Transient Shear Research Articles

Transient shear banding during startup flow: Insights from nonlinear simulations

We study the dynamics of shear startup of Johnson–Segalman and non-stretching Rolie-Poly models using nonlinear simulations. We consider startup to shear rates in both monotonic and nonmonotonic regions of the constitutive curve. For the Johnson–Segalman model, which exhibits a shear stress overshoot during startup, our nonlinear simulations show that transient shear banding is absent regardless of whether the startup shear rate is in the monotonic or nonmonotonic regions of the constitutive curve. In the latter case, while there is clearly an inhomogeneity en route to the banded state, the magnitude of the extent of banding is not substantially large compared to that of the eventual banded state. Marked inhomogeneity in the velocity profile is predicted for the nonstretching Rolie-Poly model only if the solvent to solution viscosity ratio is smaller than O(10−3), but its occurrence does not appear to have any correlation with the stress overshoot during startup. The comparison of the present nonlinear results with the results obtained within the framework of linearized dynamics show that nonlinearities have a stabilizing effect and mitigate the divergence of perturbations (as predicted within the linearized dynamics) during shear startup. We argue that the neglect of inertia in the nonlinear simulations is not self-consistent if the solvent to solution viscosity ratio is very small, and that inertial effects need to be included in order to obtain physically realistic results. Furthermore, our study demonstrates a pronounced sensitivity of shear startup in the nonstretching Rolie-Poly model when a random white noise with zero mean is used as the initial perturbation. Finally, this study clearly emphasizes that stress overshoot during shear startup does not always result in transient shear banding, notwithstanding whether the shear rates is in the monotonic or nonmonotonic part of the constitutive curve.

Rheo-optics of giant micelles: SALS patterns of cetyltrimethylammonium tosylate solutions in presence of sodium bromide

In this work, we present a systematic study based on Small-Angle Light Scattering (SALS) patterns of the simple shear flow response of semi-diluted solutions of cetyltrimethylammonium tosylate (CTAT; 5.5 wt.% - 0.12 M) in the presence of sodium bromide (NaBr) at different [NaBr]={0,0.12,0.19,0.25,0.3} M concentrations. We evidence a relationship between rheological and light scattering data that reveals a transition into a fast-breaking regime in the dynamics of wormlike micelles formed by the CTAT/NaBr system (Macías et al., 2011; Fierro et al., 2021). This transition into a micellar fast-breaking regime with salt addition ([NaBr]≥0) appears marked by the following features: (i) a decrease in the relaxation time of the material λ0, accompanied by (ii) a decrease of the viscosity level at low shear rates η0 (Macías et al., 2011; Fierro et al., 2021; Schubert et al., 2003; Alkschbirs et al., 2015; Bandyopadhyay et al., 2003). With these, (iii) the formation of butterfly-like patterns is recorded originating from concentration fluctuations, evolution that is accompanied by: (iv) shear banding in the form of non-monotonic flow curves and (v) slow oscillatory transient responses in start-up flow tests captured theoretically with the Bautista–Manero–Puig (BMP) model. In addition, the Cox–Merz rule is fulfilled at molar salt-to-surfactant ratios of R≥1.5. This results in shorter structure-recovery time-scales than the characteristic-time of the flow (Macías et al., 2011; Fierro et al., 2021; Manero et al., 2002). In the case of the elastic modulus G0, the variation was small, which suggests a transition from an entangled to a multiconnected network, as suggested by Kadoma & van Egmond (1997), Kadoma et al. (1997) and Fierro et al. (2021). From a theoretical perspective, we provide predictions for the shear–stress and the first normal-stress growth coefficients in transient start-up simple shear flow using the BMP model. Here, banding R=1.5 solutions display overshot responses at relatively high shear rates (γ̇=10−30s−1), in-line with experimental findings on the start-up flow of wormlike micellar solutions (Soltero et al., 1999; Lerouge et al., 2004; Hu & Lips 2005; Pipe et al., 2010; Mohammadigoushki et al., 2019). Our results are consistent with those reported in other investigations (Macías et al., 2011; Fierro et al., 2021; Schubert et al., 2003; Alkschbirs et al., 2015; Bandyopadhyay et al., 2003; Kadoma et al., 1997; Soltero et al., 1999; Lerouge et al., 2004; Hu & Lips 2005; López-Barrón et al., 2014) and reveal the influence of the Br−-ion used on the mechanical and optical response of the CTAT-NaBr system.

A continuum model for granular materials exhibiting a smooth transition between inelastic flow and elastic jamming

A continuum model for granular materials, which transitions from an elastic jammed phase to rapid flows, is developed using a large deformation Eulerian formulation for compressible elastic-viscoplastic response. The model incorporates constitutive functions capturing μ(I) rheology, the value of the solid fraction developed for steady-state shearing, and the transient response for shear reversal based on experiments and simulations of interacting particles using the Discrete Element Method (DEM). The present model exhibits hyperelastic response with the stress determined by measures of elastic dilatation and elastic distortional deformations. This causes the stress to naturally have a direction different from the rate of deformation tensor for transient response. Examples examine pure dilation, steady-state simple shear, transient shearing with shear reversal for constant volume and constant non-shearing components of stress, and the transient transition to jamming.

Study on cutting temperatures of SiCp ∕ Al composites for ultrasonic vibration-assisted cutting

Abstract. In order to deeply understand the cutting mechanism of SiCp / Al in ultrasonic vibration-assisted turning, a prediction model of a cutting temperature field of SiCp / Al composites in UVAC (ultrasonic vibration-assisted cutting) was established. A theoretical model of instantaneous cutting depth and transient shear angle was established considering the real-time changing cutting depth, tool front angle and shear angle characteristics of UVAC. The relationship between cutting speed, shear speed and chip flow speed in UVAC processes is revealed, as well as the shear force and the front cutter friction force. Finally, the influence of heat generated by the heat source zone and shear heat source zone on the temperature rise was calculated, and the temperature field model was established. The experiment of processing SiCp / Al composites by UVAC was carried out. SiCp / Al composites with 25 % volume fraction were turned, and the cutting temperature data were measured and recorded by an infrared thermal imaging device. The cutting speed, cutting depth and feed rate were tested by a single factor, and the changes in cutting temperature under different parameters were compared. Finally, the experimental data were compared with the theoretical values to verify the validity of the theoretical model.

Endogenous Acetylcholine Contributes to Flow-Mediated Dilation in Humans

Background & Objective. Flow-mediated dilation (FMD) is a crucial function of the vascular endothelium wherein intraluminal shear stress stimulates endothelium-dependent vasodilation mediated primarily by nitric oxide. Brachial artery FMD (baFMD) is widely evaluated in clinical research as an index of endothelial function, and impaired baFMD is an independent predictor of cardiovascular risk. Despite the physiological and clinical relevance of FMD, the mechanisms by which shear stress is transduced to endothelium-dependent vasodilation (i.e., upstream of nitric oxide production) remain unclear. baFMD corresponds closely with sensitivity to the endothelium-dependent vasodilator acetylcholine (ACh), and preclinical models suggest ACh is released from endothelial cells in response to shear stress. Therefore, the present study tested the hypothesis that activation of muscarinic ACh receptors facilitates baFMD in humans. Methods. baFMD was evaluated using Doppler ultrasound in healthy participants (n=11; 5F/6M; 23±2 y; means ± SE) under control conditions and following local inhibition of muscarinic ACh receptors via intra-arterial atropine infused proximal to the measurement site. baFMD was assessed in response to transient and sustained shear stimuli in separate trials: 1) reactive hyperemia following 5 min of arterial cuff occlusion (RH-FMD) and 2) steady-state rhythmic handgrip exercise at graded intensities (10, 15, 20, & 25% maximal voluntary contraction, MVC) to produce step increases in shear rate (EX-FMD). Blockade effcacy was confirmed via a dose-response to intra-arterial ACh infusion pre and post atropine. Results. Baseline brachial artery diameter and forearm blood flow were similar in all trials (all P>0.05). Atropine reduced peak brachial artery dilation following cuff release (RH-FMD, Δ diameter, atropine: 4.6±0.8 vs. control: 6.5±0.8%, P=0.04) and increased the shear stimulus (shear rate area under the curve, SRAUC: atropine, 29±3 vs. control: 21±3 s−1×103, P=0.01) such that the percent change in brachial diameter was diminished by 35±16% when normalized to SRAUC (baFMD:SRAUC ratio, atropine: 1.7±0.3 vs. control: 3.9±1.0, P=0.02). Similarly, in the EX-FMD trial, atropine reduced brachial artery diameter at all exercise intensities (25% MVC, percent change in diameter: atropine, 10.8±1.4 vs. control, 15.5±1.7%, P=0.04) despite a greater shear stimulus (shear rate: atropine, 10.9±1.1 vs. control, 9.3±0.9 s−1×102, P=0.01). Thus, atropine reduced the slope describing the relationship between brachial artery vasodilation (percent change) and the change in shear rate by 38±8% across all exercise intensities (slope, atropine: 1.3±0.1 vs. control: 2.4±0.4% Δ×s×10−2, P=0.007). As expected, atropine reduced brachial artery vasodilation in response to upstream infusion of ACh (1.5 μg ACh/dl forearm volume/min, Δ diameter, atropine: 2±1 vs. control: 23±4%, P=0.001). Conclusions. The results suggest endogenous ACh facilitates baFMD in response to both transient and sustained shear stimuli, which highlights a novel physiological role of endogenous ACh in vascular regulation in humans and provides insight regarding the basic mechanisms of flow-mediated vasodilation. NIH R01 HL119337 and F31 HL142240. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Comparison of transient elastography and shear wave elastography in patients with MAFLD: A single-center experience.

Metabolic-associated fatty liver disease and liver fibrosis are intimately linked to insulin resistance, type 2 diabetes, obesity, and metabolic syndrome. Transient elastography (TE) and point shear wave elastography (pSWE) were used to measure liver stiffness in patients who met the ultrasound criteria for steatotic liver diseases (SLD). This study compared two methods for estimating liver stiffness in patients with SLD, which in turn correlated with liver fibrosis. Ultrasound B-mode imaging was used to identify SLD. In total, 250 MAFLD patients were recruited. Patient characteristics, laboratory investigations, and liver stiffness measurements using TE and pSWE were assessed on the same day. In the study, 56.0% of the patients were male, with a mean age of 41.5 ± 10.7 years. The correlation between TE and pSWE was significant (Spearman's r = 0.867*, p < 0.001). The Bland-Altman Plot analysis confirmed this, with 97.5% of variations in LSM falling within 95% agreement ranges. Cohen's κ was used to assess the agreement between TE and pSWE fibrosis stages, showing almost perfect agreement (83.5% kappa agreement) and a strong association between pSWE and TE in the assessment fibrosis stages. In patients with MAFLD, TE, and SWE are reliable methods for measuring liver stiffness and can be used as non-invasive screening tools for the assessment of fibrosis in SLD.

Viscoelastic characterization of the mucus from the skin of loach (II) using linear viscoelastic property

ABSTRACT A structuralized Rivlin-Sawyers model with inhomogeneous shear-rate effect on the component of relaxation spectrum was proposed here to express the viscoelastic property of loach mucus. The linear viscoelastic property of the mucus was obtained using the experimental frequency sweep data at small strain, and the viscosity in the shear-rate decreasing region with 0.0631 s−1 as the maximum shear rate was fitted by the model with inhomogeneous shear-rate effect. Both the predictions of the steady viscosity curves with four maximum shear rates, i.e., 0.004 s−1, 0.01 s−1, 0.1 s−1, and 1 s−1, and that of the transient shear viscosity in the step rate experiment at 0.1 s−1 show reasonable characterization of the model on the viscoelasticity of the loach skin mucus.

Impact of rise and fall phases of shear on platelet activation and aggregation using microfluidics.

Blood flow disorders are often the result of the non-physiological narrowing of blood arteries caused by atherosclerosis and thrombus. The blood then proceeds through rising-peak-decreasing phases as it passes through the narrow area. Although abnormally high shear is known to activate platelets, the shear process that platelets undergo in small arteries is complex. Thus, understanding how each shear phase affects platelet activation can be used to improve antiplatelet therapy and decrease the risk of side effects like bleeding. Blood samples were sheared (68.8 ms,5200s-1) in vitro by the microfluidic technique, and platelet activation levels (P-selectin and integrin αIIbβ3) and von Willebrand factor (vWF) binding to platelets were analyzed by flow cytometry. Post-stenosis platelet aggregation was dynamically detected using microfluidic technology. We studied TXA2, P2Y12-ADP, and integrin αIIbβ3-fibrinogen receptor pathways by adding antiplatelet drugs, such as acetylsalicylic acid (ASA, an active ingredient of aspirin that inhibits platelet metabolism), ticagrelor (hinders platelet activation), and tirofiban (blocks integrin αIIbβ3 receptor) in vitro, respectively, to determine platelet activation function mediated by transient non-physiological high shear rates. We demonstrated that platelets can be activated under transient pathological high shear rates. The shear rise and fall phases influenced shear-induced platelet activation by regulating the binding of vWF to platelets. The degree of platelet activation and aggregation increased with multiple shear rise and fall phases. ASA did not inhibit shear-mediated platelet activation, but ticagrelor and tirofiban effectively inhibited shear-mediated platelet activation. Our data demonstrated that the shear rise and fall phases play an important role in shear-mediated platelet activation and promote platelet activation and aggregation in a vWF-dependent manner. Blocking integrin αIIbβ3 receptor and hindering P2Y12-ADP were beneficial to reducing shear-mediated platelet activation.

Noninvasive liver disease assessment to identify portal hypertension: Systematic and narrative reviews supporting the AASLD Practice Guideline.

Portal hypertension is a serious complication of cirrhosis, which leads to life-threatening complications. HVPG, a surrogate of portal pressure, is the reference standard test to assess the severity of portal hypertension. However, since HVPG is limited by its invasiveness and availability, noninvasive liver disease assessments to assess portal pressure, especially clinically significant portal hypertension (CSPH), are needed. We conducted a systematic review of Ovid MEDLINE(R) Ovid EMBASE, Ovid Cochrane Central Register of Controlled Trials, Ovid Cochrane Database of Systematic Reviews, and Scopus from each database's inception to April 22, 2022. We included only studies in English that examined ≥50 patients in single liver disease etiologies, which compared noninvasive tests (blood and/or imaging) to HVPG for predicting clinically significant portal hypertension (CSPH; defined as HVPG ≥ 10 mm Hg) in patients with chronic liver disease. Outcomes included measures of diagnostic test accuracy. Additionally, a narrative review of studies not eligible for the systematic review is also provided. Nine studies with 2492 patients met the inclusion criteria. There was substantial heterogeneity with regard to liver disease studied and cutoff values used to detect CSPH. Blood-based tests, including aspartate-to-platelet ratio index (APRI) (56% sensitivity and 68% specificity) and FIB-4 (54% sensitivity and 73% specificity) had low accuracy measures. Imaging-based tests (transient elastography and shear wave elastography detection of liver stiffness measurement [LSM]) had better accuracy but also had substantial variation; at 15kPa, TE sensitivity was 90%-96% and specificity was 48%-50%, while at 25kPa, its sensitivity and specificity were 57%-85% and 82%-93%, respectively. The narrative review suggested that imaging-based tests are the best available noninvasive liver disease assessment to detect CSPH; CSPH is highly unlikely to be present at an LSM ≤15kPa and likely to be present at an LSM ≥25kPa. While imaging-based noninvasive liver disease assessment appeared to have higher accuracy than blood-based tests to detect CSPH, only 9 studies fit the a priori established inclusion criteria for the systematic review. In addition, there was substantial study heterogeneity and variation in cutoffs for LSM to detect CSPH, limiting the ability to establish definitive cutoffs to detect CSPH.

Comparison of Transient Elastography and Point Shear Wave Elastography for Analysis of Liver Stiffness: A Prospective Study.

Liver stiffness measurement (LSM) by Fibroscan is the most used non-invasive method to assess liver fibrosis. Recently, point-shear wave elastography (pSWE) has been introduced as a simple alternative non-invasive test. Therefore, we aimed to compare the results of these two techniques. One hundred and eighty-four consecutive patients attending our outpatient ultrasound clinic were recruited. LSM was performed by both Fibroscan and pSWE. Statistical analysis was conducted by Spearman's test for correlation and linear regression. Bland-Altman graphs and ROC curves were drawn with area under the curve (AUC). Overall, the correlation of LS between Fibroscan and pSWE was substantial (r = 0.68, p < 0.001). Linear regression showed a coefficient b= 0.94 ± 0.02. The Bland-Altman plot found a bias of -0.10, with only 11 values exceeding the 95% confidence interval. When only considering patients with a LSM of > 10 kPa (n = 31), we found an excellent r = 0.79 (0.60-0.90, p < 0.001). A cutoff of 12.15 kPa for pSWE had sensitivity = 74.2% and specificity = 99.3% to detect relevant fibrosis, with an AUC = 0.98. The highest correlation was observed for hepatitis C (r = 0.91) and alcoholic liver disease (ALD)(r = 0.99). In conclusion, pSWE shows LSM estimation in agreement with Fibroscan in most cases, and the best concordance was observed for hepatitis C and ALD, and for higher ranges of LS.

Point Shear Wave Elastography Detected Liver Stiffness Increased in Pediatric Patient With Thalassemia Major.

Transient elastography (TE) and point shear wave elastography (pSWE) are 2 elastographic ultrasound examinations used in liver stiffness (LS) measurement. It was shown that the LS value detected by TE in pediatric β-thalassemia major patients has increased, and there was no LS evaluation obtained with pSWE in literature. Thus, in this study, it was aimed to evaluate LS with pSWE examination in children with thalassemia major and to determine LS-related parameters in these patients. Sixty-three schoolchildren with a diagnosis of β-thalassemia major and 21 healthy controls between the ages of 7 and 18 years were included. In addition to routine anamnesis, physical examination, and laboratory examinations, renal and liver ultrasounds were performed. Liver stiffness values were measured by pSWE examination. Serum levels of urea, aspartate-aminotransferase, alanine-aminotransferase, iron, and ferritin were significantly higher in patients, and serum creatinine, iron binding capacity, and hemoglobin levels were found to be significantly lower (P < 0.05 for each). Liver stiffness values were significantly higher in patients compared with healthy controls. In linear regression analysis, serum iron and iron binding capacity values were found to be closely related with LS (P < 0.001 vs. β = 0.482 and P = 0.047 vs. β = 0.237, respectively). Liver stiffness values obtained by pSWE examination increase significantly in patients. According to the results of our study, in addition to the previously known TE method, we think that the LS evaluation obtained by pSWE, a new method that can make more accurate measurements, can be used in the possible early detection of target organ damage in children with thalassemia major.

Yield stress Measurement of municipal sludge: A comprehensive evaluation of testing methods and concentration effects using a rotational rheometer

Accurate prediction and measurement of yield stress are crucial for optimizing sludge treatment and disposal. However, the differences and applicability of various methods for measuring yield stress are subjects of ongoing debate. Meanwhile, literature on measuring sludge yield stress is limited to low solid concentrations (TS <10%), understanding and studying the yield stress of medium to high solid concentration sludge is crucial due to increasingly stringent standards for sludge treatment and disposal. So, this study employed a rotational rheometer to measure sludge yield stress across a wide range of TS (4–50%) using steady shear, dynamic oscillatory shear, and transient shear. The study derived significant conclusions by comparing and summarizing the applicability and limitations of each testing method: Dynamic oscillatory shear methods, including G′-σ curve method, γ-σ curve method, and G**γc method can measure sludge yield stress ranging from 4% to 40% TS, while other methods are restricted to low or limited solid concentrations; The G' = G″ method, utilizing the intersection of G′ and G″ curves, consistently yields the highest value for yield stress when 4%≤ TS ≤ 12%; The rotational rheometer cannot measure sludge yield stress when the solid concentration exceeds 40% TS; The relationship between sludge yield stress and solid concentration is stronger as a power-law for TS ≤ 25%, transitioning to linear for higher concentrations (28%≤ TS <40%). This study systematically explores the applicability and limitations of various measurement methods for characterizing sludge yield stress across a wide range of solid concentrations, providing valuable guidance for scientific measurement and highlighting challenging research issues.

Non-Invasive Assessment of Liver Fibrosis in Hepatitis B Patients.

The aim of this review is to provide updated information on the clinical use of non-invasive serum and imaging-based tests for fibrosis assessment in chronic hepatitis B (CHB) virus infection. In recent years, non-invasive tests (NIT) have been increasingly used to determine eligibility for treatment. Liver biopsy is still considered the gold standard for assessing inflammatory activity and fibrosis staging, but it is an invasive procedure with inherent limitations. Simple serum markers such as APRI and FIB-4 are limited by indeterminate results but remain useful initial tests for fibrosis severity if imaging elastography is not available. Point-of-care US-based elastography techniques, such as vibration-controlled transient elastography or 2D shear wave elastography, are increasingly available and have better accuracy than simple serum tests for advanced fibrosis or cirrhosis, although stiffness cut-offs are variable based on E-antigen status and inflammatory activity. Current NITs have poor diagnostic performance for following changes in fibrosis with antiviral therapy. However, NITs may have greater clinical utility for determining prognosis in patients with CHB that have advanced disease, especially for the development of hepatocellular carcinoma and/or liver decompensation. Algorithms combining serum and imaging NITs appear promising for advanced fibrosis and prognostic risk stratification.

Single cavitation bubble dynamics in a stagnation flow

Jetting of collapsing bubbles is a key aspect in cavitation-driven fluid–solid interactions as it shapes the bubble dynamics and additionally due to its direct interaction with the wall. We study experimentally and numerically the near-wall collapse and jetting of a single bubble seeded into the stagnation flow of a wall jet, i.e. a jet that impinges perpendicular onto a solid wall. High-speed imaging shows rich and rather distinct bubble dynamics for different wall jet flow velocities and bubble-to-wall stand-off distances. The simulations use a volume-of-fluid method that allows us to numerically determine the microscopic and transient pressures and shear stresses on the wall. It is shown that a wall jet at moderate flow velocities of a few metres per second already shapes the bubble ellipsoidally inducing a planar and convergent jet flow. The distinct bubble dynamics allow us to tailor the wall interaction. In particular, the shear stresses can be increased by orders of magnitude without increasing impact pressures the same way. Interestingly, at small seeding stand-offs, the bubble during the final collapse stage can lift off the wall and migrate against the flow direction of the wall jet such that the violent collapse occurs away from the wall.

Note on the start-up of Couette flow for viscoelastic fluids

This paper is concerned with the numerical modeling of viscoelastic fluids in non-steady shear motions. Time-dependent solutions for three-constant differential models are obtained at the start-up of the planar Couette flows. The influences of (i) the Reynolds number, (ii) the value of κ− material parameter (the ratio between the retardation time and relaxation time), and (iii) the initial condition for the normal stress on the velocity and stresses distributions in the gap are investigated using the numerical solutions obtained with Mathematica software. The focus of the study is the analysis of the Jaumann model (characterized by the corotational derivative) in transitory simple shear rheological tests, as a function of initial conditions for stresses. The steady solutions, corroborated with the non-monotonicity of the steady flow curve, confirm the kink presence in the steady velocity distributions and the formation of shear bandings at Re ≥ 1. The analyses of the strain- and stress-controlled simulations performed at different initial and boundary conditions offer possible explanations of some spurious data recorded in shear measurements of complex viscoelastic fluids. The findings have important consequences for performing transient shear experiments; specifically, it is demonstrated that reproducibility and correlations between the tests require the control of initial normal stresses in the sample.

Rheological response of ferrofluids undergoing unsteady shear flows in the presence of a magnetic field

The rheological response of two commercial ferrofluids to transient shearing flows using a parallel disk rheometer device equipped with a magnetic cell is investigated. The basic difference between the ferrofluids is their volume fraction of magnetic particles. The first transient shear flow examined is a step-strain under the influence of a magnetic field, from which the stress relaxation functions for both magnetic fluids studied are obtained in terms of the magnetic field strength and the intensity of the step strain. The main relaxation times of both fluids are determined and shown to increase with the applied magnetic field parameter after some critical value. We also observed that the shear stress relaxes to a residual stress, which is strongly dependent on both magnetic field and strain strengths. This remarkable residual stress increases as the intensity of the magnetic field rises. In terms of the strain strength, this residual stress is found to have two interesting behaviors. First, for small values of strain, the residual stress increases linearly until a maximum is reached. Further increases in the strain strength lead to a nonlinear decrease in the residual stress. We conjecture that the linear regime is associated with a predominance of elastic deformation of the fluid microstructure while the nonlinear one to its plastic deformation or even to the structure breakup. The second experimental investigation of the magnetic fluids is carried out under the condition of oscillatory shear in a linear viscoelastic regime and in the presence of an applied magnetic field. The main viscoelastic moduli of the ferrofluids as functions of the non-dimensional frequency and the magnetic field intensity are presented. In addition, it is also shown, for both ferrofluids, that viscous and elastic characteristics are severely increased when the applied magnetic field intensity is enhanced. We also determine the shear elastic modulus for both magnetic fluids in the limit of low Deborah number as a function of the magnetic parameter. Compatibility checks between the viscous modulus and the apparent shear viscosity under conditions of the same frequency and shear rate are performed, and the first normal stress difference is calculated.

Subglacial tills: a process model based on microsedimentological clues

Abstract Subglacial sediments are subject to erosion, transport, and deposition in active, ephemeral, and spatially localized glacial environments. It is critical to determine how these mobilized sediments become immobilized in a time-transgressive process and can be frequently remobilized and reimmobilized. Microscopic sedimentary structural signatures provide invaluable information on subglacial processes and contribute to understanding till formation. Data were obtained from a series of field sites in Canada and Austria investigating the microsedimentological aspects of both alpine and continental glaciation tills to construct a conceptual model of subglacial deformation. Microstructures in these tills indicate rheological behaviors that can be summarized into a potential model for soft deforming subglacial sediments. Most microstructures noted in these subglacial till examples highlight the development of subglacial interface kinematics providing clues to till deposition mechanics, subglacial bedform development, and the processes involved in till provenance distributions. A conceptual process model of subglacial interface conditions in soft mobile sediments is developed that uses microsedimentological evidence and highlights how an active ice mass integrates with ongoing substrate deformation. In the model, interaction occurs between the ice and its sediment bed with internal sediment microstructures evolving where multiple transient shear deformation processes cause localized deformation linked to pervasive and nonpervasive sediment deformation.

Evaluating the impact of transient shear stress on platelet activation, adhesion, and aggregation with microfluidic chip technique.

When nonphysiological stenosis occurs, the transient high shear stress formed in vessels increases the risk of thrombosis and is a potential factor for cardiovascular diseases. But the platelet adhesion and aggregation behavior at nonphysiological post-stenosis and its affecting factors are not fully understood yet. In this experiment, platelet aggregation on collagen and fibrinogen at different shear stresses and different hematocrits were observed by microfluidic technology. Platelet activation (P-selectin, glycoprotein IIb/IIIa) and monocyte-platelet aggregate (MPA) levels under different shear stresses were analyzed by flow cytometry. On fibrinogen, platelets aggregate more at higher shear stress conditions. While on collagen, it becomes more difficult for platelets to form stable aggregation at higher shear stress conditions. If platelets adhere initially at low shear stress, stable platelet aggregation can be formed at subsequent high shear stress. Moreover, when the shear stress increases, platelet activity markers (P-selectin, glycoprotein IIb/IIIa and MPAs) increase significantly. Hematocrit affects the degree of platelet aggregation, and the influence of hematocrit is obvious at high shear stress. Transient high shear stress (46 ms) can effectively activate platelets. Platelet aggregation behavior was different for coated fibrinogen and collagen protein. Stable platelet adhesion at post-stenosis is more dependent on fibrinogen and platelet aggregation is stable on both fibrinogen and collagen. Hematocrit can significantly affect the formation of platelet aggregation.

Adjacent tissues modulate shear wave propagation in axially loaded tendons

Shear wave tensiometry is a noninvasive approach for gauging tendon loads based on shear wave speed. Transient shear waves are induced and tracked via sensors secured to the skin overlying a superficial tendon. Wave speeds measured in vivo via tensiometry modulate with tendon load but are lower than that predicted by a tensioned beam model of an isolated tendon, which may be due to the added inertia of adjacent tissues. The objective of this study was to investigate the effects of adjacent fat tissue on shear wave propagation measurements in axially loaded tendons. We created a layered, dynamic finite element model of an elliptical tendon surrounded by subcutaneous fat. Transient shear waves were generated via an impulsive excitation delivered across the tendon or through the subcutaneous fat. The layered models demonstrated dispersive behavior with phase velocity increasing with frequency. Group shear wave speed could be ascertained via dispersion analysis or time-to-peak measures at sequential spatial locations. Simulated wave speeds in the tendon and adjacent fat were similar and modulated with tendon loading. However, wave speed magnitudes were consistently lower in the layered models than in an isolated tendon. For all models, the wave speed-stress relationship was well described by a tensioned beam model after accounting for the added inertia of the adjacent tissues. These results support the premise that externally excited shear waves are measurable in subcutaneous fat and modulate with axial loading in the underlying tendon. The model suggests that adjacent tissues add inertia to the system, which in turn lowers shear wave speeds. This information must be considered when using tensiometry as a clinical or research tool to infer absolute tendon loading.

Molecular Processes Leading to Shear Banding in Entangled Polymeric Solutions.

The temporal and spatial evolution of shear banding during startup and steady-state shear flow was studied for solutions of entangled, linear, monodisperse polyethylene C3000H6002 dissolved in hexadecane and benzene solvents. A high-fidelity coarse-grained dissipative particle dynamics method was developed and evaluated based on previous NEMD simulations of similar solutions. The polymeric contribution to shear stress exhibited a monotonically increasing flow curve with a broad stress plateau at intermediate shear rates. For startup shear flow, transient shear banding was observed at applied shear rates within the steady-state shear stress plateau. Shear bands were generated at strain values where the first normal stress difference exhibited a maximum, with lifetimes persisting for up to several hundred strain units. During the lifetime of the shear bands, an inhomogeneous concentration distribution was evident within the system, with higher polymer concentration in the slow bands at low effective shear rate; i.e., γ˙<τR-1, and vice versa at high shear rate. At low values of applied shear rate, a reverse flow phenomenon was observed in the hexadecane solution, which resulted from elastic recoil of the molecules within the slow band. In all cases, the shear bands dissipated at high strains and the system attained steady-state behavior, with a uniform, linear velocity profile across the simulation cell and a homogeneous concentration.