Changes In Shear Rate Research Articles

Viscoelastic modeling and simulation for polymer melt flow in injection/compression molding

To describe the characteristics of compressing flow, we propose a new constitutive equation based on the Phan–Thien–Tanner (PTT) model to account for the additional stresses induced by rapid shear rate change. Analytical stress solutions are derived for shear and extensional flows to make this model describe shear thinning and strain hardening of polymers. We establish the uniform viscoelastic flow problem for injection/compression in terms of a non-isothermal compressible fluid, and construct the corresponding variational equations using the finite element method. In order to make the numerical scheme stable and efficient, a pseudo Poisson equation for pressure in weak form is founded. In addition, a two-fold iterative scheme is proposed to decouple velocity, pressure, temperature and stress. We simulate the benchmark 4:1 contraction flow to validate the proposed method and compare with the published results. Furthermore, real injection/compression molding experiments are conducted. The agreements with published and measured data shows that the proposed constitutive model accurately describes compressing flow.

AN ANALYSIS OF HYDRODYNAMIC PRESSURE DISTRIBUTION AND LOAD CARRYING CAPACITY OF A CONICAL SLIDE BEARING LUBRICATED WITH NON -NEWTONIAN SECOND GRADE FLUI

In this paper, the authors present the equations of the hydrodynamic lubrication theory for conical slide bearings lubricated with the oil with properties described by the Rivlin-Ericksen model. It is assumed, that the considered lubricating oil shows non-Newtonian properties, i.e. it is an oil for which, apart from the classic dependence of oil viscosity on pressure, temperature and operating time, there is also a change in dynamic viscosity values caused by the changes of shear rate. The method of a small parameter was used to solve the conservation of momentum, stream continuity, and energy conservation equations. The small parameter method consists in presenting the sought functions (pressure, temperature, components of the velocity vector) in the form of a uniformly convergent series expansion in powers of a constant small parameter. These functions are substituted into the system of basic equations, and then the series are multiplied by the Cauchy method. By a comparison of the coefficients with the same powers of a small parameter, we obtain systems of partial differential equations, from which the subsequent approximations of unknowns of the sought functions are determined. The small parameter method separates the non-linear system of partial differential equations and creates several linear systems of equations. The aim of this work is to derive the equations describing and allowing the determination of the temperature distribution, hydrodynamic pressure distribution, velocity vector components, load carrying capacity, friction force and friction coefficient in the gap of conical slide bearing, lubricated with the oil of the properties described by the Rivlin-Ericksen model, taking into account its viscosity changes due to time of operation.

Measurement of Drilling Fluid Rheology and Modeling of Thixotropic Behavior

AbstractDrilling fluids perform a number of important functions during a drilling operation, including that of lifting drilled cuttings to the surface and balancing formation pressures. Drilling fluids are usually designed to be structured fluids exhibiting shear thinning and yield stress behavior, and most drilling fluids also exhibit thixotropy. Accurate modeling of drilling fluid rheology is necessary for predicting friction pressure losses in the wellbore while circulating, the pump pressure needed to resume circulation after a static period, and how the fluid rheology evolves with time while in static or near-static conditions. Although modeling the flow of thixotropic fluids in realistic geometries is still a formidable future challenge to be solved, considerable insights can still be gained by studying the viscometric flows of such fluids. We report a detailed rheological characterization of a water-based drilling fluid and an invert emulsion oilbased drilling fluid. The micro structure responsible for thixotropy is different in these fluids which results in different thixotropic responses. Measurements are primarily focused at transient responses to step changes in shear rate, but cover also steady state flow curves and stress overshoots during start-up of flow. We analyze the shear rate step change measurements using a structural kinetics thixotropy model.

Thixotropic Behavior of Oil‐in‐Water Emulsions Stabilized with Ethoxylated Amines at Low Shear Rates

AbstractOil‐in‐water (O/W) emulsification is a lubricating pipeline method based on the reduction of the energy frictional loss produced during viscous flow. The flow behavior of heavy O/W emulsions formulated with nonionic surfactants is described. The effects of pH and salinity of the aqueous phase on droplet diameter, stability, and apparent viscosity of O/W emulsions were evaluated. The low‐shear Couette flow of O/W emulsions displayed intense shear‐thinning and thixotropic behavior. Thixotropy was associated to the droplet deformation energy caused by shear rate changes. The droplet deformation and alignment led to the apparent viscosity reduction compared to the fluid at rest. Thixotropic behavior is supposed to balance between the breakdown and recovery of droplet ordered structures. Emulsion formulation parameters were influenced by the aqueous phase pH, enabling to manage the emulsion properties. The droplet mean diameter of < 18 µm resulted in very stable emulsions.

Rheological Properties of BCG-CO2 Fracturing Fluid for Shale Gas

BCG-CO2 fracturing fluid system is especially suitable for shale gas stimulation with advantage of minimizing the actual water consumption, reducing the damage to formation, recovering more rapidly and efficiently, etc. A major objective of this article is to ascertain the rheological properties of BCG-CO2 fracturing fluid in a wide range of experimental conditions for guiding the selection of fracturing parameters. The experimental results showed that as the temperature reaches 35 °C, the effective viscosity of BCG-CO2 fracturing fluid has an obvious increase. Apparently the shear rate change below 1000 s−1 is sharper than that above 1000 s−1. The effective viscosity presents an increase trend with increasing foam quality under foamed conditions. It has been concluded that the key factors influencing rheological characteristics of BCG-CO2 fracturing fluid are both temperature and foam quality, and the exponential function form can be used to describe the change rule approximately.

Investigation of the Effect of Ultrasonication on the Properties of Feedstock for Production of Compounded Road Asphalts

The influence of ultrasound on the physicochemical and rheological properties of bituminous feedstocks (vacuum residues of West Siberian and heavy Arlan oils, heavy vacuum gas oil, distillate oil fraction, black oil fraction) for the manufacturing of road asphalt by compounding a blown product with unoxidized feedstock has been studied. It has been shown that ultrasonication leads to a change in the chemical group composition and decreases the kinematic viscosity and density of the feedstock components. The homogeneity of the compounded asphalts obtained has been assessed by studying dynamic viscosity with a rotational rheometer under conditions of a multiple change (from 1.5 to 30 s-1) of shear rate. For an asphalt sample obtained by conventional mixing of components, the relative change in dynamic viscosity after the shear load was 58.5%. An asphalt sample obtained by compounding with ultrasonic treatment has shown a smaller change in dynamic viscosity by 32.2% and, hence, better resistance to shear loads during use as part of asphalt concrete.

Hypercapnia‐induced Shear‐mediated Dilation in the Internal Carotid Arteries Is Blunted in Healthy Older Adults

Shear‐mediated dilation (an index of vascular function) in peripheral conduit arteries is blunted with aging. However, the effects of aging on shear‐mediated dilation in carotid arteries are unknown. We hypothesized that hypercapnia‐induced shear‐mediated dilation in the common and internal carotid arteries (CCA and ICA, respectively) would be attenuated in healthy older adults relative to young adults. Hypercapnia‐induced shear‐mediated dilation in the CCA and ICA was assessed in ten young (5F/5M, 23±1 years) and ten older (6F/4M, 68±1 years) adults. Shear‐mediated dilation was induced by two levels of hypercapnia (Low and High; +5 and +10 mmHg from individual baseline end‐tidal CO2, respectively). Doppler measurements of the CCA and ICA at low and high levels of hypercapnia were performed in a randomized order. Each trial consisted of a two‐minute baseline and three minutes of hypercapnia. Percent change in diameter (shear‐mediated dilation) and shear rate (SR) were calculated as the percent rise in the peak value from the baseline value. Hypercapnia induced an increase in SR with increasing end‐tidal CO2 in both the CCA and ICA (main effect of hypercapnia; both P<0.05), while no differences were observed in the percent change in SR between young and older adults (Interaction and main effect of age, CCA; P=0.39 and P=0.74, ICA; P=0.31 and P=0.81, respectively). Shear‐mediated dilation in the CCA was increased from low to high levels of hypercapnia in both young and older adults (main effect of hypercapnia; both P<0.01) with no differences between age group (Low vs. High, Young; 2.8±0.6 vs. 4.5±0.8%, Older; 2.0±0.3 vs. 3.7±0.6%). In contrast, shear‐mediated dilation in the ICA was enhanced from low to high levels of hypercapnia in young (4.6±0.8 to 7.9±1.2%, P=0.01), but not older adults (3.6±0.4 to 4.5±0.5%, P>0.05). Consequently, shear‐mediated dilation in the ICA at the high level of hypercapnia was lower in older adults compared to the young adults (P=0.02). Lastly, shear‐mediated dilation was correlated to the percent change in SR in the young ICA (r=0.59, P<0.01) and older ICA (r=0.49, P=0.03); however, these relationships were not present in the young CCA (P=0.95) and older CCA (P=0.30). Our data indicate that although two levels of hypercapnia induced a similar increase in SR between young and older adults, shear‐mediated dilation of the ICA to the higher level of hypercapnia was attenuated in older adults, but not in the CCA. Age‐related differences in shear‐mediated dilation were not evident during the lower level of hypercapnia trials for either carotid artery. Additionally, vasodilation in the ICA is more strongly related to the increase in SR than the CCA in both young and older adults. Together, these data may provide insight into the age‐related changes in the regulation of cerebral blood flow in healthy adults.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Effect of water content and temperature on the rheological behavior of Caoqiao heavy oil

ABSTRACTDue to the high viscosity and the complex fluid characteristics of Caoqiao heavy oil, it is difficult for its exploitation and transportation. In order to effectively develop the Caoqiao heavy oil, the effect of temperature and water content on the rheological properties of heavy oil are experimentally studied. The results show that when the water content is in the range of 0 to 30%, the viscosity of heavy oil first changes little, then increases, and finally decreases with the increase of water content. The water content and temperature have a significant effect on the fluid type of Caoqiao heavy oil. There exists a Newtonian fluid transformation temperature point. When the temperature is higher than the point, the viscosity of heavy oil is basically not affected by the change of shear rate. The higher the water content, the lower the point there is. The temperature of the transformation point is the key factor to ensure the fluidity of Caoqiao heavy oil in the formation and wellbore. The experimental results can provide a directional guide for the efficient extraction and transportation of Caoqiao heavy oil.

On the evaluation of the dynamic viscosity of non-Newtonian oil based nanofluids

The present study has evaluated the rheological behavior of an oil-based (5W50) hybrid nanofluid [magnesium oxide (65%)–multi-walled carbon nanotubes (35%)]. MgO nanoparticles with a diameter of 50 nm and multi-walled carbon nanotube nanoparticles with a diameter of 3–5 nm were selected as dispersed particles in the oil base fluid. Viscosity of nanofliud was examined at the temperature range of 5–55 °C (6 cases) and concentration of 0.05–1% (6 cases). The effect of these two parameters (temperature and volume fraction) on the viscosity was studied. The results of the curve fitting on the experimental results and achieving the proper coefficient of determination (R 2) showed non-Newtonian behavior of nanofluid in all volume fractions. The impact of shear rate changes in the range of 665.5–11,997 s−1 on the viscosity was studied. A new experimental correlation was proposed in order to predict the viscosity at different temperatures, volume fractions, and shear rates. The R-squared value was 0.999, which shows the precision of the proposed correlation.

Rheological and structural properties of tart cherry puree as affected by particle size reduction

The objective of this study was to determine the effect of micronization techniques for particle size reduction from D (v, 0.9) ≥ 800 μm to D (v, 0.9) ≤ 100 μm on the rheological and structural characteristics of tart cherry puree. Commercially frozen cherry puree was thawed and micronized using Megatron MT5000 at 15000 rpm for three cycles (M1, M2, M3; 10 min each) followed by high-pressure homogenization at 200 MPa. The particle size distribution was measured using Malvern laser particle size analyzer and total soluble and insoluble solid contents by using the AOAC official method 20.5 or 30.2. All samples were further characterized for rheology, total solids content and particle morphology. Results showed a significant reduction in apparent viscosities (2.59 Pa s to 0.0144 Pa s) with a reduction in particle size (800 μm–100 μm) and this trend was consistent in different shear rate ranges. The change in serum viscosity followed the similar trend as observed in puree viscosity with micronization. Comparative analysis of the effect of shear rate (0–1000 s−1) on viscosity showed an inverse relationship, suggesting a shear thinning behavior of the fluid. This effect on serum viscosity (0.0119 Pa s to 0.0047 Pa s) was found to be higher in the shear rate range of 0–10 s−1, as opposed to a relatively less reduction from 0.0047 Pa s to 0.0022 Pa s and 0.0015 Pa s at higher shear rates in ranges of 20–100 s−1 and 20 to 1000 s−1 respectively. The viscosity–shear rate changes were approximated by Ostwald-de Waele model (power law relation) for serum and Herschel Bulkley model for puree. The total solid content (14.20 ± 1.10 °Brix) was not significantly affected by particle size reduction. Results of this study provide useful information about the rheological properties of fruit juices/purees, which are influenced by the interactions between the dispersed phase and the serum, and are critical indicators of textural properties, mouth-feel, commercial acceptability, requirements of mixing and transport equipment.

Cardiac and haemodynamic influence on carotid artery longitudinal wall motion.

What is the central question of this study? Carotid artery longitudinal wall motion (CALM) is a bidirectional forward and backward motion of the arterial wall; however, there is no evidence in humans for what controls CALM despite proposals for pulse pressure, left ventricular motion and shear rate. What is the main finding and its importance? Carotid artery longitudinal wall motion responses were heterogeneous when manipulating sympathetic activation and endothelium-independent vasodilatation, leading to non-significant group responses. However, individual CALM responses were associated with left ventricular rotation and shear rate. These findings are important when interpreting changes in CALM in humans with acute or chronic experimental designs. Carotid artery longitudinal wall motion (CALM) has recently attracted interest as an indicator of arterial health; however, the regulation of CALM is poorly understood. We conducted a series of studies aimed at manipulating pulse pressure (PP), left ventricular (LV) motion and carotid shear rate, which have been previously suggested to regulate various components of CALM pattern and magnitude. To determine the regulatory influences on CALM, 15 healthy men (22±2years old) were exposed to three acute interventions: the serial subtraction test (SST); the cold pressor test (CPT); and exposure to sublingual nitroglycerine (NTG). The SST elicited increases in PP (P<0.01), apical LV rotation (P<0.01) and carotid shear rate (P<0.01), with no changes in CALM (P>0.05). Likewise, the CPT elicited increases in PP (P=0.01), basal LV rotation (P=0.04) and carotid shear rate (P=0.01), with no changes in CALM (P>0.05). Conversely, exposure to NTG elicited no change in PP (P=0.22), basal (P=0.65) or apical LV rotation (P=0.45), but did decrease carotid shear rate (P<0.01), without altering CALM (P>0.05). Considerable individual variability in CALM responses prompted further analyses where all three interventions were pooled for change scores. Changes in LV basal rotation were related to changes in systolic retrograde CALM (B=-0.025, P=0.03), whereas changes in carotid shear rate were related to changes in diastolic CALM displacement (B=0.0009, P=0.01). The interventions were underpinned by relationships between CALM and both LV basal rotation and local shear rate at the individual level, indicating that cardiac and haemodynamic factors may influence CALM in humans.

Measuring inline velocity profile of shampoo by electrical resistance tomography (ERT)

The rheological properties of fluid systems govern the efficiency and quality of a production process. For certain materials, i.e. non-Newtonian fluids, The change in shear rate leads to the change of local viscosity, while the conductivity also changes due to the change in micro-structure of the material by varying shear rate. This paper provides a method of measuring the local velocity of shampoo in an in-line pipeline loop using electrical resistance tomography (ERT). Boundary voltages are measured by ITS P2000 ERT system and processed into a 2-D conductivity distribution in a MATLAB based ERT tool kitEIDORS. Linear-back projection (LBP) method is used for solving the inverse problem. The change in shear rate leads to a micro-structure change in the shampoo, resulting in an annular conductivity distribution in the cross-sectional view of the pipeline. By varying the flow rate, different changes in conductivity can be obtained. A velocity profile of shampoo is calculated based on the change in conductivity, giving a method to realise in-line rheometer by ERT.

Impact of Age and Aerobic Exercise Training on Conduit Artery Wall Thickness: Role of the Shear Pattern

Hemodynamic shear stress is the frictional force of blood on the arterial wall. The shear pattern in the conduit artery affects the endothelium and may participate in the development and progression of atherosclerosis. We investigated the role of the shear pattern in age- and aerobic exercise-induced changes in conduit artery wall thickness via cross-sectional and interventional studies. In a cross-sectional study, we found that brachial shear rate patterns and brachial artery intima-media thickness (IMT) correlated with age. Additionally, brachial artery shear rate patterns were associated with brachial artery IMT in 102 middle-aged and older individuals. In an interventional study, 39 middle-aged and older subjects were divided into 2 groups: control and exercise. The exercise group completed 12 weeks of aerobic exercise training. Aerobic exercise training significantly increased the antegrade shear rate and decreased the retrograde shear rate and brachial artery IMT. Moreover, changes in the brachial artery antegrade shear rate and the retrograde shear rate correlated with the change in brachial artery IMT. The results of the present study indicate that changes in brachial artery shear rate patterns may contribute to age- and aerobic exercise training-induced changes in brachial artery wall thickness.

The interface migration in shear-banded micellar solutions

In this work, we study the diffusion of the interface between bands in wormlike micellar solutions that exhibit shear banding flow regimes, namely, systems undergoing coexistence of states of different shear rates along a constant stress plateau. The migration of the interface between bands possessing different birefringence levels is predicted by the BMP (Bautista-Manero-Puig) model in which a structural parameter (the fluidity) presents two states with differing order separated by an interface. The mechanical potential derived from the constitutive equations and a diffusion term for the structure evolution equation predict various time scales of interface migration at the inception of shear flow and under shear-rate changes along the plateau stress. It is shown that the extremes of the plateau (binodals) correspond to the minima in the mechanical potential as a function of fluidity or shear rate. We also predict the dependence of the diffusive length scale on the applied shear rate.

Flow and evolution of ice-sucrose crystal mushes.

We study the rheology of suspensions of ice crystals at moderate to high volume fractions in a sucrose solution in which they are partially soluble, a model system for a wide class of crystal mushes or slurries. Under step changes in shear rate, the viscosity changes to a relaxed value over several minutes, in a manner well fitted by a single exponential. The behavior of the relaxed viscosity is power-law shear thinning with shear rate, with an exponent of -1.76±0.25, so that shear stress falls with increasing shear rate. On longer time scales, the crystals ripen (leading to a falling viscosity) so that the mean radius increases with time to the power 0.14±0.07. We speculate that this unusually small exponent is due to the interaction of classical ripening dynamics with abrasion or breakup under flow. We compare the rheological behavior to mechanistic models based on flow-induced aggregation and breakup of crystal clusters, finding that the exponents can be predicted from liquid phase sintering and breakup by brittle fracture.

Effect of Acute Increases in Retrograde Shear on Near‐Infrared Spectroscopy Derived Microvascular Reactivity

Periods of high vascular resistance lead to increases in retrograde shear rate, this augmented shear rate has been demonstrated to acutely decrease macrovascular endothelium‐dependent flow‐mediated dilation. However, it is unknown if similar exposures to a period of retrograde shear rate negatively impacts the microvascular response via near‐infrared spectroscopy (NIRS).PurposeDetermine the effects of acute retrograde shear rate on microvascular responses in healthy, young humans. It was hypothesized that exposure to increased levels of retrograde shear rates would elicit a decrease in post‐occlusive microvascular reactivity measured via non‐invasive NIRS.Methods17 participants (22.7 ± 2.91 yrs) completed the study. Microvascular function was measured as the post‐occlusive reactive responses for tissue saturation index (TSI%) and total hemoglobin + myoglobin ([Hb] total) in the m. rectus femoris before and after a 30‐minute retrograde shear rate treatment. The retrograde shear rate in the treatment leg (T) was achieved via a cuff placed below the knee inflated to 75 mmHg, meanwhile the contralateral leg served as control (C). Shear rate was measured in the superficial femoral artery. Due to the limitations of non‐invasive 2D Doppler ultrasound interrogating smaller vessels, the evaluation of changes in shear rate in the microcirculation could not be performed.ResultsThe cuff inflation significantly increased the retrograde shear in the treatment leg (71.2 ± 46.9 s−1 vs. 156.5 ± 63.5 s‐1, P&lt;0.01) compared to the control leg (74.8 ± 58.6 s−1 vs. 66.6 ± 45.7 s−1, P=0.37). The peak post‐occlusive response for TSI% (T: 79.9 ±10.2 %; C: 76.8 ± 11.9%, P=0.15) and [Hb] total (T: 44.7± 29.7 μM C: 51.5 ± 43.3 μM, P=0.06) were not different between legs following the intervention. Similarly, the rate of change for StO2 (T: 6.0 ± 7.0 % s−1 104; C: 5.0 ± 6.0 % s−1 104) and [Hb] total (T: 4.0 ± 4.0 μM s−1 104; C: 3.0 ± 4.0 0 μM s−1 104) were not different between legs.ConclusionIncreases in retrograde shear rate achieved via application of cuff pressure does not alter post‐occlusive NIRS derived indices of microvascular reactivity. Results may be due to different retrograde shear patterns elicited along the arterial tree, limitations of NIRS, or limited effect of retrograde shear on microvascular function.

Bioethanol production process rheology

The increase of bioethanol request in Mexico forces to accelerate the construction of new production plants and to implement process improvements. Locally, the main raw material in bioethanol production process is sugar cane molasses. Nevertheless, there are different problems within the process that minimize their production potential. There is a generation of sludge and its embedding at the inner wall of the first distillation column, and its mechanical cleaning causes an attrition and damage on it. This raises greater embeddings and shorter useful life of the column. In order to improve the process efficiency, becomes necessary to study the behavior of raw material and its transformations during the process, through Rheology. Molasses and its transformations are clearly affected by changes in shear rate and temperature. Incrustation, formed in the first distillation column, presented a high consistency index value (K), a strong shear thinning behavior and viscoelastic characteristics. This confirms the high content presence of dextran gum in the incrustation sample. Both results, rheological and physical, concur with the embedding real problems inside the column. Arrhenius model was used to describe the influence of temperature on fermented mash. Activation energy was obtained as well.

A time-dependent power law viscosity model and its application in modelling semi-solid die casting of 319s alloy

The flow parameters in the conventional power law viscosity model are generally considered to be function of temperature without considering the time-dependent property of the thixotropic flow. Practically, however, semi-solid casting is a time-dependent process, especially where the solid fraction is relatively high. Therefore, the current study focusses on introducing the shear time effect into the power law model. This is achieved by considering the rate at which the structure can evolve during transitions in geometry and hence changes in shear rate. The temperature- and time-dependent flow parameters of 319s aluminium alloy are determined by isothermal compression experiments in the semi-solid state. In practice the shear strain during the geometric transition is proposed to identify the relevant time-dependent parameters for modelling the die filling process in a given geometry. Comparing with the practical thixocasting, the simulation employing this improved model works well in enabling visualization of the filling process of a complex product i.e. a turbocharger impeller. Furthermore, the non-filling defects can be successfully avoided with the aid of simulation analysis, showing the accuracy and potential improvement in properties by employing the improved viscosity model. The significance here is the development of the fundamental model, which will be applicable to other semi-solid alloys and using any software package which incorporates the power law model.

Evidence for Shear Stress–Mediated Dilation of the Internal Carotid Artery in Humans

Increases in arterial carbon dioxide tension (hypercapnia) elicit potent vasodilation of cerebral arterioles. Recent studies have also reported vasodilation of the internal carotid artery during hypercapnia, but the mechanism(s) mediating this extracranial vasoreactivity are unknown. Hypercapnia increases carotid shear stress, a known stimulus to vasodilation in other conduit arteries. To explore the hypothesis that shear stress contributes to hypercapnic internal carotid dilation in humans, temporal changes in internal and common carotid shear rate and diameter, along with changes in middle cerebral artery velocity, were simultaneously assessed in 18 subjects at rest and during hypercapnia (6% carbon dioxide). Middle cerebral artery velocity increased significantly (69±10-103±17 cm/s; P<0.01) along with shear in both the internal (316±52-518±105 1/s; P<0.01) and common (188±40-275±61 1/s; P<0.01) carotids. Diameter also increased (P<0.01) in both carotid arteries (internal: +6.3±2.9%; common: +5.8±3.0%). Following hypercapnia onset, there was a significant delay between the onset of internal carotid shear (22±12 seconds) and diameter change (85±51 seconds). This time course is associated with shear-mediated dilation of larger conduit arteries in humans. There was a strong association between change in shear and diameter of the internal carotid (r=0.68; P<0.01). These data indicate, for the first time in humans, that shear stress is an important stimulus for hypercapnic vasodilation of the internal carotid artery. The combination of a hypercapnic stimulus and continuous noninvasive, high-resolution assessment of internal carotid shear and dilation may provide novel insights into the function and health of the clinically important extracranial arteries in humans.

Thixotropic Flow Behaviour in Chemical Pulp Fibre Suspensions

This paper presents results on the thixotropic behaviour of hardwood and softwood pulp fibre suspensions. Three rheological tests including hysteresis-loops, creep tests, and step-wise experiments were used to investigate the thixotropic rheology. Both suspensions exhibited a plateau in their flow curves where a slight change of the applied shear stress led to a dramatic change in the corresponding shear rates. During creep experiments under controlled stress, they evolved either towards a rapid shear (liquid regime) or stoppage (solid regime), depending on the relative values of the imposed stress, leading to a viscosity bifurcation around a critical stress. The transient response of pulp to step changes in shear rate was marked by a characteristic time, which can be used to understand the rate of structural change for pulp suspensions. Moreover, the yielding and thixotropic behaviour of the pulp suspensions were highly dependent on shear history and the time of rest prior to the measurement.