Viscoelastic Stress Research Articles

Viscoelastic second normal stress difference dominated multiple-stream particle focusing in microfluidic channels.

Particle focusing in viscoelastic fluid flow is a promising approach for inducing particle separations in microfluidic devices. The results from theoretical studies indicated that multiple stream particle focusing can be realized with a large magnitude of the elastic second normal stress difference (N2). For dilute polymer solutions, theoretical and experimental studies show that the magnitude of N2 is never large, no matter how large the polymer molecular weight nor how high the shear rate. However, for concentrated entangled polymer solutions, the magnitude of N2 becomes large at high shear rates. Therefore, in order to test the hypothesis that N2 can be used to induce multiple particle stream focusing behavior, we perform the systematic study of the effects of increasing carrier fluid polymer concentrations in a microchannel containing fluorescent particles. In a dilute polymer solution, multiple particle stream focusing is not observed, even at high shear rates and large dimensionless Weissenberg number values (Wi ≈ 30) at which the elastic first normal stress difference (N1) and the viscosity shear-thinning should be very large, while in a concentrated entangled polymer solution, we observe that particle streams focused upon the channel centerline bifurcate to form two symmetric off-channel particle streams at higher shear rates. This particle focusing behavior is different from previous multiple-stream focusing phenomena, and that we attribute to the influence of the second normal stress difference N2. This N2 induced multiple stream focusing phenomenon provides a different approach for manipulating the particle trajectory and separation in a microchannel.

Fully dynamic earthquake sequence simulation of a fault in a viscoelastic medium using a spectral boundary integral equation method: does interseismic stress relaxation promote aseismic transients?

Along subduction interfaces or some major faults, a seismogenic layer in the upper crust is underlain by a zone of slow-slip events (SSEs) and tremors, and seismicity disappears at greater depths. The transition between seismogenic and aseismic behavior may be caused by changes in the frictional properties of the fault or changes in viscoelastic properties of the surrounding medium. Although aseismic transients have been numerically generated in previous studies by changing the frictional properties and compared to SSEs, the effect of viscoelasticity on the transition remains to be studied. In this study, we implemented interseismic viscoelastic stress relaxation in a simulation code for two-dimensional antiplane fully dynamic earthquake sequences in a uniform elastic material based on a spectral boundary integral equation method. In the implementation, we developed a suitable algorithm in which the viscoelastic relaxation is calculated by evolution of an “effective slip,” which gives viscoelastic stress change on the fault if convolved with a static Green’s function. We conducted parametric studies for a fault with a rate-weakening patch on two parameters: viscoelastic relaxation time t_{c} and characteristic length of the state evolution in the rate- and state-dependent friction law L. The behavior of the simulated fault can be classified into four classes, earthquakes (EQ), aseismic transients (AT), stable sliding (SS), and stuck (ST), in which the central part of the rate-weakening patch has a diminishingly small slip rate and is permanently locked. A phase diagram of the fault behavior shows that there are two different types of seismogenic–aseismic transition. As L increases, an EQ patch changes to an AT patch before becoming an SS patch, as has been reported in previous studies in an elastic limit. The boundary between AT and SS can be explained via linear stability analysis of a system composed of a spring, a dashpot, and sliders. As t_{c} decreases, the recurrence interval of the earthquakes diverges, and an EQ patch changes to an ST patch unless L is within a narrow range. Therefore, the transition associated with SSEs and tremors is dominated by changes in frictional properties rather than changes in viscoelastic properties.

Delaying breakup and avoiding air entrainment in curtain coating using a two-layer liquid structure

Successful curtain coating is limited by two flow instabilities, the breakup of the liquid curtain below a critical flow rate and air entrainment above a maximum substrate speed. These limitations make thin film coating at high speed a great challenge. Previous work has shown that the curtain breakup can be delayed to lower flow rates if the flowing liquid has viscoelastic behavior. However, viscoelastic stresses near the dynamic contact line destabilize the flow in that region, leading to the onset of air entrainment at lower web speeds. We propose and experimentally validate the use of a two-layer curtain coating configuration, with a shear-thinning liquid as the bottom layer and a viscoelastic liquid as the top layer, that expands the processing window and enables thin coating at high speeds.

Transient Viscosity and Afterslip of the 2015 Mw 8.3 Illapel, Chile, Earthquake

Abstract It is usually assumed that short‐term (a few years) postseismic deformation around the rupture zone is caused by continuing slip (afterslip) along the fault interface, and viscoelastic stress relaxation is only responsible for long‐term deformation. In order to verify the validity of this assumption, the initial 1.5 months postseismic displacements following the 2015 Mw 8.3 Illapel earthquake are analyzed based on a multilayered structure model. We explore the possible mechanisms, including afterslip and viscoelastic relaxation, which might have contributed to the postseismic deformation, and aim to distinguish the contributing ratio of different postseismic processes. The results show that either the models of kinematic afterslip or viscoelastic stress relaxation individually cannot match the observed horizontal and vertical postseismic displacements satisfactorily. However, a combined model considering both afterslip and viscoelastic relaxation effects can reduce the data misfit significantly and is more physically reasonable. In the preferred combined model, the transient viscosities of the lower crust and upper mantle are ∼6×1017 Pa s and ∼9×1017 Pa s, respectively. The difference between the afterslip distribution of the pure afterslip model and that of the combined model indicates that previous models based on pure elastic assumption have substantially underestimated the afterslip updip of the rupture zone, and overestimated the afterslip downdip of the rupture zone. Therefore, the role of viscoelastic stress relaxation is indispensable in the study of transient postseismic deformation following a large earthquake, which contradicts the conventional concept about deformation mechanisms of early postseismic process.

Effects of a 12-Week Chronic Stretch Training Program at Different Intensities on Joint and Muscle Mechanical Responses: A Randomized Clinical Trial.

Stretching intensity is an important variable that can be manipulated with flexibility training. However, there is a lack of evidence regarding this variable and its prescription in stretching programs. To investigate the effects of 12 weeks of knee flexor static stretching at different intensities on joint and muscle mechanical properties. A randomized clinical trial. Laboratory. A total of 14 untrained men were allocated into the low- or high-intensity group. Assessments were performed before, at 6 week, and after intervention (12 wk) for biceps femoris long head architecture (resting fascicle length and angle), knee maximal range of motion (ROM) at the beginning and maximal discomfort angle, knee maximal tolerated passive torque, joint passive stiffness, viscoelastic stress relaxation, knee passive torque at a given angle, and affective responses to training. No significant differences were observed between groups for any variable. ROM at the beginning and maximal discomfort angle increased at 6 and 12 weeks, respectively. ROM significantly increased with the initial angle of discomfort (P < .001, effect size = 1.38) over the pretest measures by 13.4% and 14.6% at the 6- and 12-week assessments, respectively, and significantly improved with the maximal discomfort angle (P < .001, effect size = 1.25) by 15.6% and 18.8% from the pretest to the 6- and 12-week assessments, respectively. No significant effects were seen for muscle architecture and affective responses. Initial viscoelastic relaxation for the low-intensity group was lower than ending viscoelastic relaxation. These results suggest that stretching with either low or high discomfort intensities are effective in increasing joint maximal ROM, and that does not impact on ROM, stiffness, fascicle angle and length, or affective response differences.

Is density enough to predict the rheology of natural sediments?

Mud is a cohesive material which contains predominantly clay minerals, water, organic matter and some amounts of silt and sand. Mud samples can have complex rheological behaviour, displaying viscoelasticity, shear-thinning, thixotropy and yield stress. In this study, influence of organic matter on the rheological behaviour of different mud samples having similar densities is investigated. Four samples, collected from different locations and depths of Port of Hamburg (Germany) were selected. Two samples with the density of about 1210 kg/m3 and two samples with the density of about 1090 kg/m3 were analysed by different rheological tests, including stress ramp-up tests, flow curves, thixotropic tests, oscillatory amplitude and frequency sweep tests. Two yield stress regions (with two yield stress values stated as “static” and “fluidic” yield stresses) were identified for all the samples, and these regions, corresponding to a structural change of the samples were significantly different from sample to sample due to the differences in organic matter content. For lower density samples, the ratio of fluidic to static yield stress increased from 3 to 4.4 while it increased from 4.4 to 5.2 in case of higher density samples, by increasing the organic matter content. The thixotropic studies showed that the mud samples having lowest organic matter content (VH and KBZ) exhibit a combination of thixotropic and anti-thixotropic behaviours. The results of frequency sweep tests revealed the solid-like character of the mud within the linear viscoelastic regime. Mud samples having higher organic matter content (RV and RT) had a higher complex modulus (417 Pa and 7909 Pa) than the ones with lower organic matter content (13 Pa and 1774 Pa), for a given density. This study demonstrated that the density only is not a sufficient criterion to predict the rheology of different mud. Furthermore, even small amounts of organic matter content change significantly the mud rheological behaviour.

Morphology and Rheological Properties of Polyacrylamide/Bentonite Organic Crosslinking Composite Gel

The use of polymer gel for water control and oil addition is a common technical method in oilfield development. The polymer and hydrated bentonite react under the action of an organic crosslinking agent to form a composite gel. The particle-size change and microstructure of the composite gel were analyzed via shear thinning, thixotropic, viscoelastic, and start-up stress rheology experiments. The experimental results show that the polyacrylamide/bentonite organic crosslinked composite gel was a gel system with bentonite as the core aggregate structure, and the large particle-size distribution was mostly increased with increasing crosslinker content. The composite gel presented shear thinning characteristics, the content of bentonite or crosslinking agent was increased, and the shear resistance was stronger at a high shear rate. The composite gel exhibited positive thixotropic properties, and the thixotropy increased with increasing bentonite content. The composite gel had good viscoelastic characteristics, the elastic characteristics of the composite gel showed more significantly with bentonite increases, and the viscosity of the composite gel showed its characteristics more significantly with the crosslinking agent increased. After loading at a rate on the composite gel, the shear stress increased significantly with time and reached its maximum value, and then the shear stress decreased and gradually stabilized.

A visco-elastic physics exercise using inquiry

In this article, we present an activity that demonstrates visco-elastic stress–strain behaviour. The experiment uses the self-folding behaviour of commercially available ‘shrink paper’ to demonstrate the Kelvin-Dashpot rheological model of visco-elasticity. We also present observations of how this activity worked when we ran it as an open inquiry activity for students. We found that students initially applied elastic models to explain the behaviour of the shrink paper but later discovered the concept of visco-elasticity through experimental observation.

Modelling the performance of devices based on thin dielectric elastomer membranes

Actuators based on thin dielectric elastomer (DE) membranes with very low modulus, high dielectric permittivity and dielectric breakdown strength have recently been used in a number of diverse applications. In order to be technologically viable, the fidelity of their responses, especially in operations over large number of cycles, and failure mechanisms need to be understood and modelled. We present a simple, uncoupled, explicit scheme for simulating actuators based on DE membranes that depends on a characterisation of the material as a electro-visco-hyperelastic one. Experimental charactersiation of the uniaxial viscoelastic stress strain response, stretch dependent dielectric permittivity and breakdown strength of a commonly used DE material has been carried out. The simulation methodology has been verified against the performance of a circular membrane based actuator subjected to a cyclic potential. The simulations reproduced the experimental responses well and also allowed us to computationally map out the parameter space for safe operation of the actuator.

Inversion for the complex elastic modulus of material from spherical wave propagation data in free field

A spherical wave experiment is conducted for polymethyl methacrylate (PMMA), with a spherical explosive charge of 0.125 g TNT as the source. The aim is to investigate the dynamic mechanical behavior of a viscoelastic material from the multiple measured radial particle velocities during the spherical wave propagation in a free field. Without using conventional mathematical models, a brief and effective method is suggested to determine the complex elastic modulus of the material through the combination of the viscoelastic spherical stress wave theory and the measured radial particle velocities at different radii in a free field. The complex elastic modulus, determined from the spherical wave data under different stress states, is compared with those evaluated through the conventional mathematical models or experimental results selected from the literature. The results show that PMMA behaves viscoelastically under spherical explosion loading. With increasing the propagation distance of spherical wave, the radial stress decreases from 682 to 40 MPa, and the radial strain decreases from 59.5 × 10−3 to 4.2 × 10−3. Meanwhile, the average storage modulus of PMMA, at high frequencies after approximately 300 kHz, decreases from 7.14 to 5.97 GPa. This indicates that the complex elastic modulus of PMMA is dependent on the stress–strain state, and the published parameters of viscoelastic materials, determined by different experimental techniques under different stress–strain states, should be used with caution.

Electromechanical Decoupling by Porous Aerogel Conducting Polymer

In the field of biointerface electronics, electromechanical decoupling—electrical performance independent of mechanical deformation—can ensure reliability of measured signals, which is critical in stable electrode. Herein, Chen et al. report such a decoupled and stretchable conductive polymer foam.

Dynamic characteristics of initially stressed viscoelastic microbeams induced by ultra-intense lasers

This work highlights the problems concerned with an initially stressed viscoelastic microbeam with small-scale thickness which is subjected to an ultra-fast laser heating and exposed to a sinusoidal varying heat. The analytical solutions for the deflection and temperature are obtained in terms of Laplace transform method. To indicate the effects of viscous damping coefficient, the time width of thermal vibration, the initial stress and the laser pulse duration parameters on the vibration characteristics of Euler–Bernoulli microbeams have been considered, and finally, numerical results are displayed. The results in here basically indicated that the viscoelasticity and initial stress effect of materials can be gradually applied to the design of the micro-electromechanical systems.

Thermocapillary motion of a Newtonian drop in a dilute viscoelastic fluid

In this work we investigate the role played by viscoelasticity on the thermocapillary motion of a deformable Newtonian droplet embedded in an immiscible, otherwise quiescent non-Newtonian fluid. We consider a regime in which inertia and convective transport of energy are both negligible (represented by the limit condition of vanishingly small Reynolds and Marangoni numbers) and free from gravitational effects. A constant temperature gradient is maintained by keeping two opposite sides of the computational domain at different temperatures. Consequently the droplet experiences a motion driven by the mismatch of interfacial stresses induced by the non-uniform temperature distribution on its boundary. The departures from the Newtonian behaviour are quantified via the “thermal” Deborah number, DeT and are accounted for by adopting either the Oldroyd-B model, for relatively small DeT, or the FENE-CR constitutive law for a larger range of DeT. In addition, the effects of model parameters, such as the concentration parameter c=1−β (where β is the viscoelastic viscosity ratio), or the extensibility parameter, L2, have been studied numerically using a hybrid volume of fluid-level set method. The numerical results show that the steady-state droplet velocity behaves as a monotonically decreasing function of DeT, whilst its shape deforms prolately. For increasing values of DeT, the viscoelastic stresses show the tendency to be concentrated near the rear stagnation point, contributing to an increase in its local interface curvature.

Transient analysis of diffusion-induced deformation in a viscoelastic electrode

In this study, we analyze the transient diffuse-induced-deformation of an electrode consisting of the conducting polymer polypyrrole (PPY) by using the theories of linear viscoelasticity and diffusion-induced stress. We consider two constitutive relationships with dependence of viscosity on strain rate: Kelvin-Voigt model and three-parameter solid model. A numerical method is used to solve the problem of one-dimensional, transient diffusion-induced-deformation under potentiostatic operation. The numerical results reveal that the maximum displacement occurs at the free surface and the maximum stress occurs at the fixed end. The inertia term causes the stress to increase at the onset of lithiation. The stress decreases with increasing lithiation time and approaches zero for prolonged lithiation. Compared with the two different constitutive relationships between the Kelvin-Viogt model and three-parameter solid model, it can be found that the spatiotemporal distribution of lithium ion concentrations in the Kelvin-Viogt model is larger than that in the three-parameter solid model at the same moment, whereas the stress of the Kelvin-Viogt model is smaller owing to more than one spring in the three-parameter solid model.

Numerical simulations on the dynamics of a particle pair in a viscoelastic fluid in a microchannel: effect of rheology, particle shape, and confinement

The dynamics of two particles suspended in a viscoelastic fluid and aligned on the centerline of a microfluidic channel is investigated by direct numerical simulations. The shear-thinning elastic fluid is modeled by the Giesekus constitutive equation. The relative particle velocity is studied by varying the interparticle distance, the Deborah number, fluid shear thinning, confinement ratio, and particle shape. Concerning the latter aspect, spherical and spheroidal particles with different aspect ratios are considered. The regimes of particle attraction and repulsion as well as the equilibrium configurations are identified and correlated with the fluid rheological properties and particle shape. The observed dynamics is related to the distribution of the viscoelastic normal stresses in the fluid between the particles. The results reported here provide useful insights into design efficient microfluidics devices to achieve particle ordering, i.e., the formation of equally spaced particle structures.

Macromolecular relaxation, strain, and extensibility determine elastocapillary thinning and extensional viscosity of polymer solutions

Delayed capillary break-up of viscoelastic filaments presents scientific and technical challenges relevant for drop formation, dispensing, and adhesion in industrial and biological applications. The flow kinematics are primarily dictated by the viscoelastic stresses contributed by the polymers that are stretched and oriented in a strong extensional flow field resulting from the streamwise gradients created by the capillarity-driven squeeze flow. After an initial inertiocapillary (IC) or viscocapillary (VC) regime, where elastic effects seem to play no role, the interplay of capillarity and viscoelasticity can lead to an elastocapillary (EC) response characterized by exponentially-slow thinning of neck radius (extensional relaxation time is determined from the delay constant). Less frequently, a terminal visco-elastocapillary (TVEC) response with linear decay in radius can be observed and used for measuring terminal, steady extensional viscosity. However, both IC/VC-EC and EC-TVEC transitions are inaccessible in devices that create stretched necks by applying a step strain to a liquid bridge (e.g., capillary breakup extensional rheometer). In this study, we use dripping-onto-substrate rheometry to obtain radius evolution data for unentangled polymer solutions. We deduce that the plots of transient extensional viscosity vs. Hencky strain (scaled by the respective values at the EC-TVEC transition) emulate the functional form of the birefringence-macromolecular strain relationship based on Peterlin's theory. We quantify the duration and strain between the IC/VC-EC and the EC-TVEC transitions using measures we term elastocapillary span and elastocapillary strain increment and find both measures show values directly correlated with the corresponding variation in extensional relaxation time.

Simulation of viscoelastic two-phase flows with insoluble surfactants

A stabilized finite element scheme is developed for computations of buoyancy driven 3D-axisymmetric viscoelastic two-phase flows with insoluble surfactants. The numerical scheme solves the Navier–Stokes equations for the fluid flow, Giesekus constitutive equation for the effects of viscoelasticity and simultaneously an evolution equation for the surfactant concentration on the interface. The interface is tracked by the coupled arbitrary Lagrangian–Eulerian (ALE) and Lagrangian approach. The interface-resolved moving meshes allow accurate incorporation of the interfacial tension force, Marangoni forces and the jumps in the material properties. Further, the tangential gradient operator technique is used to handle the curvature approximation in a semi-implicit manner. An one-level Local Projection Stabilization (LPS), which is based on an enriched approximation space and a discontinuous projection space, where both spaces are defined on a same mesh is used to stabilize the model equations. The stabilized numerical scheme allows us to use isoparametric second order conforming finite elements enriched with cubic bubble functions for velocity and viscoelastic stress, second order finite elements for surfactant concentration and discontinuous first order finite element for pressure. A number of computations are performed for a Newtonian drop rising in a viscoelastic fluid column and a viscoelastic drop rising in a Newtonian fluid column with insoluble surfactants on the interface. The influence of the Marangoni number, initial surfactant concentration and Peclet number on the dynamics of the rising drop are analyzed. The numerical study shows that a viscoelastic drop rising in a Newtonian fluid column develops an indentation around the rear stagnation point with a dimpled shape without insoluble surfactants. The presence of insoluble surfactants forces the drop to rise slowly but the drop at the tail end is pulled up more. However, a Newtonian drop rising in a viscoelastic fluid column experiences an extended trailing edge with a cusp-like shape without insoluble surfactants. The presence of surfactants pulls the tail end of the drop up slightly and makes the tail flatter with/without small undulations depending on the magnitude of the surfactant concentrations.

A turbulent duct flow investigation of drag-reducing viscoelastic FENE-P fluids based on different low-Reynolds-number models

Drag reduction of viscoelastic fluids under turbulent flow regime is an important phenomenon and well observed by many researchers. In this paper, turbulent flow characteristics under drag reduction condition are compared using four different low-Reynolds number k- ε models namely, Lam–Bremhorst, Launder–Sharma, Nagano–Hishida and Chien. The viscoelastic turbulence closure of Resende et al. (2011) is adopted. Time-averaged momentum and rheological constitutive equations based on finitely extensible nonlinear elastic-Peterlin (FENE-P) model are used and a polymeric contribution to the eddy viscosity equation is considered. The simulations are conducted for two sets of rheological parameters identified by Reτ0 = 395, β=0.9, L2 = 900 and Weτ0 = 25, 100 corresponding to drag reductions of 18% and 37%. Validation against DNS data is carried out for profiles of mean velocity, viscoelastic stress tensor, turbulent kinetic energy and its rate of dissipation. The differences are mainly limited to the viscous and buffer layer regions. While Nagano–Hishida and Chien models overpredict the DNS drag reduction results in both cases, the two other models underpredict them. The Lam–Bremhorst has the most accurate drag reduction results as a consequence of better capturing the near-wall effects.

Shear Banding in 4:1 Planar Contraction.

We study shear banding in a planar 4:1 contraction flow using our recently developed two-fluid model for semidilute entangled polymer solutions derived from the generalized bracket approach of nonequilibrium thermodynamics. In our model, the differential velocity between the constituents of the solution allows for coupling between the viscoelastic stress and the polymer concentration. Stress-induced migration is assumed to be the triggering mechanism of shear banding. To solve the benchmark problem, we used the OpenFOAM software package with the viscoelastic solver RheoTool v.2.0. The convection terms are discretized using the high-resolution scheme CUBISTA, and the governing equations are solved using the SIMPLEC algorithm. To enter into the shear banding regime, the uniform velocity at the inlet was gradually increased. The velocity increases after the contraction due to the mass conservation; therefore, shear banding is first observed at the downstream. While the velocity profile in the upstream channel is still parabolic, the corresponding profile changes to plug-like after the contraction. In agreement with experimental data, we found that shear banding competes with flow recirculation. Finally, the profile of the polymer concentration shows a peak in the shear banding regime, which is closer to the center of the channel for larger inlet velocities. Nevertheless, the increase in the polymer concentration in the region of flow recirculation was significantly larger for the inlet velocities studied in this work. With our two-fluid finite-volume solver, localized shear bands in industrial applications can be simulated.

Static Stretching Intensity Does Not Influence Acute Range of Motion, Passive Torque, and Muscle Architecture.

Although stretching exercises are commonly used in clinical and athletic practice, there is a lack of evidence regarding the methodological variables that guide the prescription of stretching programs, such as intensity. To investigate the acute effects of different stretching intensities on the range of motion (ROM), passive torque, and muscle architecture. Two-group pretest-posttest design. Laboratory. Twenty untrained men were allocated into the low- or high-intensity group. Subjects were evaluated for initial (ROMinitial) and maximum (ROMmax) discomfort angle, stiffness, viscoelastic stress relaxation, muscle fascicle length, and pennation angle. The ROM assessments showed significant changes, in both groups, in the preintervention and postintervention measures both for the ROMinitial (P < .01) and ROMmax angle (P = .02). There were no significant differences for stiffness and viscoelastic stress relaxation variables. The pennation angle and muscle fascicle length were different between the groups, but there was no significant interaction. Performing stretching exercises at high or low intensity acutely promotes similar gains in flexibility, that is, there are short-term/immediate gains in ROM but does not modify passive torque and muscle architecture.