic jamming transition was observed at a critical volume fraction of 0.485 which does not depend on the presence of the fluidizer molecule. Unexpectedly the fluidizer makes this transition to happen at lower shear rates although the yield stress has disappeared.