Laminar Shearing Research Articles

Shear-Assisted Fabrication of Block Copolymer Agglomerates with Various Morphologies in Viscous Medium.

In this work, we have investigated the effect of laminar flow shearing on the formation of block copolymer agglomerates in viscous medium. Under a laminar flow shearing, the block copolymer solution droplets were spontaneously emulsified and were then elongated into protofibers, which in turn transformed into particles with various morphologies. Besides micro-/nanorods, which were previously reported for homopolymers, sphere and sheetlike structures were unexpectedly fabricated from block copolymers depending on the solvent quality, solvent exchange rates, and the entanglement of the polymer chains. In particular, the sheet structure, fabricated from poly(ethylene glycol)-b-polystyrene (PEG-b-PS), can be fixed by UV irradiation when photo-crosslinkable azide groups were introduced onto the polystyrene block. Surprisingly, we found that the fixed sheetlike structures show demulsification capability in tens of seconds, which may have great potential application in the separation of oil from emulsions.

Role of Red Cells in Thrombosis and Hemostasis

Until recently it has been assumed that red blood cells (RBCs) play mainly a passive role in hemostasis and thrombosis, but now some studies have uncovered several novel roles for them in both of these processes. The best-known effects of RBCs in clotting in vivo are rheological, involving laminar shearing plus aggregation and deformability of RBCs. In addition, RBCs have specific receptors for platelets and fibrin(ogen) that may be involved in thrombosis. Furthermore, in the contraction of blood clots, the trapping of RBCs in the platelet-fibrin network and subsequent physical changes may be important for hemostasis and thrombosis. Contracted blood clots develop a remarkable structure, with a meshwork of fibrin and platelet aggregates on the exterior of the clot and a close-packed, tessellated array of compressed polyhedral RBCs within. The structure and properties of contracted clots vary depending on the relative amounts of platelets, fibrinogen and RBCs and the conditions of clotting. Such close-packed arrays of polyhedral erythrocytes, or polyhedrocytes, have also been observed in human arterial and especially venous thrombi taken from patients. The mechanical nature of this RBC shape change was confirmed by polyhedrocyte formation from the forces of centrifugation of blood without clotting. Many experiments suggest that polyhedrocytes are more prevalent under conditions associated with thrombosis and could be a marker of prothrombotic conditions. Consistent with this hypothesis, RBC membranes have been found to be a major source of exposure of phosphatidylserine, which forms a procoagulant surface to generate thrombin. Platelets (with their cytoskeletal motility proteins) and fibrin(ogen) (as the substrate bridging platelets for contraction) are required to generate the forces necessary to segregate platelets/fibrin from RBCs and to compress these cells into a tightly packed array. Clots formed after puncture of the mouse saphenous vein are composed primarily of polyhedrocytes, suggesting that they may be important in venous hemostasis. Moreover, the permeability of contracted clots is nearly the same as that of the endothelial cell lining, consistent with their functional significance. These results demonstrate how contracted clots form an impermeable barrier important for hemostasis and wound healing and also to restore flow past obstructive thrombi. On the other hand, they also help explain how fibrinolysis is greatly retarded after clots contract, as a result of these striking structural changes. In summary, RBCs may perform a dual role, both helping to stem bleeding but at the same time contributing to thrombosis in a variety of ways, including when the tightly packed array of RBCs in a contracted clot blocks blood flow and resists dissolution. Disclosures No relevant conflicts of interest to declare.

Hemolysis in a laminar flow-through Couette shearing device: an experimental study.

Reducing hemolysis has been one of the major goals of rotary blood pump development and in the investigational phase, the capability of hemolysis estimation for areas of elevated shear stresses is valuable. The degree of hemolysis is determined by the amplitude of shear stress and the exposure time, but to date, the exact hemolytic behavior at elevated shear stresses and potential thresholds for subcritical shear exposure remain vague. This study provides experimental hemolysis data for a set of shear stresses and exposure times to allow better estimations of hemolysis for blood exposed to elevated shearing. Heparinized porcine blood with a hematocrit of 40% was mechanically damaged in a flow-through laminar Couette shear flow at a temperature of 23°C. Four levels of shear stress, 24, 592, 702, and 842 Pa, were replicated at two exposure times, 54 and 873 ms. For the calculation of the shear stresses, an apparent viscosity of 5 mPas was used, which was verified in an additional measurement of the blood viscosity. The hemolysis measurements were repeated four times, whereby all conditions were measured once within the same day and with blood from the same source. Samples were taken at the inlet and outlet of the shear region and an increase in plasma-free hemoglobin was measured. An index of hemolysis (IH) was thereby calculated giving the ratio of free to total hemoglobin. The results are compared with data from previously published studies using a similar shearing device. Hemolysis was found to increase exponentially with shear stress, but high standard deviations existed at measurements with elevated IH. At short exposure times, the IH remained low at under 0.5% for all shear stress levels. For high exposure times, the IH increased from 0.84% at 592 Pa up to 3.57% at the highest shear stress level. Hemolysis was significant for shear stresses above ∼600 Pa at the high exposure time of 873 ms.

New Insights into the Role of Erythrocytes in Thrombus Formation

The role of erythrocytes in thrombus formation has previously been regarded as passive by their influence on rheology. Erythrocytes are known, due to their abundance and size, to push platelets to the vascular wall (laminar shearing). This results in an increased platelet delivery at the vascular wall enabling platelets to seal off a vascular damage preventing excessive blood loss. Recently, there is new evidence for erythrocytes to influence thrombus formation in multiple ways besides their effect on rheology. Several groups have shown that besides platelets, erythrocytes are the main suppliers of phosphatidylserine-exposing membranes needed for coagulation resulting in fibrin formation. In addition, our group has found that the intercellular adhesion molecule 4-αIIbβ3 interaction mediates erythrocyte-platelet interaction in flowing blood. By inhibiting this interaction, we found decreased thrombin formation and decreased incorporation of erythrocytes into a thrombus. This review will provide more in-detail information of existing and new hypotheses regarding the role of erythrocytes in thrombus formation.

Investigation of the mobile granular layer in bedload transport by laminar shearing flows

AbstractThe mobile layer of a granular bed composed of spherical particles is experimentally investigated in a laminar rectangular channel flow. Both particle and fluid velocity profiles are obtained using particle image velocimetry for different index-matched combinations of particles and fluid and for a wide range of fluid flow rates above incipient motion. A full three-dimensional investigation of the flow field inside the mobile layer is also provided. These experimental observations are compared to the predictions of a two-phase continuum model having a frictional rheology to describe particle–particle interactions. Different rheological constitutive laws having increasing degrees of sophistication are tested and discussed.

Sediment dynamics. Part 1. Bed-load transport by laminar shearing flows

We propose a two-phase model having a Newtonian rheology for the fluid phase and friction for the particle phase to describe bed-load transport in the laminar viscous regime. We have applied this continuum model to sediment transport by viscous shearing flows. The equations are shown to reduce to the momentum equation for the mixture and the Brinkman equation for the fluid velocity. This modelling is able to provide a description of the flow of the mobile granular layer. At some distance from threshold of particle motion, where the continuum approach is more realistic as the mobile layer is larger than one particle diameter, there is very little slip between the two phases and the velocities inside the mobile bed have approximately a parabolic profile. When the Poiseuille (or Couette) flow is not significantly perturbed, simple analytical results of the particle flux varying cubically with the Shields number and of the bed-load thickness varying linearly with it can then be obtained. These predictions compare favourably with experimental observations of bed-load transport in pipe flows.

A three-dimensional numerical model for incompressible two-phase flow of a granular bed submitted to a laminar shearing flow

A numerical model for the simulation of incompressible two-phase flows of a flat granular bed submitted to a laminar shearing flow is presented, considering a two-fluid model and a mixed-fluid one. The governing equations are discretized by a finite element method and a penalisation method is introduced to cope with the incompressibility constraint. A regularisation technique is used to deal with the visco-plastic behaviour of the granular phase. Validations are carried out on three flow test cases: a Bingham fluid between two infinite parallel planes, a Bingham fluid in a square lid-driven cavity and a Newtonian fluid over a granular bed in a two-dimensional configuration, for which we compare our numerical results with existing analytical or numerical results. The accuracy and efficiency of the numerical models have been compared for the two formulations of the two-phase flow model. It turns out that the two-fluid model requires ten times more CPU time than the mixed-fluid one for a comparable accuracy, which can be achieved provided one takes a smaller regularisation parameter in the latter model. Finally, three-dimensional computations are presented for the flow of a Newtonian fluid over a granular bed in a square and circular cross-section ducts.

Sedimentological processes in a scarp-controlled rocky shoreline to upper continental slope environment, as revealed by unusual sedimentary features in the Neogene Coquimbo Formation, north-central Chile

Exceptionally good outcrops of Miocene to Pliocene deposits in the vicinity of submarine Paleozoic basement scarps at Carrizalillo, north of La Serena, reveal a wealth of sedimentary features not commonly observed. The most proximal facies consist of rock fall and coarse-grained debris flow deposits directly abutting the basement wall from which they originated. Angular basement clasts are mixed with well-rounded cobbles, which probably formed as a basal gravel on a wave-cut platform at the beginning of marine flooding, subsequently accumulated at the scarp edge and were incorporated into the debris when the latter collapsed. The poor sorting, inverse grading, and protruding cobbles and boulders are classical debris flow features, with good clast imbrication indicating a laminar shearing action. A medial facies is represented by secondary channels running parallel to the major scarp about 1 km downslope of the first locality. In the largest channel, megaflutes at the base indicate the passage of highly turbulent, nondepositing flows eroding the soft, silty substrate. In the deepest, central part of the channel, a pebbly coquina shows horizontal and trough cross-stratification, with most of the bivalves oriented convex side up. Meter-scale rip-up clasts of the underlying siltstone are also present, indicating turbulent flow with a density sufficiently high to retard settling. The coquina is interpreted as a detachment deposit resulting from a hydroplaning debris flow along the central part of the channel, where the velocity and rate of pore pressure decay were highest. This deposit is overlain by fining upward, massive to horizontally stratified sandstone very similar in texture and composition to the matrix of the debris flow, suggesting its formation by surface transformation and elutriation of the latter. Along the channel margin, a basal centimeter-scale sandstone layer is virtually unaffected by the megaflute topography and clearly represents a subsequent event. It is interpreted as a basal shear carpet driven by the overlying debris flow. Within the shear carpet, a basal friction zone and an overlying collision zone containing a higher concentration of shell hash can be distinguished. The overlying debris flow deposit is represented by massive coquina with scattered, angular to rounded basement clasts. It contains disarticulated bivalves oriented with their concave side up, indicating large-scale upward fluid escape during deposition. A smaller secondary channel shows large rip-up rafts of the underlying substrate. Some rafts appear to have been plucked from the substrate by a process of sand injection from an overriding high-density sandy debris flow, which probably originated during a tsunami. Such clasts can climb upward into a laminar flow by down-current tilting and tumbling. The most distal facies occurs below a second scarp oriented more or less parallel to the present coastline, where finer-grained turbidites onlap and backlap onto the stoss and lee sides of an obstacle formed by eroded boulder conglomerates. The onlap deposits resemble inclined sandy macroforms recently described in submarine canyon settings. They are interbedded with diatom-containing, volcanic ash beds with cross-stratification dipping eastwards and containing deepwater microflora typical of continental upwelling zones.

Molecular mechanism of endothelial growth arrest by laminar shear stress.

This study was designed to elucidate the mechanism underlying the inhibition of endothelial cell growth by laminar shear stress. Tumor suppressor gene p53 was increased in bovine aortic endothelial cells subjected to 24 h of laminar shear stress at 3 dynes (1 dyne = 10 microN)/cm(2) or higher, but not at 1.5 dynes/cm(2). One of the mechanisms of the shear-induced increase in p53 is its stabilization after phosphorylation by c-Jun N-terminal kinase. To investigate the consequence of the shear-induced p53 response, we found that prolonged laminar shear stress caused increases of the growth arrest proteins GADD45 (growth arrest and DNA damage inducible protein 45) and p21(cip1), as well as a decrease in phosphorylation of the retinoblastoma gene product. Our results suggest that prolonged laminar shear stress causes a sustained p53 activation, which induces the up-regulation of GADD45 and p21(cip1). The resulting inhibition of cyclin-dependent kinase and hypophosphorylation of retinoblastoma protein lead to endothelial cell cycle arrest. This inhibition of endothelial cell proliferation by laminar shear stress may serve an important homeostatic function by preventing atherogenesis in the straight part of the arterial tree that is constantly subjected to high levels of laminar shearing.

Demixing effects in laminar shearing flows: A practically useful approach

AbstractThe phenomena of shear‐induced agglomeration and shear‐induced migration in a non‐ colloidal particulate suspension were examined in different flow fields. The shear flow of a model suspension consisting of spherical glass beads in an aqueous corn syrup with added surfactant was studied in homogeneous and non‐homogeneous flow fields (cone‐and‐plate, parallel plate and laminar flow in a tube). It was found that there was a rapid increase in the average agglomerate size followed by leveling off at long times resulting in an equilibrium particle size. This equilibrium particle size was found to be a strong function of the applied shear rate. It was further found that agglomerate data from the different flow fields superposed well, and, at least in the range of solid volume fractions considered, particle migration was negligible. This work provides a simple technique for quantifying demixing in laboratory experiments and applying the results to industrially relevant situations.

Bimolecular reactions between passively transported species

We consider the reactions A+A→0 and A+B→0 under stirring, modelled both by the baker's transformation (BT) and by laminar shearing (LS). The reactants are transported passively and react at first encounter. Under the BT the reactions follow the classical scheme closely, while for LS the decay is fluctuation dependent and shows complex forms.

Welded tuff infilling a volcanic vent at Weolseong, Republic of Korea

A plug of parataxitically welded tuff, with an elliptical (700 × 400 m) cross section, crops out in Weolseong district, Republic of Korea. It intrudes late Cretaceous granodiorite and extrusive tuffs of the Chisulryoung Volcanic Formation and displays a steeply inclined welding foliation, concordant with the intrusive contacts, and a subvertical stretching lineation. These fabrics are interpreted as resulting from laminar shearing in a boundary layer during deposition of the tuff by agglutination along the walls of a vent which served as a feeder for extrusive ignimbrites.

Biorheological methods employing the Weissenberg rheogoniometer.

The theories and spatial concepts of Karl Weissenberg have been applied to the science of biorheology by employing the Weissenberg Rheogoniometer. Two main types of experimental methods have been used for the characterization of the bulk shear properties of biorheological fluids: (A) In continuous laminar shearing motion, the tangential and normal components of stress are measured at a series of rates of shear. From these parameters, the apparent viscosity, an elastic modulus and a recoverable strain, are calculated as functions of the rate of shear. In this continuous shear experiment, the physical structure present in the material at any given rate of shear may be quite different from the material in its rest state or at other rates of shear. (B) In harmonic oscillatory motion, the material is subjected to a harmonic laminar shear about its rest state at a number of frequencies and small strain amplitudes. From these experiments the dynamic moduli of viscosity and elasticity are calculated. Preparations of biological materials, such as whole blood and systems of blood components, both in health and disease states, have been investigated using methods A and B, together with studies of surface layers of plasma proteins at interfaces.

Monte Carlo simulation of coagulation in discrete particle-size distributions. Part 1. Brownian motion and fluid shearing

A method for the Monte Carlo simulation, by digital computer, of the evolution of a colliding and coagulating population of suspended particles is described. Collision mechanisms studied both separately and in combination are: Brownian motion of the particles, and laminar and isotropic turbulent shearing motions of the suspending fluid. Steady-state distributions are obtained by adding unit-size particles at a constant rate and removing all particles once they reach a preset maximum volume. The resulting size distributions are found to agree with those obtained by dimensional analysis (Hunt 1982).

The amplitude of turbulent shear flow

A new formulation of the problem of the statistical stability of fully turbulent shear flow is proposed, in which one seeks mean fields that bound the observed flow from the stable side. In the spirit of maximum transport theory, this formulation admits a larger set of “flows” than are dynamically possible. A sequence of constraints derived from the equations of motion can narrow this set, permitting at each step the determination of a “most stable” field free of any empirical elements. Turbulent channel flow is proposed as the first application and test of this quantitative theory. Past deductive theories for this flow, from “mean field” to “transport upper bounds,” are assessed. It is shown why these theories do not retain the significant destabilizing mechanisms of the actual flow. The implications for turbulent flow of recent work on the nonlinear and three-dimensional instability of laminar shearing flow are described. In first exploration of the “decoupled mean” stability theory proposed here, approximate analytical and numerical stability methods are used to find an amplitude and structure for the averaged flow propoerties. The quantitative results differ by considerably less than two from the observed values, providing an incentive for a more complete numerical study and for further constraints on the admitted class of flows. In the language now current for nonlinear stability theory, evidence is advanced here that anN-dimensional central manifold is adjacent to the realized turbulent flow, whereN has the largest possible value compatible with the dynamical relations.

Use of trajectory analysis to study stability of colloidal dispersions in flow fields

AbstractTrajectories have been computed for two equal sized spherical particles in simple laminar shearing and in uniaxial extensional flows. Effects of interparticle attraction, electrostatic repulsion, and hydrodynamics were included.The results are pertinent to questions of colloidal stability under various conditions of flow. Particulate dispersions can react in several different ways as the intensity of shearing is increased from zero: the dispersion can remain stable; it can be redispersed, if it had been initially flocculated into a weak secondary minimum in the interparticle potential curve; it can be flocculated into a strong primary minimum in the potential curve; or, in extreme cases, it can be redispersed from the primary minimum. Results are presented which illustrate criteria for flocculation or stability to both laminar shearing and extensional flow. It is shown that hydrodynamic effects can significantly alter the criteria developed for stability of dispersions to Brownian coagulation.

An Experimental Study of Rheological Properties of Polymer Melts in Laminar Shear Flow and of Interface Deformation and Its Mechanisms in Two-Phase Stratified Flow

The stress state of four rheologically characterized polymer melts—low-density polyethylene (LDPE), high-density polyethylene (HDPE), polystyrene (PS), and poly(methyl methacrylate) (PMMA)—in laminar shearing flow was completely determined by measuring the non-Newtonian viscosity μ and the first and second normal stress differences N1 and N2. The mean N2 function was negative and about 0.1–0.3 the value of N1. The measurements were made in a Weissenberg rheogoniometer using cone- and parallel-plate geometries. Additional viscosity measurements were made in a capillary rheometer. An experimental study of interface distortion in the stratified two-phase flow has been carried out using these melts. It was found in the stratified flow experiments that the lower viscosity melts exhibited concave interfaces and encapsulated the higher viscosity melts irrespective of the relative values of N1 and N2. The rate of interface distortion and encapsulation increases with increasing viscosity differences. The pressure losses for stratified flow tended to be close to the values for the lower viscosity phase, especially for long dies. The problem of instabilities at the interface between the two melts is briefly discussed.

Measurement of Stresses Developed in Steady Laminar Shearing Flows of Viscoelastic Media

The shearing stress and both normal stress differences are reported for three solutions of polyisobutylene in decalin. The ratio of the second normal stress difference to the first, (P22−P33)/(P11−P22), is seen to be negative and to vary between −0.1 and −0.4 under the conditions studied for these media. The statistical significance and the precision of these data is considered in some detail; the results are believed to be definitive at the error levels specified. Previous measurements of this normal stress ratio, which may be shown to be free of all the major known errors, are seen to have yielded the same sign as obtained herein, for similar fluids.

A study of properties of renal microbodies of the rat.

AbstractThe in vitro response of renal microbodies and lysosomes to a variety of altered physical and chemical conditions was examined. The influence of these conditions upon the integrity of the organelles was determined as that proportion of the total activity of various “marker” enzymes (α‐hydroxy acid oxidase, catalase and D‐amino acid oxidase for microbodies, and acid phosphatase for lysosomes) which was released from the organelles and recovered in the soluble phase following treatment.Microbodies and lysosomes were separated by differential centrifugation from 15% homogenates of male, rat kidney prepared in 0.25 M sucrose. After exposure to experimental treatments, the preparation was further separated into sediment and supernatant fractions by centrifugation at 100,000 × g for 30 minutes. The sediment and supernatant fractions were assayed to determine the distribution of the “marker” enzymes.Eighty to 100% of the activities of α‐hydroxy acid oxidase and catalase were found in the supernatant after exposure of the microbody‐lysosome fraction to low concentrations of Triton‐X 100 or digitonin, or after treatment by sonic vibrations or high speed laminar shearing. Lesser but significant proportions of the total activities of these enzymes were recovered in the soluble phase after exposure of the organelles to distilled water, 0.1 M Sorensen's phosphate buffer or 0.1 M TRIS‐HCl buffer prepared in distilled water. Acid phosphatase showed lower levels of release into the supernatant under most of these conditions.When microbody‐lysosome preparations were exposed to sucrose in concentrations of 0.1 M to 0.8 M to phosphate buffer in 0.25 M sucrose or to TRIS‐HCl buffer prepared in 0.25 M sucrose, the release of α‐hydroxy acid oxidase, catalase and acid phosphatase was insignificant. However, under these conditions D‐amino acid oxidase was released into the soluble phase to a significant extent.These results suggest that α‐hydroxy acid oxidase and catalase are soluble constituents of renal microbodies while D‐amino acid oxidase is associated firmly with a sedimentable component of the organelle, and that renal microbodies are more fragile than renal lysosomes.

The rheology of greases

Two mineral oil greases and five synthetic base greases were subjected to steady laminar shearing motion in the range 0–40,000 sec−1 at the temperature of 19 ‡C, 58 ‡C and 90 ‡C. Three silicone base oils were also submitted to the same experimental programme.