On the Shear Bands and Shear Localizations in EHL Films

2004 ◽  
Author(s):  
L. Chang
Keyword(s):  
High Pressures ◽  
Glassy State ◽  
Shear Bands ◽  
Elastohydrodynamic Lubrication ◽  
Elastic Behavior ◽  
Plastic Behavior ◽  
Shear Stresses ◽  
Low Shear Stress ◽  
Recent Laboratory ◽  
Low Shear

Liquid lubricants in elastohydrodynamic lubrication (EHL) are subjected to high pressures of the magnitude of 109 N/m2 (GPa). Under these high pressures, the lubricants in room temperatures are usually in the glassy state behaving as amorphous solids. Similar to the polymers, the lubricants exhibit elastic or visco-elastic behavior under low-shear-stress loading and plastic or visco-plastic behavior with high shear stresses. Recent laboratory experiments have revealed two types of localized shear responses that can take place in the lubricant bulk. The localized shear may be active in the EHL film and may play a significant role in the responses exhibited by the contact conjunction. A typical example is the EHL traction. This paper summarizes the up-to-date research related to the shear localizations.

Rheology of Fluoride Gels

1976 ◽  
Vol 55 (3) ◽  
pp. 353-356 ◽  
Author(s):  
M. Braden ◽  
Ratna Perera
Keyword(s):  
Infrared Spectroscopy ◽  
Shear Rate ◽  
Elastic Behavior ◽  
Shear Stresses ◽  
High Shear ◽  
Shear Rates ◽  
Extreme Stress ◽  
Thickening Agents ◽  
Shear Rate Dependence ◽  
Low Shear

Six commercial fluoride gels have been studied, using a cone and plate viscometer. Also, the thickening agents have been analyzed using infrared spectroscopy. All gels showed stress thinning, which is the decrease of viscosity with shear rate. Such shear rate dependence is clinically convenient in that the gel will flow readily at the high shear stresses present when the gel is applied but will not flow readily under its own weight when on the tooth. Five materials containing hydroxyalkyl celluloses showed similar degrees of shear thinning. One material with a non-cellulosic thickener showed much more extreme stress thinning together with elastic behavior at low shear rates; such behavior may be clinically advantageous. All of the gels showed only slight temperature dependence of rheological properties.

Cohesion and detachment in biofilm systems for different electron acceptor and donors

2007 ◽  
Vol 55 (8-9) ◽  
pp. 421-428 ◽  
Author(s):  
C. Coufort ◽  
N. Derlon ◽  
J. Ochoa-Chaves ◽  
A. Liné ◽  
E. Paul
Keyword(s):  
Basal Layer ◽  
Middle Layer ◽  
Electron Donors ◽  
Shear Stresses ◽  
Average Thickness ◽  
Low Shear Stress ◽  
Erosion Test ◽  
Biofilm Structure ◽  
Initial Biofilm ◽  
Low Shear

This work deals with the cohesion and detachment in biofilm systems for two electron acceptors and for two electron donors. Biofilms were developed on plates, under very low shear stress for one month and then subjected to an erosion test for two hours in a Couette-Taylor reactor. Biofilm was characterised in terms of average thickness and residual TOC mass. It was found that the biofilm structure is very heterogeneous and stratified. The top layer, which represents 60% of the biofilm mass, is very fragile and can be easily detached; the basal layer, which represents 20% of the biofilm mass, is very cohesive and can resist shear stresses up to 13 Pa. Between these two layers, a middle layer of intermediary cohesion represents 20% of the initial biofilm mass.

Traction in Elastohydrodynamic Contacts

1988 ◽  
Vol 202 (2) ◽  
pp. 129-144 ◽  
Author(s):  
W Hirst ◽  
J W Richmond
Keyword(s):  
Shear Stress ◽  
Thermal Properties ◽  
Thermal Effects ◽  
High Pressures ◽  
Glassy State ◽  
Elastohydrodynamic Lubrication ◽  
Elastic Solid ◽  
Non Linear ◽  
Thermal Region ◽  
Very High

The magnitude of the traction in elastohydrodynamic lubrication is influenced by at least two non-Newtonian effects. At very high pressures, the lubricant becomes noticeably viscoelastic and behaves as an elastic solid when the shear stress is small. At high values of the shear stress, the relation between the stress and the shear rate becomes non-linear and, when thermal effects are insignificant, conforms to the Eyring expression for viscosity. In general, however, the elastic region, the non-linear isothermal region and the thermal region are not separate and distinct but merge gradually into each other. This makes it difficult, experimentally, to determine the magnitude of the parameters controlling the behaviour of the fluid and, conversely, to predict from them the shape of the traction curve. The present paper examines these problems and shows that the major uncertainty is caused by the dearth of knowledge of the thermal properties of fluids at very high pressures. [These have been measured in an associated investigation by Richmond et al. (1).] It is shown that, when the correct values of the thermal properties are used and allowance is also made for the merging of the various regions of the traction curve, parameters may be derived from which the shape of the traction curve can be predicted within experimental error. In the non-linear region the Eyring relation is obeyed at pressures well above the usual glass transition pressure and it is concluded that in the extreme conditions of shear stress typical of elastohydrodynamic lubrication the glassy state is not achieved.

Analysis of Particle Trajectories in Aortic Artery Bifurcations With Stenosis

1989 ◽  
Vol 111 (4) ◽  
pp. 311-315 ◽  
Author(s):  
M. Nazemi ◽  
C. Kleinstreuer
Keyword(s):  
Shear Stress ◽  
Element Model ◽  
Spherical Particles ◽  
Shear Stresses ◽  
Atherosclerotic Lesions ◽  
Aortic Artery ◽  
Diseased Segment ◽  
Low Shear Stress ◽  
Accelerated Growth ◽  
Low Shear

The fluid-particle dynamics in a two-dimensonal symmetric branching channel with local occlusions representing a diseased segment of an aortic artery bifurcation has been analyzed. The validated finite element model simulates the trajectories and landing or impact sites of spherical particles for laminar flow in bifurcation channels with generalized wall conditions. Two hypotheses relating critical wall shear stress levels and plaque formation, previously postulated by Kleinstreuer et al. (1988) and Nazemi et al. (1989), have been confirmed. Low shear stress may contribute to the onset of atherosclerotic lesions and areas of critically low and high shear stresses are susceptible to accelerated growth of plaque.

Elastohydrodynamic lubrication at high pressures II. Non-Newtonian behaviour

1979 ◽  
Vol 365 (1723) ◽  
pp. 537-565 ◽  
Keyword(s):  
High Pressure ◽  
High Pressures ◽  
Pressure Coefficient ◽  
Viscoelastic Model ◽  
Elastohydrodynamic Lubrication ◽  
Shear Stresses ◽  
Liquid Component ◽  
Hydrodynamic Behaviour ◽  
Traction Coefficient ◽  
Coefficient Of Viscosity

An experimental study has been made of the elastohydrodynamic behaviour of a range of fluids, including some of widely different pressure coefficient of viscosity. It is shown that the complex pattern of elasto-hydrodynamic behaviour observed at high pressure is due predominantly to the interplay between two non-Newtonian effects. The first is viscoelasticity. This has the effect that, at small degrees of shear, all the fluids behave as viscous liquids at low pressure and as elastic solids at high. The transition occurs when the viscosity attains 10 5 Pa s. The second is the non-linearity at high degrees of shear between the shear stress and the rate of shear. The consequence of this is that at sufficiently high rates of shear the fluid loses any elastic character and behaves as a non-Newtonian liquid. It is shown that the behaviour of the real system resembles that of a Maxwell viscoelastic model into which Eyring’s elementary expression for viscosity is incorporated to describe the non-Newtonian character of the liquid component. At high shear stresses the value of the traction coefficient {T/W ) is given by the expression T/W = α ¯ T 0 - T 0 /T 0 / p ln ( T 0 /2 n 0 s), where α ¯ is the pressure coefficient of viscosity, T 0 a characteristicshearstress, p the pressure, n 0 the viscosity at atmospheric pressure and s the rate of shear. The values of α ¯ which are appropriate relate to the actual pressures applied. These pressures are considerably in excess of those obtainable in conventional high pressure viscometry but it is shown that the appropriate value of α ¯ can be derived satisfactorily from the elastohydrodynamic experiments themselves. The fluid giving the highest value of the traction coefficient proved to be one whose pressure coefficient of viscosity was considerably greater at high pressure than at low. The results suggest that with high polymers the structural unit responsible for flow is considerably smaller than the polymer molecule itself. In contrast, certain types of molecules having branch chains appear to become entangled making the unit for flow appreciably larger than the individual molecule.

scholarly journals Computing of Low Shear Stress-Driven Endothelial Gene Network Involved in Early Stages of Atherosclerotic Process

2018 ◽  
Vol 2018 ◽  
pp. 1-12 ◽  
Author(s):  
Federico Vozzi ◽  
Jonica Campolo ◽  
Lorena Cozzi ◽  
Gianfranco Politano ◽  
Stefano Di Carlo ◽  
...  
Keyword(s):  
Gene Expression ◽  
Shear Stress ◽  
Transcription Factors ◽  
Gene Network ◽  
Bioinformatics Analysis ◽  
Atherosclerotic Disease ◽  
Cell Phenotype ◽  
Shear Stresses ◽  
Low Shear Stress ◽  
Low Shear

Background. In the pathogenesis of atherosclerosis, a central role is represented by endothelial inflammation with influx of chemokine-mediated leukocytes in the vascular wall. Aim of this study was to analyze the effect of different shear stresses on endothelial gene expression and compute gene network involved in atherosclerotic disease, in particular to homeostasis, inflammatory cell migration, and apoptotic processes.Methods. HUVECs were subjected to shear stress of 1, 5, and 10 dyne/cm2in a Flow Bioreactor for 24 hours to compare gene expression modulation. Total RNA was analyzed by Affymetrix technology and the expression of two specific genes (CXCR4 and ICAM-1) was validated by RT-PCR. To highlight possible regulations between genes and as further validation, a bioinformatics analysis was performed.Results. At low shear stress (1 dyne/cm2) we observed the following: (a) strong upregulation of CXCR4; (b) mild upregulation of Caspase-8; (c) mild downregulation of ICAM-1; (d) marked downexpression of TNFAIP3. Bioinformatics analysis showed the presence of network composed by 59 new interactors (14 transcription factors and 45 microRNAs) appearing strongly related to shear stress.Conclusions. The significant modulation of these genes at low shear stress and their close relationships through transcription factors and microRNAs suggest that all may promote an initial inflamed endothelial cell phenotype, favoring the atherosclerotic disease.

Low shear stress-induced endothelial mesenchymal transformation via the down-regulation of TET2

2021 ◽  
Vol 545 ◽  
pp. 20-26
Author(s):  
AFang Li ◽  
LiLan Tan ◽  
ShuLei Zhang ◽  
Jun Tao ◽  
Zuo Wang ◽  
...  
Keyword(s):  
Shear Stress ◽  
Down Regulation ◽  
Low Shear Stress ◽  
Mesenchymal Transformation ◽  
Low Shear

Plaque and Shear Stress Distribution in Human Coronary Bifurcations: a Multi-Slice Computed Tomography Study

2007 ◽  
Author(s):  
Alina G. van der Giessen ◽  
Jolanda J. Wentzel ◽  
Frans N. van de Vosse ◽  
Antonius F. van der Steen ◽  
Pim J. de Feyter ◽  
...  
Keyword(s):  
Computed Tomography ◽  
Shear Stress ◽  
Outer Wall ◽  
Atherosclerotic Plaques ◽  
Advanced Stage ◽  
Flow Divider ◽  
Low Shear Stress ◽  
Curved Vessels ◽  
Early Atherosclerosis ◽  
Low Shear

It is generally accepted that early atherosclerosis develops in low shear-stress (SS) regions such as the outer wall of arterial bifurcations and the inner bend of curved vessels (1). However, in clinical practice, it is common to observe atherosclerotic plaques at the flow-divider, or carina, of coronary bifurcations (2). Plaques at the carina are more frequently found in symptomatic patients, and may represent a more advanced stage of atherosclerosis. The carina is located in a region which is exposed to high SS. We hypothesize that if plaques are located in atheroprotective high SS regions, they have grown circumferentially from the atherogenic low SS regions.

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