Transient Shear Research Articles

PP.25.43] CHANGES OF INTRINSIC STIFFNESS OF THE CAROTID ARTERIAL WALL DURING THE CARDIAC CYCLE MEASURED BY SHEAR WAVE ELASTOGRAPHY IN HYPERTENSIVES COMPARED TO NORMOTENSIVES

Objective: The question whether arterial stiffness in hypertension is increased solely because of increased arterial pressure is not solved. Because measurement of arterial stiffness is highly dependent on measurement of blood pressure (BP), the development of methods independent of BP is necessary for clarifying this question. Shear wave elastography (SWE, Supersonic Imagine, Aix-en-Provence, France) is an ultrasound technique which enables to quantitatively assess local tissue stiffness by remotely generating transient shear waves into the tissue (push mode) and tracking their propagation using a very fast sampling rate (up to 10 kHz). The elastic modulus of the material is directly related to the speed of the shear waves, with no need of BP. This method has never been tested against classical echotracking (Artlab, Esaote, Maastricht, NL) and carotid to femoral pulse wave velocity (cf-PWV, Sphygmocor, AtCor, Sydney, Australia). Design and method: We included 25 subjects, 14 normotensives (NT) and 11 essential hypertensives (HT), matched for age and sex. We compared SWE to echotracking and cf-PWV. We optimized SWE algorithms for carotid wall tracking and shear wave group velocity calculation for the anterior (a-SWV) and posterior wall (p-SWV). 8 ultrasonic pushes were triggered at intervals of 200 ms in order to study the variations of stiffness during the cardiac cycle. Results: p-SWV showed no association with carotid PWV, cf-PWV nor BP. Mean a-SWV over the cardiac cycle is strongly associated with carotid PWV measured by Echotracking (r = 0.56, p = 0.003) and cf-PWV (r = 0.66, p < 0.001). a-SWV strongly increases with BP level during the cardiac cycle (p < 10–6). Similar associations between a-SWV and BP were found in NT and HT although HT had higher values of a-SWV throughout all BP levels. However, when a common BP value (100 mmHg) was considered, no significant difference was found between NT and HT, therefore showing that the wall intrinsic material stiffness is not increased in HT. Conclusions: We have demonstrated with a method independent of BP that the increased arterial stiffness in HT is entirely due to the BP increase. SWE appears as a promising technique for assessing arterial stiffness.

A new contact & slip model for tracked vehicle transient dynamics on hard ground

This study presents a new general transient contact and slip model for tracked vehicles on hard ground which is simple, accurate, and in agreement with the test results to a satisfactory level. Simulating zero track speed instances become possible with the new contact/shear model which is the major proposed improvement in addition to more accurate results for transient steering and tractive inputs. The model represents a general tracked vehicle having rear or front sprockets, with parameters for center of gravity, wheel positions, number of wheels, and track-pretention. To calculate longitudinal and lateral forces, a transient shear model is used. Shear stress under each track pad is assumed to be a function of shear displacement. The contact time formulation used in shear displacement calculation is improved to gain accuracy for transient and zero track speed conditions.The model is implemented on the Matlab/Simulink platform and verified with a comprehensive program of road tests composed of transient steering and tractive/braking scenarios. The results of the simulations and the road tests are satisfactorily similar for both constant and transient input maneuvers. Moreover, sensitivity simulations for vehicle parameters are conducted to show that the model responses are inline with the expected vehicle dynamics behaviours.

Theoretical correlation of linear and non-linear rheological symptoms of long-chain branching in polyethylenes irradiated by electron beam at relatively low doses

Dynamic and transient shear and elongation flow experiments along with gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) analysis are performed on linear low-density polyethylenes (LLDPEs) irradiated at doses below 25 kGy. GPC data indicate no changes in the molar mass distribution, and there are almost no changes in melt and crystallization temperatures, likewise. Contrary, dynamic shear rheological behavior including thermorheological complexity, type of reduced van Gurp-Palmen curves, and zero shear-rate viscosities all disclose growing levels of long-chain branching with irradiation dose. An inverse tube model is developed for binary blend of linear and star chains and used to extract the fraction of the branched components. Modeling results reveal progressive increase in the length and fraction of star chains, as evidenced by appearance of an anomalous double overshoot in the transient shear viscosities. Detection of strain hardening in extensional stress growth coefficient data, well-quantified by molecular stress function model, is also in agreement with the predictions of tube model.

One- and two-component colloidal glasses under transient shear

In concentrated colloidal mixtures different caging mechanisms exist and result in different arrested states: repulsive, attractive and asymmetric glasses as well as gel-like states. We discuss their microscopic structure, dynamics and rheological response. Special attention is given to the non-linear mechanical behaviour, in particular the transient rheological response after shear is started. Steps in both, shear rate and shear stress (creep test), are considered. The macroscopic viscoelastic response is related to the microscopic structure and dynamics on the individual-particle level.

Investigation of rheology and morphology to follow physical fibrillar network evolution through fiber spinning of PP/PA6 blend fiber

This work is aimed at investigating the influence of fibrillar morphology of deformed Polyamide 6 (PA6) droplets dispersed in Polypropylene (PP) matrix on the melt viscoelastic behavior of their blends. The blends of PP with various amounts of PA6 (1%, 6%, 10%, and 20%) were prepared by melt mixing in a co‐rotating twin screw extruder and fibrillated by fiber spinning process. Scanning Electron Microscopy revealed that the PA6 spherical droplets form fibrillar inclusions after fiber spinning. The steady and transient shear rheological responses of samples were evaluated in both linear and nonlinear ranges of deformation. Non‐terminal behavior of storage modulus at low frequency appeared as a typical characteristic of fibrillar morphology whose width and value depend on fibril growth. Storage modulus and complex viscosity of the blends containing PA6 fibrillated structure were remarkably enhanced compared to as‐extruded samples. The fibrillar‐induced elasticity of the fibers is a distinguishable behavior which was revealed by conducting transient stress and creep‐recovery measurements and upon appearing mature fibrils, elasticity of the polymer blend fibers increased significantly. POLYM. ENG. SCI., 58:1251–1260, 2018. © 2017 Society of Plastics Engineers

Evaluation of hepatic fibrosis: a review from the society of abdominal radiology disease focus panel.

Hepatic fibrosis is potentially reversible; however early diagnosis is necessary for treatment in order to halt progression to cirrhosis and development of complications including portal hypertension and hepatocellular carcinoma. Morphologic signs of cirrhosis on ultrasound (US), computed tomography (CT), and magnetic resonance imaging (MRI) alone are unreliable and are seen with more advanced disease. Newer imaging techniques to diagnose liver fibrosis are reliable and accurate, and include magnetic resonance elastography and US elastography (one-dimensional transient elastography and point shear wave elastography or acoustic radiation force impulse imaging). Research is ongoing with multiple other techniques for the noninvasive diagnosis of hepatic fibrosis, including MRI with diffusion-weighted imaging, hepatobiliary contrast enhancement, and perfusion; CT using perfusion, fractional extracellular space techniques, and dual-energy, contrast-enhanced US, texture analysis in multiple modalities, quantitative mapping, and direct molecular imaging probes. Efforts to advance the noninvasive imaging assessment of hepatic fibrosis will facilitate earlier diagnosis and improve patient monitoring with the goal of preventing the progression to cirrhosis and its complications.

Time-frequency analyses of fluid–solid interaction under sinusoidal translational shear deformation of the viscoelastic rat cerebrum

During normal extracellular fluid (ECF) flow in the brain glymphatic system or during pathological flow induced by trauma resulting from impacts and blast waves, ECF–solid matter interactions result from sinusoidal shear waves in the brain and cranial arterial tissue, both heterogeneous biological tissues with high fluid content. The flow in the glymphatic system is known to be forced by pulsations of the cranial arteries at about 1 Hz. The experimental shear stress response to sinusoidal translational shear deformation at 1 Hz and 25% strain amplitude and either 0% or 33% compression is compared for rat cerebrum and bovine aortic tissue. Time-frequency analyses aim to correlate the shear stress signal frequency components over time with the behavior of brain tissue constituents to identify the physical source of the shear nonlinear viscoelastic response. Discrete fast Fourier transformation analysis and the novel application to the shear stress signal of harmonic wavelet decomposition both show significant 1 Hz and 3 Hz components. The 3 Hz component in brain tissue, whose magnitude is much larger than in aortic tissue, may result from interstitial fluid induced drag forces. The harmonic wavelet decomposition locates 3 Hz harmonics whose magnitudes decrease on subsequent cycles perhaps because of bond breaking that results in easier fluid movement. Both tissues exhibit transient shear stress softening similar to the Mullins effect in rubber. The form of a new mathematical model for the drag force produced by ECF–solid matter interactions captures the third harmonic seen experimentally.

Cellulose nanofibril‐reinforced polypropylene composites for material extrusion: Rheological properties

Polypropylene (PP) is not typically utilized in 3D printing material extrusion because PP shrinks and warps during the printing process. Cellulose nanofibrils (CNF) have the potential to make PP 3D printer processable and also enhance mechanical properties of PP printed parts. The rheological behavior of CNF‐PP composites during material extrusion requires study because it is different from injection molding and compression molding processes. This study revealed the effects of CNF contents (3 and 10 wt%) and maleic anhydride polypropylene (MAPP) coupling agent on the rheological properties of CNF–PP composites. Morphological analysis showed that CNF agglomerated during spray drying and a spherical structure was formed. Rheological tests showed that the elastic modulus, complex viscosity, viscosity, and transient flow shear stress of PP were increased by the addition of 10 wt% CNF, while the creep strain of PP was reduced. The damping factor and stress relaxation time remained the same when 10 wt% CNF was added to the PP. Incorporation of MAPP into the CNF–PP composites impacted the rheological properties of the CNF–PP composites. Flexural strength and modulus of PP were improved by 5.9% and 26.8% by adding 10 wt% CNF compared to the control. POLYM. ENG. SCI., 2017. © 2017 Society of Plastics Engineers

Communication: Appearance of undershoots in start-up shear: Experimental findings captured by tumbling-snake dynamics.

Our experimental data unambiguously show (i) a damping behavior (the appearance of an undershoot following the overshoot) in the transient shear viscosity of a concentrated polymeric solution, and (ii) the absence of a corresponding behavior in the transient normal stress coefficients. Both trends are shown to be quantitatively captured by the bead-link chain kinetic theory for concentrated polymer solutions and entangled polymer melts proposed by Curtiss and Bird, supplemented by a non-constant link tension coefficient that we relate to the nematic order parameter. The observed phenomena are attributed to the tumbling behavior of the links, triggered by rotational fluctuations, on top of reptation. Using model parameters deduced from stationary data, we calculate the transient behavior of the stress tensor for this "tumbling-snake" model after startup of shear flow efficiently via simple Brownian dynamics. The unaltered method is capable of handling arbitrary homogeneous flows and has the promising capacity to improve our understanding of the transient behavior of concentrated polymer solutions.

Numerical determination of shear stress relaxation modulus of polymer glasses.

Focusing on simulated polymer glasses well below the glass transition, we confirm the validity and the efficiency of the recently proposed simple-average expression [Formula: see text] for the computational determination of the shear stress relaxation modulus G(t). Here, [Formula: see text] characterizes the affine shear transformation of the system at t = 0 and h(t) the mean-square displacement of the instantaneous shear stress as a function of time t. This relation is seen to be particulary useful for systems with quenched or sluggish transient shear stresses which necessarily arise below the glass transition. The commonly accepted relation [Formula: see text] using the shear stress auto-correlation function c(t) becomes incorrect in this limit.

Rheological properties of TiO2 suspensions varied by shifting the electrostatic inter-particle interactions with an organic co-solvent

The rheology of mineral suspensions is a key factor in many application fields. In particular, adjusting either the solid amount or the solvent composition appears as a possibility to control the rheological properties. These modifications should lead to variations of the electrostatic inter-particle interactions influencing thus the suspensions viscoelastic behavior. TiO2 suspensions were firstly formulated in water by adjusting the pH close to the isoelectric point of the TiO2 nanoparticles. The influence of the solid amount as well as the pH was study and coupled to the rheological properties of aqueous suspensions. In a second time, a part of water was substituted by an organic solvent (i.e. ethanol). Rheological measurements were performed and coupled to the zeta potentials of the TiO2 into different solvents. The rheological properties of TiO2 suspensions were adjusted by varying the electrostatic inter-particle interactions. The pH has to be close enough to the TiO2 isoelectric point and the particle concentration high enough to create a percolating network. With adding ethanol, transient shear thickening is suppressed and, even far from the isoelectric point, the resulting flocculated suspensions show high yield stresses over a large pH range.

Bases physiques de l’élastographie et applications au tendon calcanéen normal et pathologique

Elastosonography is an evolution of ultrasound that maps the biomechanical and elastic properties of tissues by measuring their stiffness. Over the past five years, the number of studies on elastosonography in tendon pathologies raised. Indeed, this technique is promising, but how can radiologists use it in daily practice? The purpose of this review article is to summarize the data available to date about elastosonography focused on the calcaneal tendon in order to provide physicians with an overview that may help them understand and use elastosonography routinely. The physical principles of elastosonography are described. The aspects of normal, physiological and pathologic calcaneal tendon are discussed. Transient Shear Wave Elastography (SWE), which is the most recent elastosonographic technique, allowed to quantify tendon stiffening when tendon stress increases, confirmed tendon softening in case of tendinopathy, and shown tendon strength loss when calcaneal tendon is completely ruptured. SWE is still a wide research field on viscoelastic properties of normal, pathologic, aging and operated calcaneal tendon.

Start-up inertia as an origin for heterogeneous flow.

For quite some time nonmonotonic flow curve was thought to be a requirement for shear banded flows in complex fluids. Thus, in simple yield stress fluids shear banding was considered to be absent. Recent spatially resolved rheological experiments have found simple yield stress fluids to exhibit shear banded flow profiles. One proposed mechanism for the initiation of such transient shear banding process has been a small stress heterogeneity rising from the experimental device geometry. Here, using computational fluid dynamics methods, we show that transient shear banding can be initialized even under homogeneous stress conditions by the fluid start-up inertia, and that such mechanism indeed is present in realistic experimental conditions.

In Situ Measurements of Protein Forces and Intracellular Ca2+ under Fluid Shear Stimuli

During traumatic brain injury, brain cells experience transient shear stresses that alter the forces in structural proteins. An increase in intracellular Ca2+ is seen universally in cells subjected to shear forces, but how are mechanical forces coupled to the Ca2+ influx is not well understood. We used a microfluidic chamber driven with a high-speed pressure servo to generate controlled fluid shear stress to cultured astrocytes, and simultaneously measured the intracellular responses using FRET-based force sensors actinin-cpstFRET and Ca2+ probes jRCaMP1h. We found that fluid shear generated non-uniform stresses in actinin. The time dependent force distribution is highly sensitive to the feature of the stimulus such as amplitude, duration rise time, and the frequencies of stimulation. In cells with weaker stress fibers, a rapid shear pulse (23 dyn/cm2, 2 ms rise time, 400 ms duration) produced an immediate and long-lasting increase in actinin stress at the upstream end of the cell and minimal changes at the downstream end. In contrast, a slow ramp to the same amplitude caused a minimal, and a more uniform increase in actinin stresses. Simultaneous Ca2+ imaging showed that the initial Ca2+ rise began at the upstream end of the cell where there was high strain, and it propagated to the entire cell within ∼4 s. Moreover, the Ca2+ peaked much faster (∼4 s) at the front end and slower in the center of cell body (∼10 s). This behavior occurred in ∼40% cells. In cells with abundant strong actin bundles, there was minimal response to shear stress. The actinin stress increased transiently and uniformly over the entire cell during the period of stimulation, and the Ca2+ rise appeared starting from the cell body. These results suggest that the force distribution directly alter the initial Ca2+ dynamics in different cellular domains. This work was funded by NINDS.

Ring-shear studies of till deformation: Coulomb-plastic behavior and distributed strain in glacier beds

A ring-shear device was used to study the factors that control the ultimate (steady) strength of till at high shear strains. Tests at a steady strain rate and at different stresses normal to the shearing direction yielded ultimate friction angles of 26.3 and 18.6° for tills containing 4% and 30% clay-sized particles, respectively. Other tests at steady normal stresses and variable shear-strain rates indicated a tendency for both tills to weaken slightly with increasing strain rate. This weakening may be due to small increases in till porosity. These results provide no evidence of viscous behavior and suggest that a Coulomb-plastic idealization is reasonable for till deformation. However, viscous behavior has often been suggested on the basis of distributed shear strain observed in subglacial till. We hypothesize that deformation may become distributed in till that is deformed cyclically in response to fluctuations in basal water pressure. During a deformation event, transient dilation ofdiscrete shear zones should cause a reduction in internal pore-water pressure that should strengthen these zones relative to the surrounding till, a process called dilatant hardening. Consequent changes in shear-zone position, when integrated over time, may yield the observed distributed strain.

Transient inhomogeneous flow patterns in supercooled liquids under shear.

Supercooled liquids and other soft glassy systems show characteristic spatial inhomogeneities in their local dynamical properties. Using detailed molecular dynamics simulations, we find that for sufficiently low temperatures and sufficiently high shear rates supercooled liquids also show transient inhomogeneous flow patterns (shear banding) in the start-up of steady shear flow, similar to what has already been observed for many other soft glassy systems. We verify that the onset of transient shear banding coincides quite well with the appearance of a stress overshoot for temperatures in the supercooled regime. We find that the slower bands adapt less well to the imposed deformation and therefore accumulate higher shear stresses compared to the fast bands at comparable local shear rates. Our results also indicate that the shear rates of the fast and slow bands are adjusted such that the local dissipation rate is approximately the same in both bands.

On the quantification and visualization of transient periodic instabilities in pulsatile flows

Turbulent-like flows without cycle-to-cycle variations are more frequently being reported in studies of cardiovascular flows. The associated stimuli might be of mechanobiological relevance, but how to quantify them objectively is not obvious. Classical Reynolds decomposition, where the flow is separated into mean and fluctuating velocity components, is not applicable as the phase-average is zero. We therefore expanded on established techniques and present the idea, analogous to Reynolds decomposition, to decompose a flow with transient instabilities into low- versus high frequency components, respectively, to discriminate flow instabilities from the underlying cardiac pulsatility. Transient wall shear stress and velocity signals derived from computational fluid dynamic simulations were transferred to the frequency domain. A high-pass filter was applied to subtract the 99% most-energy-containing frequencies, which gave a cut-off frequency of 25Hz. We introduce here the spectral power index, and compute the fluctuating kinetic energy, based on the high-pass filtered velocity components, both being frequency-based operators. The efficacy was evaluated in an aneurysm model for multiple flow rates demonstrating transition to turbulent-like flows. The frequency-based operators were found to better correlate with the qualitatively observed flow instabilities compared to conventional descriptors, like time-averaged wall shear stress or oscillatory shear index. We demonstrate how the high frequencies beyond the physiological range could be analyzed and/or transferred back to the time domain for quantification and visualization purposes. We have introduced general frequency-based operators, easily extendable to other cardiovascular territories based on a posteriori heuristic filtering that allows for separation, isolation, and quantification of cycle-invariant turbulent-like flows.

Are transient and shear wave elastography useful tools in Gaucher disease?

Up to now, there are no reliable biochemical markers or imaging that could reveal early tissue damage in Gaucher disease. Therefore, we addressed whether elastography technique can serve as a tool for evaluating patients with Gaucher disease. The study included 42 patients with Gaucher disease type I and 33 patients with liver cirrhosis as well as 22 healthy volunteers. Ultrasound and Doppler examination was performed on each participant prior to apply transient and 2D shear wave elastography. In Gaucher disease the median stiffness of the spleen as assessed by transient elastography (TE) and shear wave elastography (SWE) was 35KPa and 22KPa respectively in contrast to the median stiffness of healthy controls (16.95 and 17.5KPa, p=0.0028 and p=0.0002, respectively) and of patients with cirrhosis (45KPa and 34.5KPa, p=0.015 and p<0.0001 respectively). The liver stiffness in GD as measured by TE and SWE had median values of 7.1KPa and 7KPa respectively, slightly higher than in the healthy controls, but much smaller than for the cirrhotic patients (medians of 24.2KPa and 21KPa). In conclusion, a transient and shear wave elastography show a significant promise as noninvasive and reproducible tools to differentiate Gaucher disease from healthy controls and among those with splenomegaly from cirrhotic patients.

Preparation and characterisation of open-celled foams using polystyrene-b-poly(4-vinylpyridine) and poly(4-methylstyrene)-b-poly(4-vinylpyridine) diblock copolymers

In this study, the potential of amphiphilic diblock copolymers on the example of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) and poly(4-methylstyrene)-b-poly(4-vinylpyridine) (P4mS-b-P4VP) for producing open-celled foams was evaluated. The porous foam structure was realised by employing carbon dioxide and water as environmental-friendly blowing agents. Rheological measurements in shear and elongation revealed a similar strain-softening behaviour of the diblock copolymers in the melt state. The transient shear and elongational viscosity of the P4mS homopolymer agreed with the linear viscoelastic prediction. On the contrary, the employed PS homopolymer showed strain-hardening. The rheological behaviour of the PS and P4mS homopolymers is consistent with their foaming ability. The PS homopolymer led to a homogeneous, closed cell structure, while the P4mS homopolymer did not foam at all. Because of strain-softening in melt elongation, the PS-b-P4VP diblock copolymer generated homogeneous, open-celled foams throughout the whole sample. In contrast, the P4mS-b-P4VP diblock copolymer generated partially open-celled structures in coexistence with compact areas. In this work, it was demonstrated that the combination of carbon dioxide and water led to open-celled diblock copolymer foams even if the major component of the block copolymer generates homogeneous closed-celled foams or is not foamable, respectively.

Mixed Convection Flow Along a Horizontal Circular Cylinder with Small Amplitude Oscillation in Surface Temperature and Free Stream

The problem is considered on mixed convection flow due to the effect of small amplitude oscillations of a viscous incompressible fluid along a horizontal circular cylinder. Direct implicit finite-difference scheme is employed to solve the dimensionless system of partial differential equations. In case of steady flow, the solutions are presented as functions of the curvature parameter X on the entire surface of the cylinder and there is a visual comparison with the existing result. For fluctuating flow, considering Prandtl number, Pr=1.0, the results are shown graphically in terms of amplitude and phase of the Nusselt number for different values of buoyancy parameter Lambda. Due to the effect of Lambda and frequency parameter omega, streamlines and isotherms as well as transient shear stress and heat transfer are illustrated in the interplay of study.