Start-up Shear Research Articles

Retraction of "Origin of Stress Overshoot during Startup Shear of Entangled Polymer Melts".

Using Brownian dynamics simulation, we determine the chain orientation and stretching and their connection to stress overshoot in an entangled polymer melt undergoing startup shear at rates lower than the reciprocal of the Rouse time yet higher than the reciprocal of the reptation time. In this rate regime, the prevailing tube theory attributes the stress overshoot to alignment of the primitive chain. In contrast, our results reveal that there is substantial chain stretching that persists well beyond the Rouse time, and it contributes significantly to the initial stress growth. In particular, stress overshoot is found to be primarily due to chain retraction after considerable stretching rather than chain orientation.

Retraction of “Molecular Mechanisms for Conformational and Rheological Responses of Entangled Polymer Melts to Startup Shear”

ADVERTISEMENT RETURN TO ISSUEPREVRetractionNEXTORIGINAL ARTICLEThis notice is a retraction.Retraction of “Molecular Mechanisms for Conformational and Rheological Responses of Entangled Polymer Melts to Startup Shear”Yuyuan Lu, Lijia An*, Shi-Qing Wang*, and Zhen-Gang Wang*Cite this: Macromolecules 2017, 50, 6, 2602Publication Date (Web):March 15, 2017Publication History Published online15 March 2017Published inissue 28 March 2017https://doi.org/10.1021/acs.macromol.7b00493Copyright © 2017 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views1921Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (124 KB) Get e-Alerts Get e-Alerts

Long-Lived Neighbors Determine the Rheological Response of Glasses.

Glasses exhibit a liquidlike structure but a solidlike rheological response with plastic deformations only occurring beyond yielding. Thus, predicting the rheological behavior from the microscopic structure is difficult, but important for materials science. Here, we consider colloidal suspensions and propose to supplement the static structural information with the local dynamics, namely, the rearrangement and breaking of the cage of neighbors. This is quantified by the mean squared nonaffine displacement and the number of particles that remain nearest neighbors for a long time, i.e., long-lived neighbors, respectively. Both quantities are followed under shear using confocal microscopy and are the basis to calculate the affine and nonaffine contributions to the elastic stress, which is complemented by the viscoelastic stress to give the total stress. During start-up of shear, the model predicts three transient regimes that result from the interplay of affine, nonaffine, and viscoelastic contributions. Our prediction quantitatively agrees with rheological data and their dependencies on volume fraction and shear rate.

Structure-rheology relationship for a homogeneous colloidal gel under shear startup

The structural anisotropy and shear rheology of colloidal gels under startup of shear flow are calculated by Brownian dynamics simulations modified to include particle surface-mediated attractions, which is a model for thermoreversible colloidal gels. Shear-induced structural changes are analyzed in both real and reciprocal space through computation of the pair distribution function and structure factor. A distinct structural anisotropy is evident as alignment along the compressional and vorticity axes. A spherical harmonics expansion of pair distribution function is calculated to analyze structural anisotropy. In reciprocal space, structural anisotropy is quantified through an alignment factor, which shows overshoot behavior similar to the stresses. Based on the microstructure analysis, the evolution of the structural anisotropy is explained by the anisotropic rupture of the colloidal gel microstructure. This result provides evidence for how shearing creates structural anisotropy. The structural anisotro...

Nonlinear relaxation modulus via dual-frequency medium amplitude oscillatory shear (MAOS): General framework and case study for a dilute suspension of Brownian spheroids

A framework for determining the first nonlinear relaxation modulus of a viscoelastic fluid from a medium-amplitude oscillatory shear (MAOS) deformation is constructed. Knowledge of this “MAOS relaxation modulus” allows one to predict the weakly nonlinear stress response of a material under an arbitrary transient deformation via a memory integral expansion. Our framework is demonstrated by explicitly determining the MAOS relaxation modulus for a dilute suspension of Brownian spheroids subject to a dual-frequency oscillatory shear flow. Specifically, we first calculate the second normal stress difference for such a deformation from a corotational memory integral expansion. Second, the microstructural stress response of the model system of Brownian spheroids is determined via a regular perturbation expansion of the orientation distribution function at small dimensionless strain-rate amplitude, or Weissenberg number. An analytical expression for the MAOS relaxation modulus is resolved by comparing the second normal stress difference results of the memory integral expansion and microstructural stress calculation. Finally, using the MAOS relaxation modulus, we reconstruct the stress response of the model system for the start-up and cessation of simple shear and uniaxial extension. In summary, our work offers an approach to utilizing medium (and large) amplitude oscillatory shear results to predict stress dynamics of viscoelastic fluids in other transient, nonlinear flows.

Structural fingerprints of yielding mechanisms in attractive colloidal gels.

Core-Modified Dissipative Particle Dynamics (CM-DPD) with a modified depletion potential and full hydrodynamics description is used to study non-equilibrium properties of colloidal gels with short range attraction potentials at an intermediate volume fraction (ϕ = 0.2) under start-up shear deformation. Full structural and rheological analysis using the stress fabric tensor complemented by bond number and bond distribution evolution under flow reveals that similarly to dilute colloidal gels, flow-induced anisotropy and strain-induced stretching of bonds are present during the first yielding transition. Unlike in low volume fraction depletion gels however, a small fraction of bond dissociation is required to facilitate bond rotation at intermediate volume fractions. The strain at which structural stretching and anisotropy in bond distribution emerge coincides with the first maximum in the shear stress (first yielding transition). At higher strains, depending on flow strength, a second peak in stress signal appears which is attributed to the compaction and melting of clusters. In this work, for the first time we provide evidence that multibody hydrodynamic interactions are essential to predict the correct dynamics of depletion gels under flow, namely two-step yielding process.

Shear banding in Doi–Edwards fluids

We here extend to integral constitutive equations of the Doi–Edwards type the analysis of shear banding instabilities during shear startup recently performed by Moorcroft and Fielding [J. Rheol. 58, 103–147 (2014)]. Shear banding is the instability encountered in simple shear flows, whereby different shear rates coexist along the sample thickness, so that the velocity profile departs from the usually assumed linear profile. Results of both linear and nonlinear stability analysis mostly confirm those obtained by Moorcroft and Fielding [J. Rheol. 58, 103–147 (2014)] with the Rolie–Poly equation [Likhtman, A. E., and R. S. Graham, J. Non-Newtonian Fluid Mechan. 114, 1–12 (2003)] (which is an approximate differential counterpart of the Doi–Edwards integral one “[The Theory of Polymer Dynamics (Clarendon, Oxford, 1986)]”), but some significant differences are also found. Features that coincide with those predicted by the Rolie–Poly equation include: (i) For the nonmonotonic flow curve of the original Doi–Edwar...

Entangled Linear Polymer Solutions at High Shear: From Strain Softening to Hardening

The present rheological study reveals for the first time that entangled polymer solutions made of linear polystyrene or poly(methyl methacrylate) can exhibit strain hardening due to non-Gaussian stretching during startup shear at sufficiently high rates and temperatures well above their overall glass transition temperatures: Tg,solute > Texp > Tg,solution. The solutions made of high-Tg polymers only show partial yielding in the sense that both shear and normal stresses grow monotonically in time until a point of rupture, signified by an emergent cusp in the stress vs strain curve and macroscopic breakup along a shear plane. The shear softening-to-hardening transition, which occurs as a function of the applied shear rate, happens at lower equivalent rate with decreasing temperature, violating the time–temperature superposition principle.

A force-level theory of the rheology of entangled rod and chain polymer liquids. I. Tube deformation, microscopic yielding, and the nonlinear elastic limit.

We employ a first-principles-based, force-level approach to construct the anharmonic tube confinement field for entangled fluids of rigid needles, and also for chains described at the primitive-path (PP) level in two limiting situations where chain stretch is assumed to either be completely equilibrated or unrelaxed. The influence of shear and extensional deformation and polymer orientation is determined in a nonlinear elastic limit where dissipative relaxation processes are intentionally neglected. For needles and PP-level chains, a self-consistent analysis of transverse polymer harmonic dynamical fluctuations predicts that deformation-induced orientation leads to tube weakening or widening. In contrast, for deformed polymers in which chain stretch does not relax, we find tube strengthening or compression. For all three systems, a finite maximum transverse entanglement force localizing the polymers in effective tubes is predicted. The conditions when this entanglement force can be overcome by an externally applied force associated with macroscopic deformation can be crisply defined in the nonlinear elastic limit, and the possibility of a "microscopic absolute yielding" event destroying the tube confinement can be analyzed. For needles and contour-relaxed PP chains, this force imbalance occurs at a stress of order the equilibrium shear modulus and a strain of order unity, corresponding to a mechanically fragile entanglement tube field. However, for unrelaxed stretched chains, tube compression stabilizes transverse polymer confinement, and there appears to be no force imbalance. These results collectively suggest that the crossover from elastic to irreversible viscous response requires chain retraction to initiate disentanglement. We qualitatively discuss comparisons with existing phenomenological models for nonlinear startup shear, step strain, and creep rheology experiments.

Nonlinear Rheology of Random Sulfonated Polystyrene Ionomers: The Role of the Sol–Gel Transition

The linear and nonlinear rheological behaviors of nonentangled sulfonated polystyrene (SPS) ionomers near the sol–gel transition were studied. When the degree of sulfonation, p, was below the gel point, the ionomer exhibited sol-like linear viscoelastic (LVE) behavior, and shear thinning was observed for steady shear flow. For p close to the gel point, the ionomer showed power-law-like LVE behavior over a wide frequency range. Strain hardening and shear thickening behavior were observed, and their magnitudes depended on the temperature, molecular weight of the PS precursor, and the Coulomb energy of the ion pair. Above the gel point, a distinct rubbery plateau was observed in the dynamic modulus. Melt fracture occurred upon start-up shear, which prevented quantitative examination of the nonlinear rheology. The possible mechanisms for strain hardening and shear thickening near the gel point are discussed with respect to formation of large clusters that nearly percolate in space.

The relationship between rheological behavior and microstructure of nanocomposite based on PA6/NBR/clay

Microstructure, rheological properties and their relationships of PA6/NBR/nano‐clay nanocomposite have been investigated. Scanning electron microscopy (SEM) and transmission electron microscopy imaging techniques were used to study micro‐structure. The nano‐clay dispersion was measured by small angle X‐ray diffraction. Frequency sweep, steady shear, startup shear transient, and startup shear free transient experiments were carried out to study rheological characteristics of nanocomposite. SEM micrograph revealed that nano‐clay decreased the size of rubber droplets to a half in comparison with samples without nano‐clay. Storage modulus of nanocomposite containing 7% wt nano‐clay exhibited frequency independent behavior because of physical network which formed in matrix, in contrast to pure PA6 which showed terminal storage modulus at low frequencies. Shear thinning at high shear rates was observed by addition of rubber to PA6 matrix caused by deformation of rubber droplets. Nano‐clay orientation in shear flow field accelerated non‐Newtonian behavior in steady shear experiment. Transient shear viscosity studies were used for measurement the strength of nano‐clay physical network. It was found that network breakdown is more stress consuming factor in comparison with nano‐particle orientation and hydrodynamic forces. Elongation transient viscosity studies showed that critical Hencky strain for linear–nonlinear viscoelastic transition shifted to higher strains by increasing nano‐clay concentration. POLYM. COMPOS., 39:2403–2410, 2018. © 2016 Society of Plastics Engineers

Evaluating Rigid and Semiflexible Fiber Orientation Evolution Models in Simple Flows

As American vehicle fuel efficiency requirements have become more stringent due to the CAFE standards, the auto industry has turned to fiber reinforced polymer composites as replacements for metal parts to reduce weight while simultaneously maintaining established safety standards. Furthermore, these composites may be easily processed using established techniques such as injection molding and compression molding. The mechanical properties of these composites are dependent on, among other variables, the orientation of the fibers within the part. Several models have been proposed to correlate fiber orientation with the kinematics of the polymer matrix during processing, each using various strategies to account for fiber interactions and fiber flexing. However, these all require the use of empirical fitting parameters. Previous work has obtained these parameters by fitting to orientation data at a specific location in an injection-molded part. This ties the parameters to the specific mold design used. Obtaining empirical parameters is not a trivial undertaking and adds significant time to the entire mold design process. Considering that new parameters must be obtained any time some aspect of the part or mold is changed, an alternative technique that obtains model parameters independent of the mold design could be advantageous. This paper continues work looking to obtain empirical parameters from rheological tests. During processing, the fiber–polymer suspension is subjected to a complex flow with both shear and extensional behavior. Rather than use a complex flow, this study seeks to isolate and compare the effects of shear and extension on two orientation models. To this end, simple shear and planar extension are employed, and the evolution of orientation from a planar random initial condition is tracked as a function of strain. Simple shear was imparted using a sliding plate rheometer designed and fabricated in-house. A novel rheometer tool was developed and fabricated in-house to impart planar extension using a lubricated squeeze flow technique, where a low-viscosity Newtonian lubricant is applied to the solid boundaries to minimize the effect of shearing due to the no-slip boundary condition. The Folgar–Tucker model with a strain reduction factor is used as a rigid fiber model and compared against a bead–rod model (a semiflexible model) proposed by Ortman. Both models are capable of predicting the data, with the bead–rod model performing slightly better. Orientation occurs at a much faster rate under startup of planar extension and also attains a much higher degree of flow alignment when compared with startup of steady shear.

Rheological properties of nanocrystalline cellulose suspensions

Rheological behavior, including linear and nonlinear, as well as transient rheology of nanocrystalline cellulose (NCC) suspensions was studied in this work. Two kinds of polymer solutions, aqueous poly(vinyl alcohol) (PVA) with flexible chain structure and aqueous carboxymethyl cellulose (CMC) with semi-rigid chain structure, were used as the suspension media to further explore the role that the interactions among NCC and polymers played during shear flow. The results reveal that NCC has lower values of percolation threshold in the PVA solution than in the CMC one during small amplitude oscillatory shear (SAOS) flow because the flexible PVA chain has higher adsorbed level onto NCC particles than the negatively charged semi-rigid CMC chain, which is further confirmed by the Fourier transformed infrared (FT-IR) spectroscopy tests. As a result, the NCC suspension shows a weak strain overshoot in PVA solution during large amplitude oscillatory shear (LAOS) flow, which cannot be seen on the one in CMC solution. During startup shear flow, both of these two suspensions show evident stress overshoot behavior with the strain-scaling characteristics, indicating the formation of ordered long-term structure of rod-like NCC particles with self-similarity during flow. However, NCC suspension have far stronger stress overshoot response in CMC solution relative to the one in PVA solution. A possible synergy mechanism between NCC and CMC chain is hence proposed.

Shear banding in large amplitude oscillatory shear (LAOStrain and LAOStress) of polymers and wormlike micelles

We investigate theoretically shear banding in large amplitude oscillatory shear (LAOS) of polymers and wormlike micelles. In LAOStrain we find banding at low frequencies and sufficiently high strain rate amplitudes in fluids for which the underlying constitutive curve of shear stress as a function of shear rate is non-monotonic. This is the direct analogue of quasi steady state banding seen in slow strain rate sweeps along the flow curve. At higher frequencies and sufficiently high strain amplitudes we report a different but related phenomenon, which we call `elastic' shear banding. This is associated with an overshoot in the elastic (Lissajous-Bowditch) curve of stress as a function of strain. We suggest that this may arise widely even in fluids that have a monotonic underlying constitutive curve, and so do not show steady state banding under a steadily applied shear flow. In LAOStress we report banding in fluids that shear thin strongly enough to have either a negatively, or weakly positively, sloping region in the underlying constitutive curve, noting again that fluids in the latter category do not display steady state banding in a steadily applied flow. This banding is triggered in each half cycle as the stress magnitude transits the region of weak slope in an upward direction, such that the fluid effectively yields. Our numerics are performed in the Rolie-poly model of polymeric fluids, but we also provide arguments suggesting that our results should apply more widely. Besides banding in the shear rate profile, which can be measured by velocimetry, we also predict banding in the shear and normal stress components, measurable by birefringence. As a backdrop to understanding the new results on shear banding in LAOS, we also briefly review earlier work on banding in other time-dependent protocols, focusing in particular on shear startup and step stress.

Triggers and signatures of shear banding in steady and time-dependent flows

This précis is aimed as a practical field guide to situations in which shear banding might be expected in complex fluids subject to an applied shear flow. Separately for several of the most common flow protocols, it summarizes the characteristic signatures in the measured bulk rheological signals that suggest the presence of banding in the underlying flow field. It does so both for a steady applied shear flow and for the time-dependent protocols of shear startup, step stress, finite strain ramp, and large amplitude oscillatory shear. An important message is that banding might arise rather widely in flows with a strong enough time dependence, even in fluids that do not support banding in a steadily applied shear flow. This suggests caution in comparing experimental data with theoretical calculations that assume a homogeneous shear flow. In a brief postlude, we also summarize criteria in similar spirit for the onset of necking in extensional filament stretching.

Curve-fitting equation for prediction of the start-up stress overshoot of an oil-based drilling fluid

Drilling fluids usually gel at rest in order to avoid cuttings to precipitate over the drill bit when circulation is interrupted. At flow start-ups, pumping pressures higher than the steady-state circulation pressure are usually required to surpass the gel strength. The gel-liquid transition may have significant importance, especially in ultra-deep waters where high pressures and low temperatures take place. In the current work, controlled shear rate rheometric tests were conducted to investigate the yielding of an oil based drilling fluid. An algebraic equation, that accounts for both shear rate and shear history, is proposed to predict the gel breaking. This equation requires less fitting parameters than the current structural kinetic models available in literature, and is quite useful to represent the pressure peaks that take place during drilling fluid flow start-ups. The equation fit to rheometric data and is able to predict satisfactorily the start-up shear stress of a gelled drilling fluid at different shear rates.

Flow of DNA in micro/nanofluidics: From fundamentals to applications.

Thanks to direct observation and manipulation of DNA in micro/nanofluidic devices, we are now able to elucidate the relationship between the polymer microstructure and its rheological properties, as well as to design new single-molecule platforms for biophysics and biomedicine. This allows exploration of many new mechanisms and phenomena, which were previously unachievable with conventional methods such as bulk rheometry tests. For instance, the field of polymer rheology is at a turning point to relate the complex molecular conformations to the nonlinear viscoelasticity of polymeric fluids (such as coil-stretch transition, shear thinning, and stress overshoot in startup shear). In addition, nanofluidic devices provided a starting point for manipulating single DNA molecules by applying basic principles of polymer physics, which is highly relevant to numerous processes in biosciences. In this article, we review recent progress regarding the flow and deformation of DNA in micro/nanofluidic systems from both fundamental and application perspectives. We particularly focus on advances in the understanding of polymer rheology and identify the emerging research trends and challenges, especially with respect to future applications of nanofluidics in the biomedical field.

Start-up shear of concentrated colloidal hard spheres: Stresses, dynamics, and structure

The transient response of model hard sphere glasses is examined during the application of steady rate start-up shear using Brownian dynamics simulations, experimental rheology and confocal microscopy. With increasing strain, the glass initially exhibits an almost linear elastic stress increase, a stress peak at the yield point and then reaches a constant steady state. The stress overshoot has a nonmonotonic dependence with Peclet number, Pe, and volume fraction, φ, determined by the available free volume and a competition between structural relaxation and shear advection. Examination of the structural properties under shear revealed an increasing anisotropic radial distribution function, g(r), mostly in the velocity-gradient (xy) plane, which decreases after the stress peak with considerable anisotropy remaining in the steady-state. Low rates minimally distort the structure, while high rates show distortion with signatures of transient elongation. As a mechanism of storing energy, particles are trapped within a cage distorted more than Brownian relaxation allows, while at larger strains, stresses are relaxed as particles are forced out of the cage due to advection. Even in the steady state, intermediate super diffusion is observed at high rates and is a signature of the continuous breaking and reformation of cages under shear.

Viscoelasticity and nonlinear simple shear flow behavior of an entangled asymmetric exact comb polymer solution

We report upon the characterization of the steady-state shear stresses and first normal stress differences as a function of shear rate using mechanical rheometry (both with a standard cone and plate and with a cone partitioned plate) and optical rheometry (with a flow-birefringence setup) of an entangled solution of asymmetric exact combs. The combs are polybutadienes (1,4-addition) consisting of an H-skeleton with an additional off-center branch on the backbone. We chose to investigate a solution in order to obtain reliable nonlinear shear data in overlapping dynamic regions with the two different techniques. The transient measurements obtained by cone partitioned plate indicated the appearance of overshoots in both the shear stress and the first normal stress difference during start-up shear flow. Interestingly, the overshoots in the start-up normal stress difference started to occur only at rates above the inverse stretch time of the backbone, when the stretch time of the backbone was estimated in analogy with linear chains including the effects of dynamic dilution of the branches but neglecting the effects of branch point friction, in excellent agreement with the situation for linear polymers. Flow-birefringence measurements were performed in a Couette geometry, and the extracted steady-state shear and first normal stress differences were found to agree well with the mechanical data, but were limited to relatively low rates below the inverse stretch time of the backbone. Finally, the steady-state properties were found to be in good agreement with model predictions based on a nonlinear multimode tube model developed for linear polymers when the branches are treated as solvent.

Polylactide/acetylated nanocrystalline cellulose composites prepared by a continuous route: A phase interface-property relation study

A ‘continuous route’ was developed in this work for the preparation of nanocrystalline cellulose (NCC) filled polylactide (PLA) composites. It combines several separated steps, including extraction of NCC, surface acetylation of NCC, and final composite preparation, into a continuous process, without traditional freeze drying. The obtained PLA composites were then studied in terms of phase interface structure, rheological and mechanical properties. The results reveal that surface acetylation of NCC can improve its affinity to PLA evidently. The thickened interfacial layer makes the system filled with modified NCC show lower percolation threshold than the one filled with pristine NCC; and the former presents a typical strain-scaling stress overshoot behavior in the start-up shear flow because the network structure of modified NCC presents stronger characteristics of self-similarity. The phase interface adhesion also plays an important role in the mechanical behavior of PLA/NCC composites, which is further revealed by the nanomechanical analysis using atom force microscopy.