Polymer Stretch Research Articles

An open-source anisotropic k−ε−v2−f model for turbulent viscoelastic duct flows

A novel open-source anisotropic k−ε−v2−f model is presented for turbulent viscoelastic duct flow with dilute polymeric solutions described by the finitely extensible nonlinear elastic-Peterlin constitutive model. The turbulence model for channel and square duct flow of Newtonian fluids is adapted to incorporate the polymeric terms within the governing equations. All the required non-linear terms are validated with simple closure models and are assessed a priori against independent direct numerical simulation data in fully developed channel flow. The NLTij term, which accounts for the interaction between fluctuating components of the conformation tensor and the velocity gradient tensor, is modeled with the mean flow direction, ti, and wall-normal, ni, present in the Newtonian model, based on the streamwise alignment of mean polymer stretch. The implicit polymer effects on pressure–strain are assessed with a simple ad hoc closure accounting for the reduced near-wall production of turbulent kinetic energy. The same closure is also adapted for the spanwise Reynolds stress predictions of polymer-enhanced secondary flow. The model performs well in channel flow and captures low, intermediate, and high drag reduction features for a wide range of rheological parameters. The capabilities are extended for square ducts (or any regular polygon) due to the symmetric modeling of the closure models, which can predict the mean streamwise and secondary flow features associated with second normal Reynolds stress differences. Accessible codes and models are crucial for the advancement and improvement of turbulent viscoelastic models, and an OpenFOAM C++ code package is developed and freely available on GitHub (https://github.com/MikeMcDermott-Code/v2f).

Mechanism of coupling polymer thickness and interfacial interactions on strength and toughness of non-covalent nacre-inspired graphene nanocomposites

Nanoconfinement resulting from interfacial attraction leads to the formation of polymer interphases with outstanding stiffness, which profoundly impacts the toughness and strength of non-covalent nacre-inspired nanocomposites with “brick-and-mortar” structures. Nonetheless, challenges arise due to discrepancies in interphase thickness characterization and ambiguity in nanoscopic foundations of interfacial stiffening, which hinders systematic understanding of strengthening and toughening mechanisms. To address these issues, we conduct coarse-grained molecular dynamics simulations to investigate the effect of polymer phase thickness and interfacial strength on the toughness and strength of graphene-polyethylene nanocomposites. Our results indicate that when the interfacial adhesion is equal to 0.12 J/m2, nanocomposites possessing a polyethylene phase thickness of 3 nm exhibit significantly greater yield strength (590.8 ± 3.3 MPa) than those of other thicknesses, while nanocomposites with polyethylene phase thickness of 13 nm exhibit the highest toughness (83.3 ± 1.7 MJ/m3). When the interfacial interaction adhesion increases from 0.12 to 1.75 J/m2, both the yield strength and toughness of nanocomposites with polymer thickness 3 nm experience a three-phase increase, reaching up to 1442.3 ± 107.7 MPa and 119.8 ± 8.4 MJ/m3, respectively. The aforementioned improvement in both strength and toughness can be attributed to the nanoconfinement originating from strong interfacial adhesion, which enhances the stiffness and strength of the polymer glues filling the gaps of graphene “bricks”. Furthermore, the deviation of the simulation results from the shear-lag model with mechanical properties of bulk polymer can be remedied with the nanoconfinement effect and polymer stretch taken into account. Our findings provide timely guidance for future design of non-covalent nacre-inspired graphene-based polymer nanocomposites with high toughness and strength.

Switchable Polyacrylic Acid Polyelectrolyte Brushes for Surface Plasmon Resonance Applications.

Imaging wide-field surface plasmon resonance (SPR) microscopy sensors based on polyacrylic acid polyelectrolyte brushes (PAA PEBs) were designed to enhance the sensitivity of nano-object detection. The switching behavior of the PAA PEBs against changes in the pH values was investigated by analyzing the chemical, morphological, optical, and electrical properties. At pH ~1, the brushes collapse on the surface with the dominance of carboxylic groups (COOH). Upon the increase in the pH to nine, the switching process completes, and the brushes swell from dissociating most of the COOH groups and converting them into COO- groups. The domination of the negatively charged COO- groups increases the electrostatic repulsion in the polymer chains and stretches the brushes. The sensitivity of the SPR sensing device was investigated using a theoretical approach, as well as experimental measurements. The signal-to-noise ratio for a Au layer increases from six to eighteen after coating with PAA PEBs. In addition, the linewidth of the recorded image decreases from six pixels to five pixels by using the Au-PAA layers, which results from the enhanced spatial resolution of the recorded images. Coating a Au-layer with PAA PEBs enhances the sensitivity of the SPR sensing device, and improves the spatial resolution of the recorded image.

Steady-state extensional rheology of a dilute suspension of spheres in a dilute polymer solution

It is well-known and intuitive that addition of a dilute volume concentration of spheres increases the viscosity of a Newtonian liquid. We find that a viscoelastic liquid undergoing uniaxial extensional flow at small $D\phantom{\rule{0}{0ex}}e$ (extension rate times the polymer relaxation time) experiences a larger increase in viscosity than the Newtonian case due to wakes of extra polymer stretch produced downstream of the spheres. However, when $D\phantom{\rule{0}{0ex}}e\ensuremath{\gtrsim}0.6$, the highly stretched polymer in the bulk fluid collapses near the particle surface and the suspension deforms more easily than the particle-free viscoelastic fluid.

Arrow-shaped elasto-inertial rotating waves.

We present direct numerical simulations of the Taylor-Couette flow of a dilute polymer solution when only the inner cylinder rotates and the curvature of the system is moderate ([Formula: see text]). The finitely extensible nonlinear elastic-Peterlin closure is used to model the polymer dynamics. The simulations have revealed the existence of a novel elasto-inertial rotating wave characterized by arrow-shaped structures of the polymer stretch field aligned with the streamwise direction. This rotating wave pattern is comprehensively characterized, including an analysis of its dependence on the dimensionless Reynolds and Weissenberg numbers. Other flow states having arrow-shaped structures coexisting with other types of structures have also been identified for the first time in this study and are briefly discussed. This article is part of the theme issue 'Taylor-Couette and related flows on the centennial of Taylor's seminal Philosophical Transactions paper (Part 2)'.

A thermodynamically consistent constitutive equation describing polymer disentanglement under flow

We derive a thermodynamically consistent framework for incorporating entanglement dynamics into constitutive equations for flowing polymer melts. We use this to combine the convected constraint release (CCR) dynamics of Ianniruberto–Marriccui into a finitely extensible version of the Rolie–Poly model, and also include an anisotropic mobility as in the Giesekus model. The reversible dynamics are obtained from a free energy that describes both a finitely extensible conformation tensor and an ideal gas of entanglements along the chain. The dissipative dynamics give rise to coupled kinetic equations for the conformation tensor and entanglements, whose coupling terms describe shear-induced disentanglement. The relaxation dynamics of the conformation tensor follow the GLaMM and Rolie–Poly models, and account for reptation, retraction, and CCR. We propose that the relaxation time τν for entanglement recovery is proportional to the Rouse time τR which governs polymer stretch within the tube. This matches recent molecular dynamics simulations and corresponds to relaxing the entanglement number before the entire polymer anisotropy has relaxed on the longer reptation time τd. Our model suggests that claimed signatures of slow re-entanglement on the reptation time in step-strain experiments may be interpreted as arising from anisotropies in reptation dynamics.

A Simple Strengthening Method for Preventing Collapsed of Vulnerable Masonry Infills

A series of structural tests were conducted to examine the seismic performance of masonry infills strengthened with particular materials on infilled reinforced concrete (RC) frame structures. Six 1:4 scaled-down RC frame specimens had been prepared, including one brick-infilled frame without strengthening and five brick infills strengthened with innovative strengthening materials. The materials were steel wire mesh, chicken hexagonal wire mesh, plastic wire mesh, fiber-reinforced polymer (FRP), and plastic stretch film. The strengthening was diagonally applied on both surfaces of the masonry infill. The steel wire mesh, chicken hexagonal wire mesh, and plastic wire mesh were sewn using steel wire, while the FRP sheet was glued using epoxy resin and the plastic stretch film was glued using synthetic rubber adhesive. The specimens were tested following the FEMA 461 standard testing protocol, which involved applying lateral static cyclic loading to the specimens. The displacement transducer apparatus measured the deformations of the specimens, and crack propagation was observed during experimental works. The experimental results showed that most specimens exhibited an increase in their lateral strength, secant stiffness, deformation capacity, and energy dissipation. Among all prepared specimens, the specimen using plastic stretch film showed the best and most promising results, i.e., long deformation and steady lateral strength after yielding. This result suggests that using plastic stretch for strengthening can increase ductility performance. It is expected to withstand earthquake shaking, has low application costs, and is feasible for application even by unskilled local laborers.

An exploration of polymer presence in magneto‐hydrodynamic flow and heat transfer past a magnetized stretching surface with effects of Joule heating

AbstractThe consequences of polymer presences on magneto‐hydrodynamic flow and heat transfer of a viscous fluid induced by a magnetized permeable heated stretching surface are studied in this article. The Lorentz force and Joule heating are also taken into consideration. The Oldroyd‐B model of polymers is used to study polymer presence which adds an extra stress in the mathematical model due to the polymer stretching. An iterative numerical scheme is opted to solve the set of ruling equations of the problem. Polymers stretch in the boundary layer region due to high velocity gradients. Polymer stretching reduces the zero‐shear viscosity near the surface but the polymeric viscosity (the total contribution of viscosity due to polymer addition) still remains positive, which cause velocity to reduce in the boundary layer region. Polymers are found to have no impact on the bulk. Skin friction along with heat and magnetic fluxes at the surface are evaluated with increasing polymer concentration. Zero polymer concentration represents the absence of polymers. Reduction in heat and magnetic fluxes and drag at the surface is observed in consequence of polymer addition. Polymer effects on skin friction, heat flux, and magnetic flux with respect to different values of magnetic force parameter, magnetic Prandtl number, magnetic interaction parameter, and the Eckert number are also thoroughly analyzed and graphically presented.

Coherent Structures in Plane Channel Flow of Dilute Polymer Solutions with Vanishing Inertia.

When subjected to sufficiently strong velocity gradients, solutions of long, flexible polymers exhibit flow instabilities and chaotic motion, often referred to as elastic turbulence. Its mechanism differs from the familiar, inertia-driven turbulence in Newtonian fluids and is poorly understood. Here, we demonstrate that the dynamics of purely elastic pressure-driven channel flows of dilute polymer solutions are organized by exact coherent structures that take the form of two-dimensional traveling waves. Our results demonstrate that no linear instability is required to sustain such traveling wave solutions and that their origin is purely elastic in nature. We show that the associated stress profiles are characterized by thin, filamentlike arrangements of polymer stretch, which is sustained by a solitary pair of vortices. We discuss the implications of the traveling wave solutions for the transition to elastic turbulence in straight channels and propose ways for their detection in experiments.

Universality of dilute solutions of ring polymers in the thermal crossover region between θ and athermal solvents

Due to their unique topology of having no chain ends, dilute solutions of ring polymers exhibit behavior distinct from their linear chain counterparts. The universality of their static and dynamic properties, as a function of solvent quality z in the thermal crossover regime between θ and athermal solvents, is studied here using Brownian dynamics simulations. The universal ratio URD of the radius of gyration Rg to the hydrodynamic radius RH is determined, and a comparative study of the swelling ratio αg of the radius of gyration, the swelling ratio αH of the hydrodynamic radius, and the swelling ratio αX of the mean polymer stretch X along the x-axis, for linear and ring polymers, is carried out. The ratio URD for dilute ring polymer solutions is found to converge asymptotically to a constant value as z→∞, which is a major difference from the behavior of solutions of linear chains, where no such asymptotic limit exists. Additionally, the ratio of the mean stretch along the x-axis to the hydrodynamic radius, (X/RH), is found to be independent of z for polymeric rings, unlike in the case for linear polymers. These results indicate a fundamental difference in the scaling of static and dynamic properties of rings and linear chains in the thermal crossover regime.

Self-assembly of hydrophobically associating amphiphilic polymer with surfactant and its effect on nanoemulsion

A new scheme for controlling the rheological properties and stability of oil-in-water nanoemulsions is presented based on the polymer-surfactant self-assembly in solution. Specifically, amphiphilic property, hydrophobic association and thermo-responsive behavior is imparted to the polymer by using surfactant as a main function monomer to introduce hydrophilic and hydrophobic groups on the polymer. The polymer can cooperate with the surfactant to form oil-water interfacial film without compromising nanoemulsion formation based on its self-assembly properties. And there can be a moderate increase of the association between the hydrophobic groups on different polymers with increasing temperature based on their hydrophobicity. These behaviors will affect the rheological properties and stability of the nanoemulsion. The thermo-association mechanism of polymer, interaction between polymer and surfactant, and the effects of the polymer on nanoemulsions were analyzed. Test results show that, the thermo-association occurs when the polymer concentration is greater than 0.2%, and the association begins to be obvious when the temperature is higher than 50 °C, which would increase the viscosity of solution. The thermo-association may be attributed to the fact that the curled side chains of the polymer stretch and the hydrophobically association is intensified under higher temperature. The self-assembly of polymer and surfactant increase the interface film dilational modulus of the emulsions, which is conducive to the stability of nanoemulsion droplets. This polymer and the thermo-association mechanism could be applied in the study of smart emulsion and temperature-resistant emulsion.

Tollmien-Schlichting route to elastoinertial turbulence in channel flow

Direct simulations of two-dimensional channel flow of a viscoelastic fluid have revealed the existence of a family of Tollmien-Schlichting (TS) attractors that is nonlinearly self-sustained by viscoelasticity [Shekar et al., J.Fluid Mech. 893, A3 (2020)]. Here, we describe the evolution of this branch in parameter space and its connections to the Newtonian TS attractor and to elastoinertial turbulence (EIT). At Reynolds number $Re=3000$, there is a solution branch with TS-wave structure but which is not connected to the Newtonian solution branch. At fixed Weissenberg number, $Wi$ and increasing Reynolds number from 3000-10000, this attractor goes from displaying a striation of weak polymer stretch localized at the critical layer to an extended sheet of very large polymer stretch. We show that this transition is directly tied to the strength of the TS critical layer fluctuations and can be attributed to a coil-stretch transition when the local Weissenberg number at the hyperbolic stagnation point of the Kelvin cat's eye structure of the TS wave exceeds $\frac{1}{2}$. At $Re=10000$, unlike $3000$, the Newtonian TS attractor evolves continuously into the EIT state as $Wi$ is increased from zero to about $13$. We describe how the structure of the flow and stress fields changes, highlighting in particular a "sheet-shedding" process by which the individual sheets associated with the critical layer structure break up to form the layered multisheet structure characteristic of EIT.

Exact Traveling Wave Solutions in Viscoelastic Channel Flow.

Elasto-inertial turbulence (EIT) is a new, two-dimensional chaotic flow state observed in polymer solutions with possible connections to inertialess elastic turbulence and drag-reduced Newtonian turbulence. In this Letter, we argue that the origins of EIT are fundamentally different from Newtonian turbulence by finding a dynamical connection between EIT and an elasto-inertial linear instability recently found at high Weissenberg numbers [Garg etal., Phys. Rev. Lett. 121, 024502 (2018)PRLTAO0031-900710.1103/PhysRevLett.121.024502]. This link is established by isolating the first known exact coherent structures in viscoelastic parallel flows-nonlinear elasto-inertial traveling waves (TWs)-borne at the linear instability and tracking them down to substantially lower Weissenberg numbers where EIT exists. These TWs have a distinctive "arrowhead" structure in the polymer stretch field and can be clearly recognized albeit transiently in EIT as well as being attractors for EIT dynamics if the Weissenberg number is sufficiently large. Our findings suggest that the dynamical systems picture in which Newtonian turbulence is built around the coexistence of many (unstable) simple invariant solutions populating phase space carries over to EIT, though these solutions rely on elasticity to exist.

Lift and drag force on a spherical particle in a viscoelastic shear flow

We present a comprehensive 3D numerical study of particles with imposed velocities relative to the local bulk flow (termed “slip velocities”) in a viscoelastic shear flow. We consider the force on a spherical particle sedimenting, a spherical bubble rising, and a spherical neutral squirmer swimming in an imposed viscoelastic shear flow. We demonstrate that any particle moving with a slip velocity in the flow or gradient direction of the shear flow experience a lateral lift force. We calculate and compare the magnitude and direction of the lift force in all situations. At small Deborah (De) and Weissenberg (Wi) numbers, our results show good agreement with an existing perturbation theory for rigid particles (Einarsson and Mehlig, 2017 [1]) and new perturbation theories for drops and for squirmers respectively. Our simulations extend these results to higher De and Wi regimes. Through our simulations, we uncover the physical mechanism of the lateral force on all particles. For rigid particles, we find the lift force arises from an imbalance in polymer stress on either side of the particle, which in turn is due to the imbalance of polymer stretch surrounding the particle. If this lift force is not balanced by an external force, a lateral drift velocity arises. We further consider the implication of this lateral drift for rigid particles hydrodynamically forced in a viscoelastic Poiseuille flow, where particles can migrate either toward the channel center plane or toward the wall, depending on whether the direction of the applied force is in the direction aligned or opposite to the direction of the Poiseuille flow, respectively. We study both the migration of a single particle as well as a suspension of particles in channel flow. Even with the addition of hydrodynamic interactions, we show that particles forced in the direction of the Poiseuille flow migrate towards the channel center.

Critical-Layer Structures and Mechanisms in Elastoinertial Turbulence.

Simulations of elastoinertial turbulence (EIT) of a polymer solution at low Reynolds number are shown to display localized polymer stretch fluctuations. These are very similar to structures arising from linear stability (Tollmien-Schlichting modes) and resolvent analyses, i.e., critical-layer structures localized where the mean fluid velocity equals the wave speed. Computations of self-sustained nonlinear Tollmien-Schlichting waves reveal that the critical layer exhibits stagnation points that generate sheets of large polymer stretch. These kinematics may be the genesis of similar structures in EIT.

Polymer extrudate-swell: From monodisperse melts to polydispersity and flow-induced reduction in monomer friction

This paper describes finite-element simulations and associated experimental studies of extrudate swell for near-monodisperse and polydisperse polystyrenes. The tube-model based Rolie-Poly constitutive model, when extended to include a reduction of monomeric friction at high extension rates, makes much-improved predictions of extrudate swell at high Weissenberg number. This is especially significant for near-monodisperse polymers where rheological features are unchanged by the effects of polydispersity. Extension of this molecular rheology scheme to a polydisperse constitutive model addresses extrusion experiments on polydisperse polystyrenes inside a multipass rheometer, accounting for experimental data up to Rouse Weissenberg numbers of 50. We, therefore, show that from a measurement of polymer molecular weight distribution, it is possible to predict extrudate swell over a broad range of processing conditions for polydisperse polymers and realistic extrusion processes. Small changes in the capillary length to diameter ratio have little effect on extrudate swell in this range of Weissenberg number. This is because the capillary residence time is sufficiently long for a steady state to be reached within the polymer stretch relaxation time, which controls the most decisive physics responsible for extrudate swell.

Stretchy hydrogel heals like muscle

When we work out, the strain breaks fibers in our muscles. Our body repairs those fibers with a steady supply of amino acids that it knits into proteins, ultimately building us stronger muscles. Scientists have now made a muscle-like hydrogel that works the same way, strengthening itself when mechanically stressed. The work, they say, could lead to longer-lasting tires or soft robots made of flexible plastic that can repair themselves and even grow. The stretchy hydrogel material comes from a team led by Hokkaido University’s Jian Ping Gong and Tasuku Nakajima and takes advantage of polymer mechanochemistry, in which mechanical force initiates a chemical reaction. The soft yet tough hydrogel is made up of 85% water and two intertwined, cross-linked polymer networks. One of the networks—made of poly(2-acrylamido-2-methylpropanesulfonic acid) sodium salt—is taut. The other network—made of polyacrylamide—is slack. When pulled, the slack polymer stretches out, which prevents the hydrogel from

Modelling flow-enhanced crystallisation during fused filament fabrication of semi-crystalline polymer melts

Achieving better control in fused filament fabrication (FFF) relies on a molecular understanding of how thermoplastic printing materials behave during the printing process. For semi-crystalline polymers, the ultimate crystal morphology and how it develops during cooling is crucial to determining part properties. Here crystallisation kinetics are added to a previously-developed model, which contains a molecularly-aware constitutive equation to describe polymer stretch and orientation during typical non-isothermal FFF flow, and conditions under which flow-enhanced nucleation occurs due to residual stretch are revealed. Flow-enhanced nucleation leads to accelerated crystallisation times at the surface of a deposited filament, whilst the bulk of the filament is governed by slower quiescent kinetics. The predicted time to 10% crystallinity, t10, is in quantitative agreement with in-situ Raman spectroscopy measurements of polycaprolactone (PCL). The model highlights important features not captured by a single measurement of t10. In particular, the crystal morphology varies cross-sectionally, with smaller spherulites forming in an outer skin layer, explaining features observed in full transient crystallisation measurements. Finally, exploitation of flow-enhanced crystallisation is proposed as a mechanism to increase weld strength at the interface between deposited filaments.

Easy-to-make polymer stretches, sticks, self-heals

Easy-to-make polymer stretches, sticks, self-heals

Origami-shaped conducting polymer stretches like an accordion

Origami-shaped conducting polymer stretches like an accordion