Volume 59, Issue 2, March 2015
Index of content:
The dynamic motion of single bubbles with unique shapes rising freely in hydrophobically modified alkali-soluble emulsion polymer solutions59(2015); http://dx.doi.org/10.1122/1.4904911View Description Hide Description
The motion of single bubbles rising through hydrophobically modified alkali-soluble emulsion polymer (HASE) solutions is experimentally examined. In this study, we prepare 1.0, 1.1, and 1.6 wt. % HASE solutions and the bubble rise motion is explored depending on the value of the suspending viscosity. Consequently, in a HASE solution with high-viscosity, we find threadlike thin and long branches formed at the trailing edge of the bubble. For moderate-viscosity, we observe bubbles with long and large trailing skirts. These are different from those for Newtonian fluid systems. In the case of low-viscosity, many thread-shaped tails are formed along the bottom edge of the bubble. We observe that threadlike tails irregularly or regularly (unidirectionally) rotate depending on the bubble size along the bottom edge of the bubble. In addition, the bubble rise motion is considered based on modified physical parameters and the terminal bubble rise velocity. In our experimental conditions, it is reported that a jump discontinuity of the terminal bubble rise velocity has not occurred.
Interfacial and rheological properties of PLA/PBAT and PLA/PBSA blends and their morphological stability under shear flow59(2015); http://dx.doi.org/10.1122/1.4905714View Description Hide Description
Blends of 75 wt. % amorphous polylactide (PLA) with 25 wt. % poly[(butylene succinate)-co-adipate] (PBSA) and poly[(butylene adipate)-co-terephthalate] were separately prepared using an internal batch mixer. The morphology and viscoelastic properties of these two blends were analyzed and compared. Annealing did not cause any pronounced morphology changes nor a subsequent modification of the viscoelastic behavior for both blends. However, applying a shear for 20 min at a rate of 0.05 s−1 induced significant droplet coalescence in both blends, although the changes in the viscoelastic response were more prominent in the PLA/PBSA blend. It was also shown that applying a shear for 10 and 20 min at a rate of 0.2 s−1 caused a slight droplet coalescence and minor changes in the viscoelastic behavior of both blends. Moreover, the Palierne model was used to calculate the interfacial tensions between the blend components. It was also utilized to estimate the droplet size after applying annealing and shearing.
A constitutive model for entangled polymers incorporating binary entanglement pair dynamics and a configuration dependent friction coefficient59(2015); http://dx.doi.org/10.1122/1.4905921View Description Hide Description
Following recent work [e.g., J. Park et al., J. Rheol. 56, 1057–1082 (2012); T. Yaoita et al., Macromolecules 45, 2773–2782 (2012); and G. Ianniruberto et al., Macromolecules 45, 8058–8066 (2012)], we introduce the idea of a configuration dependent friction coefficient (CDFC) based on the relative orientation of Kuhn bonds of the test and surrounding matrix chains. We incorporate CDFC into the “toy” model of Mead et al. [Macromolecules 31, 7895–7914 (1998)] in a manner akin to Yaoita et al. [Nihon Reoroji Gakkaishi 42, 207–213 (2014)]. Additionally, we incorporate entanglement dynamics (ED) of discrete entanglement pairs into the new Mead–Banerjee–Park (MBP) model in a way similar to Ianniruberto and Marrucci [J. Rheol. 58, 89–102 (2014)]. The MBP model predicts a deformation dependent entanglement microstructure which is physically reflected in a reduced modulus that heals slowly following cessation of deformation. Incorporating ED into the model allows “shear modification” to be qualitatively captured. The MBP model is tested against experimental data in steady and transient extensional and shear flows. The MBP model captures the monotonic thinning of the extensional flow curve of entangled monodisperse polystyrene (PS) melts [A. Bach et al., Macromolecules 36, 5174–5179 (2003)] while simultaneously predicting the extension hardening found in PS semidilute solutions where CDFC is diluted out [P. K. Bhattacharjee et al., Macromolecules 35, 10131–10148 (2002)]. The simulation results also show that the rheological properties in nonlinear extensional flows of PS melts are sensitive to CDFC but not to convective constraint release (CCR) while those for shear flows are influenced more by CCR. The monodisperse MBP toy model is generalized to arbitrary polydispersity.
Time- and shear history-dependence of the rheological properties of a water-soluble extract from the fronds of the black tree fern, Cyathea medullaris59(2015); http://dx.doi.org/10.1122/1.4905006View Description Hide Description
A shear-thickening polysaccharide extracted from the New Zealand black tree fern was characterized for its time-dependent rheology and the effects of shear-history. The material exhibited antithixotropic behavior at shear rates between 4 and 10 s−1. The parallel growth of normal stresses with time at these shear rates indicated the development of an elastic network under shear. The formation of association structures is proposed to take place via cooperative zipping of stretched chains where associative groups are exposed only on shearing. Hysteresis effects were also observed at shear rates between 4 and 10 s−1. This was probably caused by rapid structure formation at these shear rates, which cannot be easily disrupted at subsequent lower shear rates (<10 s−1). Preshearing at high shear rates (∼1000 s−1) fully stretched out and aligned the polysaccharide chains, exposing all sites for intermolecular association at appropriate lower shear rates.
Simultaneous morphological and rheological measurements on polypropylene: Effect of crystallinity on viscoelastic parameters59(2015); http://dx.doi.org/10.1122/1.4906121View Description Hide Description
A rotational rheometer equipped with an optical module is adopted to characterize the evolution of rheological parameters and at the same time the morphology development during isothermal crystallization. This allowed the determination of the effect of crystallinity on the viscoelastic functions of an isotactic polypropylene. A linear multimode Maxwell model was then applied to obtain the modulus and relaxation time of each mode as a function of the measured crystallinity degree. It was found that at low crystallinity levels, the increase of moduli with crystallinity is about the same for all the modes whereas, when crystallinity degree rises, the increase of moduli is higher for the modes characterized by higher relaxation times. Concerning the relaxation times, it was found that the increase due to crystallinity is about the same for all the modes and reaches a factor of about 10 when relative crystallinity degree is close to 10%. The effect of crystallinity on complex viscosity was also assessed by determining a shift factor due to crystallinity. It was observed that the data collected at the lowest frequencies need higher shift factors with respect to the other ones. This was ascribed to the fact that more than one shift factor is needed to describe the effect on moduli, and low frequencies stir modes with higher relaxation times, which undergo a moduli increase larger than modes with shorter relaxation times.
Unexpected maximum in the concentration dependency of the relaxation frequency of Brownian stress in a colloidal suspension59(2015); http://dx.doi.org/10.1122/1.4906227View Description Hide Description
Concentrated and semidiluted sheared suspensions of silica nanoparticles in Diglycidyl Ether of Bisphenol A have recently been shown to exhibit a low-frequency relaxation process of the shear moduli measured by oscillatory rheology. This process, which is slower than the structural α-process of the matrix, was interpreted as Brownian stress relaxation resulting from strain-induced perturbations of the isotropic filler distribution. In this paper, we extend the rheological investigation of the low-frequency anomaly to ultra-diluted DGEBA/silica suspensions. We illustrate that the Brownian relaxation process depends in a complex manner on the filler volume concentration: For very dilute systems, the relaxation frequency increases with the concentration, whereas for semidilute or concentrated systems, the opposite behavior can be observed. This nonmonotonic dependency of the relaxation frequency leads to a maximum of the relaxation frequency at a volume concentration around 0.133. It can no longer be modeled by Peclet frequencies, since the classical Peclet frequencies depend only on a single concentration dependent physical quantity, viz., the suspension viscosity. A modified Peclet frequency depending on the suspension viscosity and the average surface-to-surface distance between the fillers as a structural, concentration dependent length scale allows for an accurate description of the Brownian relaxation for all concentrations.
59(2015); http://dx.doi.org/10.1122/1.4906621View Description Hide Description
The competition between filler-filler interaction and filler-rubber interaction during the dispersion process of silica-filled rubber has been investigated. Several complementary techniques were carried out going from local observations of the dispersion to a global view given from linear and nonlinear rheological measurements in order to lead to a better estimation of the dispersion kinetics. It has been shown that reinforcement evolves with mixing time. A direct link between the bound rubber amount and reinforcement indicators was found, revealing a replacement of strong filler-filler interactions by weak rubber-filler ones. As a result, rheological reinforcement can be cast under the form of a universal power law by introducing an effective interacting surface between fillers. Finally, a kinetic model of the rubber reinforcement has been developed on the basis of the competition between filler dispersion mechanism and rubber physical adsorption.
59(2015); http://dx.doi.org/10.1122/1.4906543View Description Hide Description
The motion of a spherical Brownian “probe” particle addressed by an external force immersed in shear flow of a colloidal dispersion of spherical neutrally buoyant “bath” particles is quantified. The steady-state nonequilibrium microstructure of bath particles around the probe—induced by the applied force and ambient shear—is calculated to first order in the volume fraction of bath particles, ϕ. The distortion to the equilibrium microstructure caused by the moving probe is characterized by a Péclet number Pef (a dimensionless pulling force), and the distortion due to the shear flow is represented by another Péclet number Pes (a dimensionless shear rate). Matched asymptotic expansions are employed to quantify the microstructure at small Péclet numbers; specifically, within the distinguished limits . The nonequilibrium microstructure is subsequently utilized to compute the average rectilinear velocity of the probe through , for an arbitrary orientation of the external force to the shear flow. Here, Us is the Stokes velocity of the probe in a pure Newtonian fluid. It is also shown that to the torque-free probe simply rotates with the ambient shear; a modification to the angular velocity of the probe is at most . In particular, a probe forced along the flow axis of shear is demonstrated to experience a cross-streamline drift velocity of , to leading order, which acts to propel the particle to streamlines of the ambient shear that move in the same direction as the external force. A mathematical connection between this result and cross-streamline drift of a particle in a Newtonian fluid at small, but nonzero, Reynolds numbers is drawn. The magnitude of the cross-streamline drift velocity is found to be sensitive to the degree of hydrodynamic interactions between the probe and bath particles, which are tuned via an excluded-annulus model. It is also demonstrated that a probe forced along the vorticity axis of the shear experiences a shear-driven enhancement in rectilinear velocity of , to leading order: This nonanalytic dependence originates from the microstructural deformation in the shear dominated (outer) region far from the probe. A connection of this finding to recent work on particle sedimentation in orthogonal shear flow of viscoelastic liquids is discussed.
59(2015); http://dx.doi.org/10.1122/1.4906544View Description Hide Description
A simulation method is proposed to explore the effect of particle size polydispersity in magnetorheology including Brownian motion. The method aims to extend the classical particle-level simulation methodology developed by Klingenberg et al. [J. Chem. Phys. 91, 7888–7895 (1989)] for the case of polydisperse magnetorheological (MR) fluids. The simulation study concerns the aggregation kinetics at rest as well as the rheological behavior under start-up of steady shear and dynamic oscillatory shear tests at increasing strain amplitudes. Results demonstrate that the effect of polydispersity is only relevant at the transition regime between magnetostatic to hydrodynamic control of the suspension structure. The yielding behavior is correlated to the structural characteristics (radial distribution functions, pair correlation functions, and angular connectivities) of the MR fluids before the onset of flow. A more abrupt transition is observed for polydisperse MR fluids because interparticle links are weaker in this case if compared to monodisperse suspensions in spite of the fact that polydisperse MR fluids exhibit a larger connectivity.
59(2015); http://dx.doi.org/10.1122/1.4907743View Description Hide Description
In this work, the rheo-optical response of multiwalled carbon nanotube (MWCNT) suspensions was analyzed. Dichroism was obtained using a polarization-modulation technique in parallel disks and for the first time for these particles in a Couette flow geometry. MWCNTs were dispersed in a Newtonian epoxy matrix, at different concentrations covering the dilute and semidilute regimes. Measurements of dichroism were performed as functions of shear rate and nanotube concentration. Surprisingly, the ultimate average orientation angle with respect to the flow direction was far from zero degree, even at high Peclet (Pe) numbers in very dilute suspensions. To explain this peculiar behavior, a new model for flexible rods, valid in the dilute regime, is proposed. It is based on the development of Strautins and Latz [Rheol. Acta 46, 1057–1064 (2007)] that considers flexible rods made of beads and connectors. We modified their bending potential that allows only straight rods at equilibrium with a harmonic cosine expression. This simple modification changes drastically the behavior of these flexible particles that exhibit a non-negligible orientation in the vorticity direction under steady state.
Semianalytical methods for the determination of the nonlinear parameter of nonlinear viscoelastic constitutive equations from LAOS data59(2015); http://dx.doi.org/10.1122/1.4907976View Description Hide Description
Various viscoelastic constitutive equations have been developed to describe nonlinear viscoelastic flows. Most equations contain two kinds of parameters: Nonlinear and linear ones. The linear parameters correspond to relaxation time spectrum and can be determined from linear viscoelastic data. Meanwhile, the nonlinear parameters cannot be determined by linear viscoelastic data. The determination of the nonlinear parameters requires both reliable nonlinear data and complex procedures for fitting numerical solution of differential equations to the nonlinear data. If an analytical solution of viscoelastic model is available then dramatic reduction of difficulty is expected in the determination of the nonlinear parameters. Previous studies on analytical solution of large amplitude oscillatory shear (LAOS) are based on series expansion which is effective up to only third harmonic. Since it is practically impossible to obtain the analytical solution of higher order than fifth, we suggest a new method which extracts semianalytical solutions (SAS) for some relevant quantities of LAOS from the numerical solutions of nonlinear viscoelastic constitutive equations: The Giesekus model and the Phan-Thien/Tanner model. The SAS includes the effects of higher harmonics which cannot be achieved by low-order series expansion technique. The series expansion is applicable to limited Wi ≪ 1 but any De, while the SAS are applicable to De < 1 but any Wi. The methods developed here are helpful not only for the estimation of nonlinear parameters of viscoelastic models but also for the investigation of the origin of strain-frequency superposition in LAOS.
59(2015); http://dx.doi.org/10.1122/1.4903346View Description Hide Description
Rheologists have expectations for signatures of linear viscoelastic properties, such as shapes of G′(ω) and G″(ω). Medium amplitude (or asymptotically-nonlinear) oscillatory shear (MAOS) provides additional nonlinear rheological information with low dimensional, well-defined material functions [Ewoldt and Bharadwaj, Rheol. Acta 52, 201–209 (2013)]. Here, we develop expectations of signatures (or fingerprints) for the four asymptotically-nonlinear material functions associated with MAOS, [e 1](ω), [e 3](ω), [v 1](ω), [v 3](ω). Although the linear fingerprints may be identical for different models, the asymptotically-nonlinear fingerprints may be different in magnitude, frequency-scaling, curve shapes, and sign changes. To perform the analysis, we collect/translate a library of available analytical strain-controlled MAOS fingerprints for seven different constitutive models. Using this library, we identify general trends and highlight key differences of asymptotic-nonlinear viscoelasticity. Asymptotic nonlinearities for all models considered here obey the terminal regime inter-relations and frequency scaling predicted by Bharadwaj and Ewoldt [J. Rheol. 58, 891–910 (2014)]. Unlike the positive linear viscoelastic measures, at least one of the four asymptotic nonlinearities changes signs with Deborah number (De). Following sign interpretations of Ewoldt and Bharadwaj [Rheol. Acta 52, 201–209 (2013)], we show that nonlinearities tend to be driven by strain-rates at small De, and by strains at large De, a trend observed for nearly all the constitutive models studied here, the exception being the model for dilute rigid dumbbell suspensions of Bird et al. [J. Chem. Phys. 140, 074904 (2014)]. Some constitutive models exhibit multiple sign changes at intermediate De and there may be no universal behavior of asymptotically-nonlinear fingerprints in this regime. Therefore, frequency-dependent signatures can be material-specific. This will allow inverse problems to infer structure, select models, and fit model parameters using asymptotically-nonlinear signatures. To illustrate this aspect, we demonstrate a fingerprint matching exercise with experimental measurements on a transiently cross-linked hydrogel system. We find that currently available model fingerprints can match the qualitative magnitudes and frequency dependence, but not the signs of the experimental transient network response.