Uniform Shear Stress Research Articles

Preparation of black pigment with the Couette–Taylor vortex for electrophoretic displays

Carbon black is widely used as a black pigment in electrophoretic displays. The carbon black is required to control the density and surface charge depending on dielectric fluid to achieve a black image under the electric field. Both of them are closely related to the uniform polymer coating on the carbon black. Batch reactors have been used to carry out the polymerization on the carbon black. However, coating uniformity is not adequate when polymer coating is performed with a conventional batch system because it is difficult to maintain a uniform fluidic motion and mass transport throughout the batch reactor. Thus, a Couette–Taylor vortex reactor, generating uniform shear stress and fluidic motion, is used to coat the carbon black with a polymer. In this study, the influences of process variables in the Couette–Taylor vortex reactor on the properties of the synthesized black pigments are investigated. Because of strong agitation and uniform fluidic motion driven by the Couette–Taylor vortex, more uniform and smaller black pigments are obtained, resulting in the enhanced movement of the black pigment under an electric field. The productivity of the pigment is also improved along with the enhanced properties of the black pigments when using a continuous Couette–Taylor vortex reactor.

Black Pigment Preparation Via Couette-Taylor Vortex for Electrophoretic Display Application

Electrophoretic display realizes the images using the movements of black and white pigments in the dielectric fluid under the external electric field. In electrophoretic display, carbon black is widely used as a black pigment. To improve the motion of black pigments in the dielectric fluid, the density and surface charge of pigments should be controlled. The most effective way to modify the property of pigments is to coat the polymer on the carbon black. The density and the surface charge are determined by the kind and the amount of coated polymer.The polymerization reaction takes place on the surface of the carbon black as shown in Fig 1. At first, the polymerizable functional group was attached on the surface of carbon black which enabled the further polymerization. After the surface treatment, the radical polymerization was applied to grow the polymers on carbon black. However, the homogeneous polymerization, i.e. not on the surface of carbon black, can occur. If the homogeneous reaction takes place severely, the control of pigments’ properties can be failed. Thus, the selectivity of polymerization is determined by the dispersion of carbon blacks in the solution. The carbon blacks with improved dispersion provide more active surface area for polymerization step, therefore, the selectivity can be improved.A batch reactor has been used for the polymerization on carbon black.1,2 However, the uniformity of polymer coating is occasionally inadequate because of non-uniform and weak fluidic motion, resulting in the wide deviations of density and size of black pigments. Furthermore, the productivity of black pigments is relatively low with a batch reactor. Therefore, the preparation method or reactor should be developed to improve the property of black pigments as well as to increase the productivity of pigments.In this study, the Couette-Taylor vortex reactor,3,4,5 which induces the uniform shear stress and homogeneous mixing, is applied to polymerization step on carbon black. Fig. 2 shows the conventional Couette-Taylor vortex reactor, consisting of two parallel cylinders. The inner cylinder rotates, developing Couette-Taylor vortex between two cylinders. The development of Couette-Taylor vortex can be predicted based on Taylor number, which is a function of inner cylinder’s rotating speed. This vortex increases the dispersion of reactants as well as uniformity of fluidic motion. Furthermore, the continuous production is enabled by using a Couette-Taylor vortex reactor. As displayed in Fig. 2, the reactants can be injected through the inlet of reactor, and the products can be collected at the outlet. The productivity is determined by the axial flow rate, i.e. feeding rate, as indicated in Fig. 2. Because of its advantages related to the enhanced fluidic motion, it is expected that the properties and productivity of black pigments can be improved at once.In this presentation, we studied the properties of black pigments prepared in a batch reactor, batch Couette-Taylor vortex reactor, and continuous Couette-Taylor vortex reactor. The changes in the size, density, and distribution according to the process variables were investigated. In continuous Couette-Taylor vortex reactor, the influence of the axial flow rate was also clarified. Finally, we applied three black pigments obtained from each process to electrophoretic displays with white TiO2 particles, and the response times of electrophoretic motion under the electric field were compared.

Mechanoresponsive networks controlling vascular inflammation.

Atherosclerosis is a chronic inflammatory disease of arteries that develops preferentially at branches and bends that are exposed to disturbed blood flow. Vascular function is modified by flow, in part, via the generation of mechanical forces that alter multiple physiological processes in endothelial cells. Shear stress has profound effects on vascular inflammation; high uniform shear stress prevents leukocyte recruitment to the vascular wall by reducing endothelial expression of adhesion molecules and other inflammatory proteins, whereas low oscillatory shear stress has the opposite effects. Here, we review the molecular mechanisms that underpin the effects of shear stress on endothelial inflammatory responses. They include shear stress regulation of inflammatory mitogen-activated protein kinase and nuclear factor-κB signaling. High shear suppresses these pathways through the induction of several negative regulators of inflammation, whereas low shear promotes inflammatory signaling. Furthermore, we summarize recent studies indicating that inflammatory signaling is highly sensitive to pulse wave frequencies, magnitude, and direction of flow. Finally, the importance of systems biology approaches (including omics studies and functional screening) to identify novel mechanosensitive pathways is discussed.

Fluid Shear Stress Increases Neutrophil Activation via Platelet-Activating Factor

Leukocyte exposure to hemodynamic shear forces is critical for physiological functions including initial adhesion to the endothelium, the formation of pseudopods, and migration into tissues. G-protein coupled receptors on neutrophils, which bind to chemoattractants and play a role in neutrophil chemotaxis, have been implicated as fluid shear stress sensors that control neutrophil activation. Recently, exposure to physiological fluid shear stresses observed in the microvasculature was shown to reduce neutrophil activation in the presence of the chemoattractant formyl-methionyl-leucyl-phenylalanine. Here, however, human neutrophil preexposure to uniform shear stress (0.1–2.75 dyn/cm2) in a cone-and-plate viscometer for 1–120 min was shown to increase, rather than decrease, neutrophil activation in the presence of platelet activating factor (PAF). Fluid shear stress exposure increased PAF-induced neutrophil activation in terms of L-selectin shedding, αMβ2 integrin activation, and morphological changes. Neutrophil activation via PAF was found to correlate with fluid shear stress exposure, as neutrophil activation increased in a shear stress magnitude- and time-dependent manner. These results indicate that fluid shear stress exposure increases neutrophil activation by PAF, and, taken together with previous observations, differentially controls how neutrophils respond to chemoattractants.

Stress Concentration of a Microvoid Embedded in an Adhesive Layer during Stress Transfer

AbstractThis paper presents an elastic solution for one microvoid embedded in an adhesive layer close to a stiff bonding surface, which can be approximated as a semiinfinite domain with a fixed boundary. A Green’s function for a force in a semiinfinite solid with a fixed boundary condition is used to derive the stress field. By using the equivalent inclusion method, the microvoid has been replaced by the adhesive, but an eigenstrain is introduced to simulate the material mismatch. Through the stress equivalent condition, the eigenstrain is solved, and the stress concentration caused by the microvoid is evaluated considering the stress transfer caused by shear and tensile forces. A uniform normal stress and uniform shear stress are applied in the far field. The formulation is verified with finite-element simulation and can be extended to the case of multiple microvoids.

A Microfluidic-Based Multi-Shear Device for Investigating the Effects of Low Fluid-Induced Stresses on Osteoblasts

Interstitial fluid flow (IFF) within the extracellular matrix (ECM) produces low magnitude shear stresses on cells. Fluid flow-induced stress (FSS) plays an important role during tissue morphogenesis. To investigate the effect of low FSS generated by IFF on cells, we developed a microfluidic-based cell culture device that can generate multiple low shear stresses. By changing the length and width of the flow-in channels, different continuous low level shear stresses could be generated in individual cell culture chambers. Numerical calculations demonstrate uniform shear stress distributions of the major cell culture area of each chamber. This calculation is further confirmed by the wall shear stress curves. The effects of low FSS on MC3T3-E1 proliferation and differentiation were studied using this device. It was found that FSS ranging from 1.5 to 52.6 µPa promoted MC3T3-E1 proliferation and differentiation, but FSS over 412 µPa inhibited the proliferation and differentiation of MC3T3-E1 cells. FSS ranging from 1.5 to 52.6 µPa also increased the expression of Runx2, a key transcription factor regulating osteoblast differentiation. It is suggested that Runx2 might be an important regulator in low FSS-induced MC3T3-E1 differentiation. This device allows for detailed study of the effect of low FSS on the behaviors of cells; thus, it would be a useful tool for analysis of the effects of IFF-induced shear stresses on cells.

Alteration of human umbilical vein endothelial cell gene expression in different biomechanical environments

Biomechanical environments affect the function of cells. In this study we analysed the effects of five mechanical stimuli on the gene expression of human umbilical vein endothelial cells (HUVECs) in mRNA level using real-time PCR. The following loading regimes were applied on HUVECs for 48 h: intermittent (0-5 dyn/cm(2) , 1 Hz) and uniform (5 dyn/cm(2) ) shear stresses concomitant by 10% intermittent equiaxial stretch (1 Hz), uniform shear stress alone (5 dyn/cm(2) ), and intermittent uniaxial and equiaxial stretches (10%, 1 Hz). A new bioreactor was made to apply uniform/cyclic shear and tensile loadings. Three endothelial suggestive specific genes (vascular endothelial growth factor receptor-2 (VEGFR-2, also known as FLK-1), von Willebrand Factor (vWF) and vascular endothelial-cadherin (VE-cadherin)), and two smooth muscle genes (α-smooth muscle actin (α-SMA) and smooth muscle myosin heavy chain (SMMHC)) were chosen for assessment of alteration in gene expression of endothelial cells and transdifferentiation toward smooth cells following load applications. Shear stress alone enhanced the endothelial gene expression significantly, while stretching alone was identified as a transdifferentiating factor. Cyclic equiaxial stretch contributed less to elevation of smooth muscle genes compared to uniaxial stretch. Cyclic shear stress in comparison to uniform shear stress concurrent with cyclic stretch was more influential on promotion of endothelial genes expression. Influence of different mechanical stimuli on gene expression may open a wider horizon to regulate functions of cell for tissue engineering purposes.

The Influence of Cyclic and Uniform Shear Stresses Concurrent with Cyclic Stretch on the Gene Expression of Human Umbilical Vein Endothelial Cells

The Influence of Cyclic and Uniform Shear Stresses Concurrent with Cyclic Stretch on the Gene Expression of Human Umbilical Vein Endothelial Cells

Design and validation of a new fixture for the shear testing of cellular solids

The design and validation of a new fixture for the shear testing of cellular solids are presented. The fixture is an extended version of a picture-frame shear fixture (EPF) and is suited for comparatively thick rectangular block specimens. The stress state in the specimen is analyzed using a detailed finite element model. The finite element model is based on a 3D CAD model and incorporates friction in the revolute joints. Using specimens with low stiffness, a nearly pure and uniform shear stress state is induced in the specimen. A correction factor for the shear stress is derived which takes into account the friction in the joints and the nonuniformity of the shear stress distribution in the gauge section. The shear response of the polymer foam Rohacell® 200 WF is determined in order to demonstrate the capabilities of the EPF. The strain state is analyzed by means of digital image correlation and is detected to be very pure and uniform on the specimen’s surface, as predicted by the numerical analysis. The shear modulus obtained with the EPF is in good agreement with the calculated shear modulus derived from tensile tests on the same material. In addition, there is only little scatter of the strength values over the tested specimens which further confirms the accuracy of the new fixture.

A novel in vitro loading system to produce supraphysiologic oscillatory fluid shear stress

A multi-well fluid loading (MFL) system was developed to deliver oscillatory subphysiologic to supraphysiologic fluid shear stresses to cell monolayers in vitro using standard multi-well culture plates. Computational fluid dynamics modeling with fluid-structure interactions was used to quantify the squeeze film fluid flow between an axially displaced piston and the well plate surface. Adjusting the cone angle of the piston base modulated the fluid pressure, velocity, and shear stress magnitudes. Modeling results showed that there was near uniform fluid shear stress across the well with a linear drop in pressure across the radius of the well. Using the MFL system, RAW 264.7 osteoclastic cells were exposed to oscillatory fluid shear stresses of 0, 0.5, 1.5, 4, 6, and 17Pa. Cells were loaded 1h per day at 1Hz for two days. Compared to sub-physiologic and physiologic levels, supraphysiologic oscillatory fluid shear induced upregulation of osteoclastic activity as measured by tartrate-resistant acid phosphatase activity and formation of mineral resorption pits. Cell number remained constant across all treatment groups.

Self-similar evolution of a body eroding in a fluid flow

Erosion of solid material by flowing fluids plays an important role in shaping landforms, and in this natural context is often dictated by processes of high complexity. Here, we examine the coupled evolution of solid shape and fluid flow within the idealized setting of a cylindrical body held against a fast, unidirectional flow, and eroding under the action of fluid shear stress. Experiments and simulations both show self-similar evolution of the body, with an emerging quasi-triangular geometry that is an attractor of the shape dynamics. Our fluid erosion model, based on Prandtl boundary layer theory, yields a scaling law that accurately predicts the body's vanishing rate. Further, a class of exact solutions provides a partial prediction for the body's terminal form as one with a leading surface of uniform shear stress. Our simulations show this predicted geometry to emerge robustly from a range of different initial conditions, and allow us to explore its local stability. The sharp, faceted features of the terminal geometry defy the intuition of erosion as a globally smoothing process.

Computer simulations of structure, dynamics, and phase behavior of colloidal fluids in confined geometry and under shear

Using computer simulations, colloidal systems in different external fields are investigated. Colloid-polymer mixtures, described in terms of the Asakura-Oosawa (AO) model, are considered under strong confinement. Both in cylindrical and spherical confinement, the demixing transition of the three-dimensional AO model is rounded and, using Monte Carlo simulations, we analyze in detail the consequences of this rounding (occurrence of multi-domain states in cylindrical geometry, non-equivalence of conjugate ensembles due to different finite-size corrections in spherical geometry etc.). For the case of the AO model confined between two parallel walls, spinodal decomposition is studied using a combination of molecular dynamics simulation and the multiparticle collision dynamics method. This allows us to investigate the influence of hydrodynamic interactions on the domain growth during spinodal decomposition. For a binary glass-forming Yukawa mixture, non-linear active micro-rheology is considered, i.e. a single particle is pulled through a deeply supercooled liquid. The diffusion dynamics of the pulled particle is analyzed in terms of the van Hove correlation function. Finally, the Yukawa mixture in the glass state, confined between walls, is studied under the imposition of a uniform shear stress. Below and around the yield stress, persistent creep in the form of shear-banded structures is observed.

Onset of flow in a confined colloidal glass under an imposed shear stress

A confined colloidal glass, under the imposition of a uniform shear stress, is investigated using numerical simulations. Both at macro- and microscales, the consequent dynamics during the onset of flow is studied. When the imposed stress is gradually decreased, the time scale for the onset of steady flow diverges, associated with long-lived spatial heterogeneities. Near this yield-stress regime, persistent creep in the form of shear-banded structures is observed.

Two Circular Inclusions with Rate-Dependent Interfacial Sliding under Longitudinal Shear

We investigate the time-dependent response of an infinite matrix reinforced by two circularly cylindrical elastic inclusions with viscous sliding interfaces under longitudinal shear. The loadings include remote uniform shear stresses and a screw dislocation fixed in the matrix. The results for remote uniform loading are further used to predict the effective properties of a hybrid composite containing a dilute and random dispersion of perfectly aligned or randomly oriented inclusion pairs. The time-dependent image force acting on the dislocation due to its interaction with the two inclusions is derived, and its far-field asymptotic approximation is obtained. Interestingly, it is found that the asymptotic expression of the image force is intrinsically connected with the expression of the effective shear moduli of the hybrid composite.

Effect of Column Disorder on Carrier Transport in Columnar Discotic Liquid Crystal Evaluated by Applying Precisely Controlled Shear Stress

The effect of column disorder on carrier drift mobility in columnar discotic liquid crystals has been investigated by applying a precisely controlled oscillating shear stress. Drift mobilities on the order of 10-1 cm2·V-1·s-1 were confirmed for positive and negative carriers in the columnar phase of 1,4,8,11,15,18,22,25-octahexylphthalocyanine in a well-aligned homeotropic geometry, in which the columnar axis was perfectly perpendicular to substrates with an electrode. A slight tilt of the columnar axis upon applying shear stress led to a marked decrease in electronic carrier mobility from 10-1 to less than 10-6 cm2·V-1·s-1, and transport was only confirmed for positive ion carriers. This result indicates that a uniform shear stress blocks the carrier transport path in the entire area of the electrode, and one-dimensional carrier transport path along the columns is easily hindered in columnar discotic liquid crystals.

The impact of handgrip exercise duty cycle on brachial artery flow-mediated dilation

Endothelial function is essential for vasoprotection and regulation of vascular tone. Using handgrip exercise (HGEX) to increase blood flow-associated shear stress is an increasingly popular method for assessing brachial artery endothelial function via flow-mediated dilation (FMD). However, different exercise duty cycles [ratio of handgrip relaxation: contraction (seconds)] produce different patterns of brachial artery shear stress with distinct antegrade/retrograde magnitudes. To determine the impact of HGEX duty cycle on brachial artery %FMD, three distinct duty cycles were employed while maintaining a uniform mean shear stress. Brachial artery diameter and mean blood velocity were assessed via echo and Doppler ultrasound in 16 healthy male subjects. Shear stress was estimated as shear rate (SR = blood velocity/brachial artery diameter) and the target mean SR during HGEX was 75 s(-1). Subjects performed three 6-min HGEX trials on each of 2 days (like trials averaged). In each trial, subjects performed one of the three randomly ordered HGEX duty cycles (1:1, 3:1, 5:1). %FMD was calculated from baseline to the end of HGEX and (subset N = 10) during each minute of HGEX. Data are mean ± SD. As intended, mean SR was uniform across duty cycles (6 min HGEX average: 72.9 ± 4.9s(-1), 72.6 ± 3.6s(-1), 72.8 ± 3.5 s(-1), p = 0.835), despite differences in antegrade/retrograde SR (p < 0.001). End-exercise %FMD (4.0 ± 1.3 %, 4.1 ± 2.2 %, 4.2 ± 1.4 %, p = 0.860) and %FMD during exercise (p = 0.939) were not different between duty cycles. These data indicate that the endothelium responds to the mean shear stress and is not specifically sensitive to the contraction/relaxation or retrograde shear stress created by a range of HGEX protocols.

Microvascular permeability to water is independent of shear stress, but dependent on flow direction

Endothelial cells in a cultured monolayer change from a "cobblestone" configuration when grown under static conditions to a more elongated shape, aligned with the direction of flow, after exposure to sustained uniform shear stress. Sustained blood flow acts to protect regions of large arteries from injury. We tested the hypothesis that the stable permeability state of individually perfused microvessels is also characteristic of flow conditioning. In individually perfused rat mesenteric venular microvessels, microvascular permeability, measured as hydraulic conductivity (Lp), was stable [mean 1.0 × 10(-7) cm/(s × cmH2O)] and independent of shear stress (3-14 dyn/cm(2)) for up to 3 h. Vessels perfused opposite to the direction of normal blood flow exhibited a delayed Lp increase [ΔLp was 7.6 × 10(-7) cm/(s × cmH2O)], but the increase was independent of wall shear stress. Addition of chondroitin sulfate and hyaluronic acid to perfusates increased the shear stress range, but did not modify the asymmetry in response to flow direction. Increased Lp in reverse-perfused vessels was associated with numerous discontinuities of VE-cadherin and occludin, while both proteins were continuous around the periphery of forward-perfused vessels. The results are not consistent with a general mechanism for graded shear-dependent permeability increase, but they are consistent with the idea that a stable Lp under normal flow contributes to prevention of edema formation and also enables physiological regulation of shear-dependent small solute permeabilities (e.g., glucose). The responses during reverse flow are consistent with reports that disturbed flows result in a less stable endothelial barrier in venular microvessels.

Fluid shear stress sensitizes cancer cells to receptor-mediated apoptosis via trimeric death receptors

Cancer metastasis, the process of cancer cell migration from a primary to distal location, typically leads to a poor patient prognosis. Hematogenous metastasis is initiated by intravasation of circulating tumor cells (CTCs) into the bloodstream, which are then believed to adhere to the luminal surface of the endothelium and extravasate into distal locations. Apoptotic agents such as tumor necrosis factor apoptosis-inducing ligand (TRAIL), whether in soluble ligand form or expressed on the surface of natural killer cells, have shown promise in treating CTCs to reduce the probability of metastasis. The role of hemodynamic shear forces in altering the cancer cell response to apoptotic agents has not been previously investigated. Here, we report that human colon cancer COLO 205 and prostate cancer PC-3 cells exposed to a uniform fluid shear stress in a cone-and-plate viscometer become sensitized to TRAIL-induced apoptosis. Shear-induced sensitization directly correlates with the application of fluid shear stress, and TRAIL-induced apoptosis increases in a fluid shear stress force- and time-dependent manner. In contrast, TRAIL-induced necrosis is not affected by the application fluid shear stress. Interestingly, fluid shear stress does not sensitize cancer cells to apoptosis when treated with doxorubicin, which also induces apoptosis in cancer cells. Caspase inhibition experiments reveal that shear stress-induced sensitization to TRAIL occurs via caspase-dependent apoptosis. These results suggest that physiological fluid shear forces can modulate receptor-mediated apoptosis of cancer cells in the presence of apoptotic agents.

Magnetic resonance imaging of the rheology of ionic liquid colloidal suspensions

The rheology, and underpinning colloidal interactions, of ionic liquid (IL) dispersions of colloidal silica have been investigated using bulk rheological measurements with magnetic resonance (MR) velocity and relaxation measurements. Two ionic liquids were investigated: tetradecyl(trihexyl)phosphonium bistriflamide ([P6,6,6,14][NTf2]) and 1-butyl-methylimidizolium tetrafluoroborate ([C4mim][BF4]), in the absence and presence of hydrophilic silica nanoparticles (Aerosil 200). Bulk rheology was probed using measurements of shear stress and viscosity as a function of shear rate in a cone-and-plate rheometer. Local rheology was probed using MR velocity imaging of flow in Couette and cone-and-plate cells. Velocity profiles were extracted from the Couette measurements and fitted using a power-law model. Newtonian rheology was observed for both ILs in the absence of dispersed silica. For the dispersion of 15% silica in [C4mim][BF4], bulk rheology and MR velocity imaging measurements showed Newtonian behaviour at low shear rates ( 10 s−1). For the dispersion of 5% silica in [P6,6,6,14][NTf2], more complex rheology was observed in the flow curve, which was suggestive of shear-banding. This was investigated further using the MR velocity profiles in a Couette cell and velocity images in a cone-and-plate cell, which both showed the coexistence of regions of sheared and unsheared fluid. The sheared fluid was found to be highly shear-thinning and close inspection of the flow profile at the interface between sheared and unsheared fluid suggested that the behaviour was shear-banding rather than shear-localisation. This was further confirmed by the velocity images in the cone-and-plate rheometer, which showed sheared and unsheared fluid in a uniform shear stress environment.

Assessment of penetrometry technique for measuring the yield stress of muds and granular pastes

We discuss the possibility of using penetrometry technique for measuring the yield stress of concentrations made of grains immersed in a colloidal phase, such as concrete or muds. In that aim we used model materials made by suspending glass beads at different concentrations in a kaolin-water paste. We then show that a uniform shear stress develops along the object (plate or cylinder) beyond the entrance length. This shear stress plotted versus the object velocity exhibits a shape similar to the flow curve of the material determined from rheometry. For materials exhibiting the typical flow curve of a simple yield stress fluid, i.e. at bead concentrations smaller than 30 %, the stress associated with an inflection point located at low velocities of this curve appears to correspond to the material yield stress. At larger concentrations of beads the suspensions have a more complex behaviour likely affected by its granular nature at a local scale and the possibility of migration or frictional effects, so that neither conventional rheometry nor penetrometry provide relevant data. We conclude by describing two practical penetrometry techniques for precisely measuring the yield stress of simple pastes.