Volume 57, Issue 4, July 2013
Index of content:
57(2013); http://dx.doi.org/10.1122/1.4811184View Description Hide Description
57(2013); http://dx.doi.org/10.1122/1.4808439View Description Hide Description
This note is concerned about the circumstances, under which flow induced crystallization can take place. In first instance, a discrimination must be made between two temperature ranges. One of these ranges is between the equilibrium melting point of ideal crystals and the melting temperature of spherulitic structures. The other range is below the temperature, where spherulites melt. A second question is, whether one has to do with homogeneous (sporadic) nucleation or with heterogeneous nucleation on so-called defects. In other words, does flow actually cause sporadic nucleation in a homogeneous medium or does it merely activate kind of already existing dormant entities. A third point is the mechanism of the growth of nuclei. The applied specific work is of importance.
Steady states in extensional flow of strain hardening polymer melts and the uncertainties of their determination57(2013); http://dx.doi.org/10.1122/1.4803932View Description Hide Description
The existence of a plateau, a maximum or even an overshoot of the elongational viscosity of polymer melts as a function of time or extension, respectively, is a hot topic of scientific discussions. Although not essential for the relation of elongational properties to applications, the feature is crucial for a verification of various theories. From a critical review of sophisticated comparative stressing and creep experiments on a low density polyethylene (LDPE), the existence of a pronounced stationary elongational viscosity becomes very probable. These conclusions are supported by investigations on a long-chain branched polypropylene. For an LDPE of higher molar mass with stress maxima caused by local necking within the sample, the maximum viscosities agree with the corresponding steady-state viscosities obtained from creep experiments performed by the same rheometer. The decisive role of the uniformity of the sample deformation for the accuracy of elongational experiments is pointed out. From the findings presented, it becomes evident that a careful control of the sample uniformity is the basic condition for a critical assessment of differing results from the various experimental methods found in the literature.
57(2013); http://dx.doi.org/10.1122/1.4804198View Description Hide Description
We report on the shear flow start-up and the relaxation upon flow cessation of anionically synthesized comb polymers of different chemistries. The experimental data, obtained with a cone partitioned-plate geometry in order to avoid artifacts, showed that the start-up shear flow of combs exhibits systematic dependencies on the branching structure. They were interpreted by invoking dynamic dilution and hierarchical relaxation, which are known to control the linear viscoelastic response. For all combs studied here, the backbones remained entangled after dynamic dilution due to branch relaxation. We combined the important molecular parameters (i.e., the number and molar mass of the branches) into a single parameter, the number of entanglements of the dynamically diluted backbone, ZBB DIL., which we found to be the main scaling parameter for the observed nonlinear flow behavior. The steady viscosities as function of Weissenberg number were less shear-thinning compared to linear analogues, and the higher the amount of branching, the less thinning they became, reflecting a broader relaxation spectrum, and being consistent with the behavior of commercial branched polymers. The strain at maximum viscosity was higher for combs in comparison to linear polymers, a finding attributed to nonlinear hierarchical relaxation. The maximum in viscosity (scaled with steady viscosity) became lower with increased degree of branching due to the action of dynamic dilution. The viscosity peaks became broader for combs with an increased degree of branching, which is again a reflection of a broader relaxation spectrum. The initial relaxation rate upon cessation of steady shear increased with shear rate and seemed to reflect the loss of entanglements of combs in steady shear due to the action of convective constraint release. The relaxation was found to be independent of branching structure, suggesting that for the time ranges considered here, the loss of orientation of the backbones scales with the longest relaxation time, and is hence an effect of linear relaxation mechanisms (i.e., mainly reptation of the backbone).
57(2013); http://dx.doi.org/10.1122/1.4804358View Description Hide Description
This work is devoted to the stick–slip instabilities that appear in the shear flow of highly concentrated suspensions of magnetic microparticles. The effect of the applied magnetic field strength was analyzed in details. With this aim, homogeneous suspensions of iron microparticles with concentration near the limit of maximum-packing fraction were prepared, and shear-flow measurements were performed in a controlled-rate mode using a rheometer provided with a rough parallel-plate geometry. For each given value of the shear rate, the time evolution of the shear stress was monitored for at least 20 min. Saw-tooth-like stress oscillations, typical of stick–slip instabilities, were obtained at low enough shear rate values. The measurements were restricted to small enough oscillations, at which the rheometer was still able to maintain the shear rate constant. From the microscopic viewpoint, these stick–slip instabilities principally appear due to the periodic failure and healing of the field-induced particle structures, as inferred from experimental observations. This hypothesis is corroborated by a theoretical model developed on the basis of the balance of the magnetic and hydrodynamic torques over the particle structures, allows us to predict the correct order of magnitude of the main parameters of the stick–slip instabilities, including the amplitude and period of the stress oscillations.
57(2013); http://dx.doi.org/10.1122/1.4810019View Description Hide Description
In this work, we studied a pressure-driven flow of a magnetorheological suspension through a cylindrical tube in the presence of a nonuniform magnetic field perpendicular to the tube and varying along its axis. The flow was realized with the help of a commercial capillary rheometer in a controlled-velocity mode. Experimental pressure–flow rate curves exhibited a local minimum, and flow instabilities were observed in the range of flow rates corresponding to the decreasing branch of these curves. The nonmonotonic behavior of the flow curves is attributed to the interplay between the hydrodynamic dissipation and the interaction between particle aggregates and walls. Our theoretical model, based on the particle flux conservation, correctly predicts the shape of the pressure–flow rate curves and indicates the speed range within which flow instabilities are expected. These instabilities are manifested by somewhat regular oscillations of the pressure difference and of the outlet flow rate at a constant imposed piston speed. Visualization of particle structures in a transparent tube revealed that the flow oscillations were governed by both the suspension compressibility and the stick–slip of the aggregates on the tube walls. This study is motivated by the problem of particle clogging in magnetorheological smart devices employing nonuniform magnetic fields.
57(2013); http://dx.doi.org/10.1122/1.4805093View Description Hide Description
Nonlinear oscillation shear has become an important method to study the complex fluids. However, choosing the suitable material functions is not as simple as that in linear viscoelasticity. A framework is suggested in this work to account for the stress–strain and stress–strain rate relationship based on the concept of mean stress and mean strain (rate) in the Lissajous curves. The applications of such framework in an imposed oscillatory shear strain and an imposed oscillatory shear stress are clearly demonstrated. The intracycle nonlinear modulus and viscosity are defined from the slope of the stress–strain curve and the stress–strain rate curve, respectively. The intercycle nonlinear behaviors are obtained from the strain (rate) amplitude dependence of the zero mean strain (rate) modulus or viscosity. We justify the strain-hardening/softening and shear-thickening/thinning behaviors of nonlinear moduli and viscosities by using typical constitutive models like Bingham model and the Maxwell model. It is found that the modulus (viscosity) defined from the stress–mean strain (rate) curves is the most physical reasonable quantity. In addition, we apply the new analysis method to two yield stress fluids, which reveals critical balance between aging and shear rejuvenation.
57(2013); http://dx.doi.org/10.1122/1.4807857View Description Hide Description
We examine carefully the accuracy of stochastic rotational dynamics (SRD) simulations for isolated polymer chains in a solvent, where SRD incorporates hydrodynamic interaction (HI) through momentum exchange (collisions) between polymer beads and solvent beads, both of which are assigned mass. We show that the main error is due to the inertial effect that finite bead mass has on polymer hydrodynamics. We find that the inertial effect is negligible when , the radius of gyration of the polymer chain is much larger than , the distance over which bead inertia is lost due to collisions with solvent. For moderate HI, good agreement is found between the rotational relaxation time simulated by SRD with that from normal-mode analysis and from Brownian dynamics (BD) simulations, even for short five-bead chains. For dominant HI, for short chains, we can minimize the inertial effect by varying the ratio of polymer to solvent bead mass. For long chains ( ) SRD and BD relaxation times agree, but are larger than those from normal-mode analysis due to neglect of fluctuating HI in the latter. We also find that, using the same parameters, the SRD method can reproduce the BD results obtained by Jendrejack et al. for a λ-DNA chain in viscosified water.
57(2013); http://dx.doi.org/10.1122/1.4808054View Description Hide Description
We present a detailed comparison of the rheology of concentrated hard and soft-sphere suspensions using a variety of techniques including large-amplitude oscillatory shear (LAOS). While the soft spheres are jammed and exhibit permanent contact, the hard-sphere suspensions are below close packing where particle collisions lead to an effective modulus. Oscillatory shear measurements are used to determine the strain-dependent viscoelastic moduli and yield stress. A recent scheme is applied to interpret LAOS data in terms of a sequence of physical processes [Rogers et al., J. Rheol. 55, 435–458 (2011a)], revealing different characteristics of yielding, flow, and structural rejuvenation in the two systems. While for hard spheres, yielding and flow are governed by the breaking and rejuvenation of the nearest neighbor cage; for soft spheres, the particle compliance gives rise to a much more gradual yielding. We address the effect of particle softness directly by measuring the single-particle modulus with atomic force microscopy, and linking it to the suspension modulus via the pair correlation function determined by microscopy.
57(2013); http://dx.doi.org/10.1122/1.4808411View Description Hide Description
In this work, we extend the classical analysis of concentration fluctuations in polymer solutions under shear flow to consider the same phenomenology under extensional flow. Experimental work by van Egmond and Fuller [Macromolecules 26, 7182–7188 (1993)] revealed a four-lobe scattering pattern for a polystyrene solution in a planar extensional flow field. Similar to earlier results found in shear, they find the existence of finite-wavelength peak intensity locations. To investigate this phenomenon, we couple stress and concentration using a two-fluid model with fluctuations driven by thermal noise incorporated through a canonical Langevin approach. The polymer stress is governed by the Rolie–Poly model augmented with finite extensibility to account for large stretching of chains at high Weissenberg numbers. Perturbing the equations about homogeneous planar extensional flow for weak amplitude inhomogeneities, but arbitrary flow strength, we solve for the steady correlations. The resulting structure factor undergoes a pattern transition for increased strain rates. At small Weissenberg numbers, we predict fluctuation enhancement along the stretching axis (abnormal butterfly pattern). At , a four-lobe substructure emerges accompanied by finite-wavelength scattering peaks located along the 45° axis, as observed in experiment. However, these peaks rotate toward the compression axis under the increasing effect of advection as the flow strength increases, and, at very large stretching rates, fluctuation enhancement occurs along the compression axis (normal butterfly pattern). These latter changes were not seen experimentally. In addition, the peak intensity varies nonmonotonically with Weissenberg number exhibiting two local maxima at extension rates corresponding to the inverse reptation and Rouse times, respectively, providing a clear relationship between the extensional rheology and the concentration fluctuation spectrum.
57(2013); http://dx.doi.org/10.1122/1.4809732View Description Hide Description
We study the local and global rheology of non-Brownian suspensions in a solvent that is not density-matched, leading to either creaming or sedimentation of the particles. Both local and global measurements show that the incomplete density matching leads to the appearance of a critical shear rate above which the suspension is homogenized by the flow, and below which sedimentation or creaming happens. We show that the value of the critical shear rate and its dependence on the experimental parameters are governed by a competition between the viscous and the gravitational forces, and present a simple scaling model that agrees with the experimental results from different types of experiments (local and global) in different setups and systems.
57(2013); http://dx.doi.org/10.1122/1.4811477View Description Hide Description
We present experimental data and numerical modeling of a nonlinear phenomenon in active magnetic microbead rheology that appears to be common to entangled polymer solutions (EPS). Dynamic experiments in a modest range of magnetic forces show (1) a short-lived high viscosity plateau, followed by (2) a bead acceleration phase with a sharp drop in apparent viscosity, and (3) a terminal steady state that we show resides on the shear-thinning slope of the steady-state flow curve from cone and plate data. This latter feature implies a new protocol to access the nonlinear steady-state flow curve for biological EPS available only in microliter-scale volumes. We use the moment-closure form of the Rolie–Poly kinetic model for EPS hydrodynamics, together with a decoupling approximation that obviates the need for a full three-dimensional (3D) flow solver, to qualitatively reproduce this dynamic experimental sequence. We thereby explain the phenomenon in terms of entangled polymer physics, and show how the nonlinear event (acceleration and termination on the shear-thinning response curve) is tunable by the interplay between molecular-scale mechanisms (relaxation via reptation and chain retraction) and magnetic force controls. The experimental conditions mimic movement of cilia tips, bacteria, and sperm in mucus barriers, implying a physiological relevance of the phenomenon and compelling further quantitative kinetic-flow 3D numerical modeling.
57(2013); http://dx.doi.org/10.1122/1.4802052View Description Hide Description
Bingham plastics exhibit complex behaviors, depending on both geometrical and rheological factors, and are difficult to characterize systematically. This is particularly true in the case of transient flows, where solidlike and fluidlike behaviors coexist in an intermittent fashion. The aim of this contribution is to study the slump of Bingham columns under gravity, while varying systematically and independently both the geometry of the system and the rheological parameters. To do so, numerical experiments are carried out in two dimensions with a non-Newtonian Navier–Stokes code, the Gerris flow solver, using a volume-of-fluid approach. We are able to determine the slump height and the spreading of the column after motion ceased. These characteristics are related to the rheological properties and initial shape through scaling relationships. The results are compared with previous scalings and prediction from the literature. A discussion ensues on the importance of the normalization choice and of unambiguous discrimination between geometrical and material factors.