Nonaffine Network Structural Model for Molten Low-Density Polyethylene and High-Density Polyethylene in Oscillatory Shear

We propose molten polymer's entanglement network deformation to be nonaffine and use transient network structural theory with the revised Liu's kinetics rate equation and the revised upper convected Maxwell constitutive equation to establish a nonaffine network structural constitutive model for studying the rheological behavior of molten Low Density Polyethylene (LDPE) and High Density Polyethylene (HDPE) in oscillatory shear. As a result, when the strain amplitude or frequency increases, the shear stress amplitude increases. At the same time, the accuracy of the nonaffine network model is higher than that of affine network model. It is clear that there is a small amount of nonaffine network deformation for LDPE melts which have long chain branches, and there is a larger amount of nonaffine network deformation in oscillatory shear for HDPE melts which has no long chain branches. So we had better consider the network deformation nonaffine when we establish the constitutive equations of polymer melts in oscillatory shear.

Correlation Between Yield Stress of Silty Mud Sediments and Continuous Oscillatory Shearing Properties

Analyzing the rheological properties of silty beds subjected to continuous oscillatory shear loading is crucial for understanding the morphological deformation of the seabed and ensuring safety in geological and marine engineering applications.In this study,the effects of oscillatory shearing properties on the yield stress(S_(u))of silty sediments were quantitatively investigated.The effects of oscillatory shear strength(0-3),water content(26.6%-70.84%),and particle diameter(8.79-50μm)were examined extensively through a series of laboratory tests.The results indicated that the three aforementioned parameters were the major factors that affected the rheological characteristics of silty sediments.Furthermore,their effects could be elucidated using the yield stress of cohesive sediments as the indicator parameter.The ratio of yield stress(S_(u)/S_(u0))varied as the oscillatory shear strength increased up to a critical value,Λ_(cr).S_(u)bsequently,the ratio remained at a constant value.It was deduced that the yield stress decreased with increasing oscillatory shear strength forΛ<Λ_(cr),when the sediments were in a non-equilibrium fluidization stage.WhenΛ>Λ_(cr),the sediments entered an equilibrium fluidization stage,and the yield stress remained almost constant,irrespective of the oscillatory shear strength.Furthermore,during the equilibrium fluidization stage,it was observed that the ratio S_(u)/S_(u0)did not vary with water content but decreased as the particle diameter increased.Finally,based on regression analysis of the experimental data for non-equilibrium and equilibrium fluidization stages,a correlation between yield stress of silty sediments and continuous oscillatory shearing properties was proposed.This correlation can aid in understanding the changes in solid resistance and assessing safety in piling engineering.Furthermore,it can provide a theoretical guidance for reducing soil resistance in marine structures using mechanical vibrations.

A revisit of strain-rate frequency superposition of dense colloidal suspensions under oscillatory shears

Strain-rate frequency superposition(SRFS) is often employed to probe the low-frequency behavior of soft solids under oscillatory shear in anticipated linear response. However, physical interpretation of an apparently well-overlapped master curve generated by SRFS has to combine with nonlinear analysis techniques such as Fourier transform rheology and stress decomposition method. The benefit of SRFS is discarded when some inconsistencies of the shifted master curves with the canonical linear response are observed. In this work, instead of evaluating the SRFS in full master curves, two criteria were proposed to decompose the original SRFS data and to delete the bad experimental data. Application to Carabopol suspensions indicates that good master curves could be constructed based upon the modified data and the high-frequency deviations often observed in original SRFS master curves are eliminated. The modified SRFS data also enable a better quantitative description and the evaluation of the apparent structural relaxation time by the two-mode fractional Maxwell model.

G3BP2 regulates oscillatory shear stress-induced endothelial dysfunction

GTPase-activating SH3 domain-binding protein 2(G3BP2)is a mediator that responds to environmental stresses through stress granule formation and is involved in the progression of chronic diseases.However,no studies have examined the contribution of G3BP2 in the oscillatory shear stress(OSS)-induced endothelial dysfunction.Here we assessed the effects of G3BP2 in endothelial cells(ECs)function and investigated the underlying mechanism.Using shear stress apparatus and partial ligation model,we identified that stress granulerelated genes in ECs could be induced by OSS with RNA-seq,and then confirmed that G3BP2 was highly and specifically expressed in athero-susceptible endothelia in the OSS regions.G3bp2e/eApoee/e mice had significantly decreased atherosclerotic lesions associated with deficiency of G3BP2 in protecting endothelial barrier function,decreasing monocyte adhesion to ECs and inhibiting the proinflammatory cytokine levels.Furthermore,loss of G3BP2 diminished OSS-induced inflammation in ECs by increasing YAP nucleocytoplasmic shuttling and phosphorylation.These data demonstrate that G3BP2 is a critical OSS regulated gene in regulating ECs function and that G3BP2 inhibition in ECs is a promising atheroprotective therapeutic strategy.

Effect of Oscillatory Shear on the Mechanical Properties and Crystalline Morphology of Linear Low Density Polyethylene

In this study, effects of oscillatory shear with different frequencies （0-2.5 Hz） and amplitudes （0-20 mm） on the mechanical properties and crystalline morphology of linear low density polyethylene （LLDPE） were investigated. It was found that the mechanical properties of LLDPE are improved because of the more perfect crystalline structure when LLDPE crystallizes under low-frequency and small-amplitude （0.2 Hz/4 mm） oscillatory shear. The mechanical properties can be further improved by increasing either the frequency or the amplitude of oscillatory shear. The Young＇s modulus and tensile strength of LLDPE are improved by 27% and 20%, respectively, when the frequency is increased to 2.5 Hz and the amplitude is maintained at 4 mm; while the Young＇s modulus and tensile strength are improved by 49% and 47%, respectively, when the amplitude is increased to 20 mm and the frequency is remained as 0.2 Hz. The crystallinity and microstructure of LLDPE under different oscillatory shear conditions were investigated by using differential scanning calorimetry, wide angle X-ray diffraction and scanning electron microscopy to shed light on the mechanism for the improvement of mechanical properties.

Strain Softening of Styrene-Isoprene-Styrene Copolymers under Large Amplitude Oscillatory Shear for Clarifying Payne Effect in Rubbers and Their Nanocomposites

To illustrate mechanisms of Payne effect in rubbers and their nanocomposites experiencing large amplitude oscillatory shear(LAOS),comparison studies were performed in styrene-isoprene-styrene(SIS)copolymers and their selectively crosslinked materials at temperatures below and above glass transition temperature of the polystyrene(PS)phase.It was found that under periodic dynamic shear,the strain softening is reversible when the polyisoprene(PI)phase,either crosslinked or not,is restricted by hard PS domains but it shows hysteresis once the PS domains disassociate.The strain softening can happen at the time scale of intrinsic Rouse relaxation of elastically active network strands.Critical stress of strain softening scales with number density of elastically active network strands,a simple relation being verified not only in the selectively crosslinked SIS copolymers but also in PI gum vulcanizates and carbon black filled PI compounds.Payne effect is traditionally used to term strain softening of highly filled rubber vulcanizates under LAOS deformation while evidenced herein is that the Payne effect of highly filled rubber vulcanizates shares the mechanism being common to the strain softening of SIS copolymers.

Large Amplitude Oscillatory Shear Studies on the Strain-stiffening Behavior of Gelatin Gels

Linear and nonlinear viscoelasticity of gelatin solutions was investigated by rheology. The dynamic mechanical properties during the sol-gel transition of gelatin followed the time-cure superposition. The fractal dimension df of the critical gel was estimated as 1.76, which indicated a loose network. A high sol fraction ws = 0.61 was evaluated from the plateau modulus by semi-empirical models. Strain-stiffening behavior was observed under large amplitude oscillatory shear（LAOS） for the gelatin gel. The strain and frequency dependence of the minimum strain modulus GM, energy dissipation Ed, and nonlinear viscoelastic parameter NE was illustrated in Pipkin diagrams and explained by the strain induced helix formation reported previously by others. The BST model described the strain-stiffening behavior of gelatin gel quite well, whereas the Gent and worm-like chain network models overestimated the strain-stiffening at large strains.

Some exact solutions of the oscillatory motion of a generalized second grade fluid in an annular region of two cylinders

The velocity field and the associated shear stress corresponding to the longitudinal oscillatory flow of a generalized second grade fluid, between two infinite coaxial circular cylinders, are determined by means of the Laplace and Hankel transforms. Initially, the fluid and cylinders are at rest and at t = 0＋ both cylinders suddenly begin to oscillate along their common axis with simple harmonic motions having angular frequencies Ω1 and Ω2. The solutions that have been obtained are presented under integral and series forms in terms of the generalized G and R functions and satisfy the governing differential equation and all imposed initial and boundary conditions. The respective solutions for the motion between the cylinders, when one of them is at rest, can be obtained from our general solutions. Furthermore, the corresponding solutions for the similar flow of ordinary second grade fluid and Newtonian fluid are also obtained as limiting cases of our general solutions. At the end, the effect of different parameters on the flow of ordinary second grade and generalized second grade fluid are investigated graphically by plotting velocity profiles.

Non-linear Viscoelastic Rheological Properties of PCC/PEG Suspensions

The shear thinning and shear thickening rheological properties of PCC/PEG suspension were investigated with the increase of oscillatory amplitude stress at different constant frequencies. The results show that the complex viscosity was initially independent of stress amplitude and obvious shear thinning occurred, then dramatic shear thickening took place after reaching the minimum viscosity. Typically, in a constant frequency of 5 rad/s, the elastic modulus, viscous modulus, and tanδ （δ is the out-of-phase angle） vs. the stress amplitude was investigated. It is found that the elastic modulus initially appeared to be independent of stress amplitude and then exhibited a rapid decrease, but the viscous modulus was independent of amplitude stress at lower amplitude stress. After reaching the minimum value the viscous modulus showed a rapid increase. On the other hand, tanδ increased from 0.6 to 92, which indicates that the transition from elastic to viscous had taken place and tanδ showed a steep increase when shear thickening occurred. Lissajous plots are shown for the dissipated energy vs. different maximum stress amplitude in the shear thinning and shear thickening regions. The relationship of dissipated energy vs. maximum stress amplitude was determined, which follows a power law. In the shear thinning region the exponent was 1.91, but it steeply increases to 3.97 in the shear thickening region.

Haemodynamic Analysis of the Relationship between the Morphological Alterations of the Ascending Aorta and the Type A Aortic-Dissection Disease

Type A aortic dissection(AD)is one of the most serious cardiovascular diseases,whose risk predictors are controversial.The purpose of this research was to investigate how elongation accompanied by dilation of the ascending aorta(AAo)affects the relevant haemodynamic characteristics using image-based computational models.Five elongated AAos with different levels of dilation have been reconstructed based on the centerlines data of an elderly and an AD patient.Numerical simulations have been performed assuming an inflow waveform and a Windkessel model with three elements for all outflow boundaries.The numerical results have revealed that the elongation of AAo can disturb the systolic helical flow pattern between the root of AAo and the aortic arch.The helical flow inside the AAo starts to develop into a vortex flow when the elongated AAo becomes dilated.The vortex gives rise to a localized oscillatory shear index at the ostia of the brachiocephalic artery(BA)and the inner curve of the aortic arch.This study suggests that abnormal growth of AAo,especially accompanied by its moderate dilation,can be considered as morphological risk factors of AD.

Exact solutions for the flow of second grade fluid in annulus between torsionally oscillating cylinders

The velocity field and the associated shear stress corresponding to the torsional oscillatory flow of a second grade fluid, between two infinite coaxial circular cylinders, are determined by means of the Laplace and Hankel transforms. At time t = 0, the fluid and both the cylinders are at rest and at t = 0 ＋ , cylinders suddenly begin to oscillate around their common axis in a simple harmonic way having angular frequencies ω 1 and ω 2 . The obtained solutions satisfy the governing differential equation and all imposed initial and boundary conditions. The solutions for the motion between the cylinders, when one of them is at rest, can be obtained from our general solutions. Furthermore, the corresponding solutions for Newtonian fluid are also obtained as limiting cases of our general solutions.

THE EFFECTS OF PHYSIOLOGICAL FLOW WAVEFORM ON WALL SHEAR STRESS IN CAROTID BIFURCATION MODEL

Numerical models of carotid bifurcation were constructed using a combination of tuning-fork bifurcation and straight or curved common carotid. The different inlet velocity profiles of the common carotid were generated for Bloch flow waveform and Holdsworth flow waveform, respectively. The effects of the different flow waveform for the common carotid on Wall Shear Stress （WSS） and Oscillatory Shear Index （OSI） of carotid bifurcation were studied by CFD method. The results show that the physiological flow waveform of curved common carotid has a significant effect on OSI. In particular, the OSI on the outer walls of carotid sinus and external carotid becomes higher in the inward-curved common carotid for Holdsworth flow waveform. But, in both cases of low WSS and high OSI, the effects of flow waveforms are smaller than those of the curved common carotid. The study reveals that the exact knowledge of the physiological flow waveform, vascular geometry and inlet velocity profile is important for hemodynamic numerical simulation of artery bifurcation.

Enhanced External Counterpulsation Inducing Arterial Hemodynamic Variations and Its Chronic Effect on Endothelial Function

To make clear the precise hemodynamic mechanism underlying the anti-atherogenesis benefit of enhanced external couterpulsation(EECP) treatment, and to investigate the proper role of some important hemodynamic factors during the atherosclerotic progress, a comprehensive study combining long-term animal experiment and numerical solving was conducted in this paper. An experimentally induced hypercholesterolemic porcine model was developed and the chronic EECP intervention was subjected. Basic hemodynamic measurement was performed in vivo, as well as the arterial endothelial samples were extracted for physiological examination. Meanwhile, a numerical model was introduced to solve the complex hemodynamic factors such as WSS and OSI. The results show that EECP treatment resulted in significant increase of the instant levels of arterial WSS, blood pressure, and OSI. During EECP treatment, the instant OSI level of the common carotid arteries over cardiac cycles raised to a mean value of 8.58 ×10-2±2.13 ×10-2. Meanwhile, the chronic intervention of EECP treatment significantly reduced the atherosclerotic lesions in abdominal aortas and the endothelial cellular adherence. The present study suggests that the unique blood flow pattern induced by EECP treatment and the augmentation of WSS level in cardiac cycles may be the most important hemodynamic mechanism that contribute to its anti-atherogenesis effect. And as one of the indices that cause great concern in current hemodynamic study, OSI may not play a key role during the initiation of atherosclerosis.

Influence of Kinematic Viscosity of Base Oil on Magnetorheological Grease

In order to explore the effect of kinematic viscosity of base oil on the rheological properties of magnetorheological(MR)grease,MR grease samples containing 70%(mass fraction)carbonyl iron powder and carrier liquid separately with kinematic viscosity of 10mm^(2)/s,100mm^(2)/s,350mm^(2)/s and 500mm^(2)/s were prepared.The influence of kinematic viscosity of carrier liquid on the settlement performance of MR grease was analyzed.The rheological properties of MR grease were tested and analyzed under steady-state shear and oscillatory shear modes.The results show that the kinematic viscosity of base oil has a significant effect on the settling stability of MR grease.The zero-field viscosity of MR grease increases with the increase of the kinematic viscosity of the base oil,and the sedimentation performance is better.The colloid stability of MR grease is poor after the kinematic viscosity of base oil is lower than a threshold,and static oil bleed will occur immediately.In addition,the shear stress of MR grease increases with the increase of magnetic induction.When the shear rate is less than 10 s−1,the shear stress increases rapidly with the increase of shear rate.When the shear rate further increases,the shear stress tends to a stable value.The reason is that the thickener fibers in MR grease are subjected to the shear stress between laminar flows,the entanglement occurs,which makes MR grease exhibit shear thinning,and its rheological properties conform to the Herschel-Bulkley constitutive model.In the process of preparation,some carbonyl iron powder will be embedded into the thickener fiber,which shows different magnetic saturation phenomena due to shear thinning under steady-state shear and oscillatory shear.

Numerical simulation to study the impact of compliance mismatch between artificial and host blood vessel on hemodynamics

Small-diameter artificial blood vessels are prone to cause intimal hyperplasia(IH)after transplantation,which leads to restenosis and low long-term patency rates.The main biomechanical factor for IH formation is the compliance mismatch between the artificial and host blood vessels which can cause abnormal hemodynamics.Although there have been many studies on vascular compliance mismatches,however,little attention has been paid to the effect of the degree of compliance mismatch between graft and the host vessel on hemodynamics.At present,the research on compliance mismatch between the artificial and host blood vessels is still very limited,especially with regard to the specific impact of the compliance mismatch degree on hemodynamics.Therefore,three end-to-end anastomosis models(the compliance of the artificial blood vessel is lower than,similar to,and higher than that of the host blood vessel,called model 1,model 2,model 3,respectively)were constructed and simulated in this study.Simulation results showed that the radial displacement difference between the artificial and host blood vessels were 0.281 mm,0.183 mm and 0.485 mm in model 1,model 2 and model 3,respectively.A low-velocity recirculation zone near the distal anastomosis was formed in model 1 which resulted in excessively low TAWSS(9.261 E-5Pa)and high OSI(0.497).Similarly,a low-velocity recirculation zone near the proximal anastomosis was formed in model 3 and lead to low TAWSS(6.007 E-4Pa)and high OSI(0.480).However,there was no low-velocity recirculation zone near the anastomosis stoma in model 2.The results are instructive for the design and preparation of artificial blood vessels.

Pulsatile blood flow through stenosed artery with axial translation

The study of pulsatile blood flow through axisymmetric stenosed artery subject to an axial translation has been attempted with hematocrit concentration-dependent blood viscosity. The heart contraction and subsequent relaxation generate periodic pressure gradient in blood flow and translation in the artery can be represented by Fourier series. Numerical data required for computing Fourier harmonics for the pressure gradient and acceleration in the artery has been simulated from pressure waveform graph and biplanar angiogram. Velocity field has been obtained by solving governing equation using variational Ritz method. The hemodynamic indicators WSS, AWSS, OSI, RRT are derived and computed numerically. The effects of thickness of stenosis, and hematocrit concentration index on these indicators are computed and analyzed through graphs.