Time‐dependent finite element simulations of a shear‐thinning viscoelastic fluid with application to blood flow

A new finite element method is developed to simulate time‐dependent viscoelastic shear‐thinning flows characterized by the generalized Oldroyd‐B model. The focus of the algorithm is improved stability through a free‐energy dissipative scheme by using low‐order piecewise‐constant finite element approximations for stress. The algorithm is further modified by incorporating a pressure‐projection method, a DG‐upwinding scheme, a symmetric interior penalty DG method to solve the elliptic pressure‐update equation and a geometric multigrid preconditioner. The improved stability and cost to accuracy is compared when using higher order discontinuous bilinear approximation, where in addition, we consider the influence of a slope limiter for these elements. The algorithm is applied to the 2D start‐up‐driven cavity problem, and the stability of the free energy is illustrated and compared between element choices. An application of the model to modelling blood in small arterioles and channels is considered by simulating pulsatile blood flow through a stenotic arteriole. The individual influences of viscoelasticity and shear‐thinning within the generalized Oldroyd‐B model are investigated by comparing results to the Newtonian, generalized Newtonian and Oldroyd‐B models. Copyright © 2014 John Wiley & Sons, Ltd.     

Single‐Walled Carbon Nanotubes in Highly Viscous Media: A Comparison between the Dispersive Agents [BMIM][BF4], L121, and Triton X‐100

Dispersions of single‐walled carbon nanotubes (SWNTs) have been prepared by using the room‐temperature ionic liquid [BMIM][BF₄] (1‐butyl‐3‐methylimidazolium tetrafluoroborate), the triblock copolymer Pluronic L121 [poly(ethylene oxide)₅‐poly(propylene oxide)₆₈‐poly(ethylene oxide)₅] and the non‐ionic surfactant Triton X‐100 (TX100) in the pure state. The size of the SWNTs aggregates and the dispersion degree in the three viscous systems depend on the sonication time, as highlighted by UV/Vis/NIR spectroscopy and optical microscopy analysis. A nonlinear increase in conductivity can be observed as a function of the SWNTs loading, as suggested by electrochemical impedance spectroscopy. The generation of a three‐dimensional network of SWNTs showing a viscoelastic gel‐like behavior above a critical percolation concentration has been found at 25 °C in all the investigated systems by oscillatory rheology measurements.     

Insight into shear‐induced modification for improving processability of polymers: Effect of shear rate on the evolution of entanglement state

Many experimental results have revealed that the re‐entanglement kinetics of disentangled polymers is much slower than that predicted by tube theory. This retarded recovery of fully entangled state is of practical significance that shear‐induced modification may offer a way to improve processability for a polymer by reducing viscosity. This work tried to figure out the shear‐rate dependence variation of viscosity in the view of evolution of entanglement state through disentanglement and re‐entanglement, aiming to provide fundamental insights into application prospect of shear‐induced modification in preparing “in‐pellet” disentangled polymers prior to final processing. High‐density polyethylene was sheared on a parallel‐plate rotational rheometer with a linearly increased shear rate. Results showed that higher shear rate could induce further disentanglement, resulting in a lower viscosity with a reduction rate up to 93.7%, larger molecular weight between entanglements M_e , and longer re‐entanglement time. Additionally, less entanglement would give a larger lamellar thickness of sheared samples after nonisothermal crystallization. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 598–606     

Regulation of shear stress on rolling behaviors of HL-60 cells on P-selectin

Circulating leukocytes in trafficking to the inflammatory sites, will be first tether to, and then roll on the vascular surface. This event is mediated through specific interaction of P-selectin and P-selectin glycoprotein ligand-1(PSGL-1), and regulated by hemodynamics. Poor data were reported in understanding P-selectin-mediated rolling. With the flow chamber technique, we herein observed HL-60 cell rolling on P-selectin with or without 3% Ficoll at various wall shear stresses from 0.05 to 0.4 dyn/cm2. The results demonstrated that force rather than transport regulated the rolling, similar to rolling on L- and E-selectin. The rolling was accelerated quickly by an increasing force below the optimal shear threshold of 0.15 dyn/cm2 first and then followed by a slowly decelerating phase starting at the optimum, showing a catch-slip transition and serving as a mechanism for the rolling. The catch-slip transition was completely reflected to the tether lifetime and other rolling parameters, such as the mean and fractional stop time. The narrow catch bond regime stabilized the rolling quickly, through steeply increasing fractional stop time to a plateau of about 0.85. Data presented here suggest that the low shear stress threshold serves as a mechanism for most cell rolling events through P-selectin.     

Mechanistic model for deformation of polymer nanocomposite melts under large amplitude shear

We report the mechanical response of a model nanocomposite system of poly(styrene) (PS)-silica to large-amplitude oscillatory shear deformations. Nonlinear behavior of PS nanocomposites is discussed with the changes in particle dispersion upon deformation to provide a complete physical picture of their mechanical properties. The elastic stresses for the particle and polymer are resolved by decomposing the total stress into its purely elastic and viscous components for composites at different strain levels within a cycle of deformation. We propose a mechanistic model which captures the deformation of particles and polymer networks at small and large strains, respectively. We show, for the first time, that chain stretching in a polymer nanocomposite obtained in large amplitude oscillatory deformation is in good agreement with the nonlinear chain deformation theory of polymeric networks. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Relation between interfacial shear strength and compressive strength of carbon fiber/epoxy resin composite strands

The mechanism with which the fiber-matrix interfacial strength exerts its influence on the compressive strength of fiber reinforced composites has been studied by measuring the axial compressive strength of carbon fiber/epoxy resin unidirectional composite strands having different levels of interfacial shear strength. The composite strands are used for experiments in order to investigate the compressive strength which is not affected by the delamination taking place at a weak interlayer of the laminated composites. The interfacial strength is varied by applying various degrees of liquid-phase surface treatment to the fibers. The efficiency of the compressive strength of the fibers utilized in the strength of the composite strands is estimated by measuring the compressive strength of the single carbon filaments with a micro-compression test. The compressive strength of the composite strands does not increase monotonically with increasing interfacial shear strength but showes lower values at higher interfacial shear strengths. With increasing interfacial shear strength, the suppression of the interfacial failure in the misaligned fiber region increases the compressive strength, while at higher interfacial shear strengths, the enhancement of the crack sensitivity decreases the compressive strength.     

Two negative minima of the first normal stress difference in a cellulose‐based cholesteric liquid crystal: Helix uncoiling

The shear rate dependence of material functions such as shear viscosity (η) and the first normal stress difference (N₁) were given and interpreted earlier by Kiss and Porter. Their widely accepted work revealed the possibility of having a negative minimum of N₁ for polymeric liquid crystals. In this work, we disclose for the first time the evidence of two negative N₁ minima on a sheared cellulosic lyotropic system. The lower shear rate minimum is ascribed to the uncoiling of the cholesteric helix, as theoretically predicted earlier. Our findings contribute also to the understanding of the other minimum already reported in the literature and attributed to the nematic director tumbling mode. Moreover, the elastic change that the LC‐HPC sample undergoes during the helix unwinding of the cholesteric structure is also by means of oscillatory measurements. This study is a contribution for the understanding of the structure‐properties relationship linked with the complex rheological behavior of chiral nematic cellulose‐based systems and may help to improve their further processing. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 821–830     

MRI investigation of granular interface rheology using a new cylinder shear apparatus

The rheology of granular materials near an interface is investigated through proton magnetic resonance imaging. A new cylinder shear apparatus has been inserted in the magnetic resonance imaging device, which allows the control of the radial confining pressure exerted by the outer wall on the grains and the measurement of the torque on the inner shearing cylinder. A multi-layer velocimetry sequence has been developed for the simultaneous measurement of velocity profiles in different sample zones, while the measurement of the solid fraction profile is based on static imaging of the sample. This study describes the influence of the roughness of the shearing interface and of the transverse confining walls on the granular interface rheology.     

Synthesis,Characterization and Comparison of Properties of Novel Fluorinated Polyimides Derived from Bisphenol–A‐di(phthaleic anhydride)

Four new poly(etherimide)s have been synthesized by reaction with commercially available bisphenol‐A‐(diphthaleic anhydride) (BPADA) with four different kinds of diamines, namely 4,4′‐bis(p‐aminophenoxy‐3,3″‐trifluoromethyl) terphenyl,4,4′‐bis(3″‐trifluoromethyl‐p‐aminobiphenyl ether)biphenyl,2,6‐bis(3′‐trifluoromethyl‐p‐aminobiphenyl ether)pyridine, 2,5‐bis(3′‐trifluoromethyl‐p‐aminobiphenylether)thiopene. The poly(etherimide)s are named as 1a, 1b, 1c and 1d, respectively. The synthesized polyimides show good solubility in various organic solvents. The polyimide films had low water absorption of 0.19–0.30% and low dielectric constant of 2.79–3.1 at 1 MHz. These polyimides showed very high thermal stability with decomposition temperature (5% wt loss) up to 522°C in nitrogen. Transparent thin films of these polyimides exhibited tensile strength up to 97 MPa, a modulus of elasticity up to 1.56 GPa and elongation at break up to 20%.     

Rheological Behavior of Dope Solutions of Poly(acrylonitrile‐co‐itaconic acid) in N,N‐dimethylformamide: Effect of Polymer Molar Mass

The rheological behavior of dope solutions of poly(acrylonitrile‐co‐itaconic acid) or poly(AN‐co‐IA) is important from the point of view of deriving the spinning conditions for good quality special acrylic fibers. The viscosity of the resin dope is dictated by the polymer concentration, molar mass, temperature and shear force. The dynamic shear rheology of concentrated poly(AN‐co‐IA) polymer dope solutions in N, N‐dimethylformamide, in the molar mass (M¯_v) range of 1×10⁵ to 1×10⁶ g/mol, was investigated in the shear rate (γ′) range of 1×10¹ to 5×10⁴ min^?1. An empirical relation between η and M¯_v was found to exist at constant shear rate. The dope viscosity was dependent on the molar mass and the shear rate at a given temperature (T) and concentration. The polymer molar mass index of dope viscosity (m) was calculated as functions of concentration (c), shear rate and temperature. The m values increased with shear rate and temperature. A master equation relating m, with shear rate and temperature was derived for a given dope concentration. At higher shear rates, m tends to the value of 3.4, which is close to the molar mass index of viscosity reported for molten thermoplastics. m increased significantly with shear rate and nominally with temperature, while an increase in concentration decreased it. The onset of shear thinning of the dope shifted to a lower shear rate regime with an increase in polymer concentration and the molar mass. For a given value of molar mass, the increase in viscosity of the dope solution with polymer concentration was dependent on the shear rate.