scholarly journals Effect of contact geometry on the friction of acrylamide hydrogels with different surface structures

Friction ◽  
2020 ◽  
Author(s):  
Wenrui Liu ◽  
Rok Simič ◽  
Yuhong Liu ◽  
Nicholas D. Spencer
Keyword(s):  
Shear Stress ◽  
Tribological Behavior ◽  
Contact Geometry ◽  
Surface Structures ◽  
Wide Range ◽  
Order Of Magnitude ◽  
Inlet Zone ◽  
Convergent Inlet ◽  
Hydrodynamic Thickness ◽  
Flat Contact

AbstractPolyacrylamide (PAAm) hydrogels with brush-covered or crosslinked surfaces were produced and their tribological behavior was studied over a wide range of sliding speeds for two different contact geometries: sphere-on-flat and flat-pin-on-flat. Irrespective of the contact geometry, the brushy hydrogel surfaces displayed up to an order of magnitude lower coefficients of friction μ (COF) compared to the crosslinked surfaces, even achieving superlubricity (μ < 0.01). In general, a hydrogel sphere showed a lower coefficient of friction than a flat hydrogel pin at a similar contact pressure over the entire range of sliding speeds. However, after normalizing the friction force by the contact area, the shear stress of hydrogels with either crosslinked or brushy surfaces was found to be similar for both contact geometries at low speeds, indicating that hydrogel friction is unaffected by the contact geometry at these speeds. At high sliding speeds, the shear stress was found to be lower for a sphere-on-flat configuration compared to a flat-pin-on-flat configuration. This can be attributed to the larger equivalent hydrodynamic thickness due to the convergent inlet zone ahead of the sphere-on-flat contact, which presumably enhances the water supply in the contact, promotes rehydration, and thus reduces the friction at high sliding speeds compared to that measured for the flat-pin-on-flat contact.

scholarly journals Electrodeposited Hydroxyapatite-Based Biocoatings: Recent Progress and Future Challenges

Coatings ◽  
2021 ◽  
Vol 11 (1) ◽  
pp. 110
Author(s):  
Mir Saman Safavi ◽  
Frank C. Walsh ◽  
Maria A. Surmeneva ◽  
Roman A. Surmenev ◽  
Jafar Khalil-Allafi
Keyword(s):  
Recent Progress ◽  
Tribological Behavior ◽  
Metallic Materials ◽  
Industrial Processing ◽  
Component Size ◽  
Polymeric Particles ◽  
Operational Variables ◽  
Wide Range ◽  
Future Challenges ◽  
Coating Methods

Hydroxyapatite has become an important coating material for bioimplants, following the introduction of synthetic HAp in the 1950s. The HAp coatings require controlled surface roughness/porosity, adequate corrosion resistance and need to show favorable tribological behavior. The deposition rate must be sufficiently fast and the coating technique needs to be applied at different scales on substrates having a diverse structure, composition, size, and shape. A detailed overview of dry and wet coating methods is given. The benefits of electrodeposition include controlled thickness and morphology, ability to coat a wide range of component size/shape and ease of industrial processing. Pulsed current and potential techniques have provided denser and more uniform coatings on different metallic materials/implants. The mechanism of HAp electrodeposition is considered and the effect of operational variables on deposit properties is highlighted. The most recent progress in the field is critically reviewed. Developments in mineral substituted and included particle, composite HAp coatings, including those reinforced by metallic, ceramic and polymeric particles; carbon nanotubes, modified graphenes, chitosan, and heparin, are considered in detail. Technical challenges which deserve further research are identified and a forward look in the field of the electrodeposited HAp coatings is taken.

Soft EHL Lubrication of Complex Multiphase Fluids

2005 ◽  
Author(s):  
J. de Vicente ◽  
H. A. Spikes ◽  
J. R. Stokes
Keyword(s):  
Complex Fluids ◽  
Steel Ball ◽  
Film Forming ◽  
Wide Range ◽  
Multiphase Fluids ◽  
Water Based ◽  
Complex Rheology ◽  
Lubrication Properties ◽  
Flat Contact ◽  
Ehl Lubrication

The lubrication properties of a series of multiphase water-based fluids of complex rheology and microstructure, including o/w emulsions, have been studied in a rolling-sliding steel ball-on-elastomer flat contact. The results show that friction curves of Newtonian fluids made over a wide range of entrainment speeds and viscosity can be used to identify the prevailing mechanisms of lubrication for more complex fluids and, for emulsions, to show the predominant film-forming phase.

Roughness Element Geometry Required for Wind Tunnel Simulations of the Atmospheric Wind

1977 ◽  
Vol 99 (3) ◽  
pp. 480-485 ◽  
Author(s):  
I. S. Gartshore ◽  
K. A. De Croos
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Wind Tunnel ◽  
Roughness Element ◽  
Three Dimensional ◽  
Wall Stress ◽  
Wide Range ◽  
Drag Plate ◽  
Rough Boundaries ◽  
Single Figure

Using a data correlation for the wall stress associated with very rough boundaries and a semi-empirical calculation method, the shape of boundary layers in exact equilibrium with the roughness beneath them is calculated. A wide range of roughness geometries (two- and three-dimensional elements) is included by the use of equivalent surfaces of equal drag per unit area. Results can be summarized in a single figure which relates the shape factor of the boundary layer (its exponent if it has a power law velocity profile) to the height of the roughness elements and their spacing. New data for one turbulent boundary layer developing over a long fetch of uniform roughness is presented. Wall shear stress, measured directly from a drag plate is combined with boundary layer integral properties to show that the shear stress correlation adopted is reasonably accurate and that the boundary layer is close to equilibrium after passing over a streamwise roughness fetch equal to about 350 times the roughness element height. An example is given of the way in which roughness geometry may be chosen from calculated equilibrium results, for one particular boundary layer thickness and a shape useful for simulating strong atmospheric winds in a wind tunnel.

scholarly journals Aluminum metal matrix composites a review of reinforcement; mechanical and tribological behavior

2018 ◽  
Vol 7 (2.4) ◽  
pp. 117 ◽  
Author(s):  
Pranav Dev Srivyas ◽  
M S. Charoo
Keyword(s):  
Metal Matrix ◽  
Tribological Behavior ◽  
Coefficient Of Thermal Expansion ◽  
Aluminum Matrix ◽  
Weight Ratio ◽  
High Hardness ◽  
Aluminum Matrix Composites ◽  
Matrix Composites ◽  
Wide Range ◽  
Structural Applications

This review aims to explore the fundamental mechanical and tribological behavior Aluminum matrix composites (AMCs) reinforced with different reinforcements. Aluminum matrix composites are considered to be the new emerging class of materials which are having the tailored properties for specific applications. AMCs are the advanced engineering materials having superior properties as comparison to other conventional aluminum alloys. AMCs exhibits attractive properties such as high hardness, better yield strength, strength to weight ratio, high thermal conductivity, low coefficient of thermal expansion, superior wear and corrosion resistance. In recent times, because of these properties they have repealed keen interest for various potential applications in aerospace, automotive and various other structural applications.. Extensive research and development has been made in the Al-based MMCs with every possible alloy and different reinforcements so as to get the material of desired properties. By suitable use of different reinforcements in the Al metal matrix a wide range of properties combination can be obtained. The fundamental mechanical and tribological behavior of different reinforcements under dry and wet lubricated sliding conditions is recently being studied. It is reported that various reinforcement were successfully employed to decrease friction and wear in various applications. A comprehensive review is provided with the aim to analyze such properties of different reinforcements. 

scholarly journals Entropy-Based Shear Stress Distribution in Open Channel for all types of Flow using Experimental Data

2021 ◽  
Author(s):  
Hae Seong Jeon ◽  
Ji Min Kim ◽  
Yeon Moon Choo
Keyword(s):  
Shear Stress ◽  
Stress Distribution ◽  
Turbulent Flow ◽  
Correlation Coefficient ◽  
Open Channel ◽  
Shear Stress Distribution ◽  
Entropy Theory ◽  
Tractive Force ◽  
Design Standards ◽  
Wide Range

Abstract Korea’s river design standards set general design standards for river and river-related projects in Korea, which systematize the technologies and methods involved in river-related projects. This includes measurement methods for parts necessary for river design, but do not include information on shear stress. Shear Stress is to one of the factors necessary for river design and operation. Shear stress is one of the most important hydraulic factors used in the fields of water especially for artificial channel design. Shear stress is calculated from the frictional force caused by viscosity and fluctuating fluid velocity. Current methods are based on past calculations, but factors such as boundary shear stress or energy gradient are difficult to actually measure or estimate. The point velocity throughout the entire cross section is needed to calculate the velocity gradient. In other words, the current Korea’s river design standards use tractive force, critical tractive force instead of shear stress because it is more difficult to calculate the shear stress in the current method. However, it is difficult to calculate the exact value due to the limitations of the formula to obtain the river factor called the tractive force. In addition, tractive force has limitations that use empirically identified base value for use in practice. This paper focuses on the modeling of shear stress distribution in open channel turbulent flow using entropy theory. In addition, this study suggests shear stress distribution formula, which can be easily used in practice after calculating the river-specific factor T. and that the part of the tractive force and critical tractive force in the Korea’s river design standards should be modified by the shear stress obtained by the proposed shear stress distribution method. The present study therefore focuses on the modeling of shear stress distribution in open channel turbulent flow using entropy theory. The shear stress distribution model is tested using a wide range of forty-two experimental runs collected from the literature. Then, an error analysis is performed to further evaluate the accuracy of the proposed model. The results revealed a correlation coefficient of approximately 0.95–0.99, indicating that the proposed method can estimate shear stress distribution accurately. Based on this, the results of the distribution of shear stress after calculating the river-specific factors show a correlation coefficient of about 0.86 to 0.98, which suggests that the equation can be applied in practice.

scholarly journals URMAP, an ultra-fast read mapper

2020 ◽  
Author(s):  
Robert C. Edgar
Keyword(s):  
Read Mapping ◽  
Mapping Algorithm ◽  
Sequencing Technologies ◽  
Large Size ◽  
Mapping Software ◽  
Biological Studies ◽  
Wide Range ◽  
Order Of Magnitude ◽  
Comparable Accuracy ◽  
Generation Sequencing

AbstractMapping of reads to reference sequences is an essential step in a wide range of biological studies. The large size of datasets generated with next-generation sequencing technologies motivates the development of fast mapping software. Here, I describe URMAP, a new read mapping algorithm. URMAP is an order of magnitude faster than BWA and Bowtie2 with comparable accuracy on a benchmark test using simulated paired 150nt reads of a well-studied human genome. Software is freely available at https://drive5.com/urmap.

scholarly journals Impact of change in erosion rate and landscape steepness on hillslope and fluvial sediments grain size in the Feather River Basin (Sierra Nevada, California)

2014 ◽  
Vol 2 (2) ◽  
pp. 1047-1092 ◽  
Author(s):  
M. Attal ◽  
S. M. Mudd ◽  
M. D. Hurst ◽  
B. Weinman ◽  
K. Yoo ◽  
...  
Keyword(s):  
Grain Size ◽  
River Basin ◽  
Sierra Nevada ◽  
Sediment Flux ◽  
Fluvial Sediments ◽  
Sediment Grain Size ◽  
Erosion Rates ◽  
Wide Range ◽  
Feather River ◽  
Order Of Magnitude

Abstract. The characteristics of the sediment transported by rivers (e.g., sediment flux, grain size distribution – GSD –) dictate whether rivers aggrade or erode their substrate. They also condition the architecture and properties of sedimentary successions in basins. In this study, we investigate the relationship between landscape steepness and the grain size of hillslope and fluvial sediments. The study area is located within the Feather River Basin in Northern California, and studied basins are underlain exclusively by tonalite lithology. Erosion rates in the study area vary over an order of magnitude, from > 250 mm ka−1 in the Feather River canyon to < 15 mm ka−1 on an adjacent low relief plateau. We find that the coarseness of hillslope sediment increases with increasing hillslope steepness and erosion rates. We hypothesize that, in our soil samples, the measured ten-fold increase in D50 and doubling of the amount of fragments larger than 1 mm when slope increases from 0.38 to 0.83 m m−1 is due to a decrease in the residence time of rock fragments, causing particles to be exposed for shorter periods of time to processes that can reduce grain size. For slopes in excess of 0.7 m m−1, landslides and scree cones supply much coarser sediment to rivers, with D50 and D84 more than one order of magnitude larger than in soils. In the tributary basins of the Feather River, a prominent break in slope developed in response to the rapid incision of the Feather River. Downstream of the break in slope, fluvial sediment grain size increases, due to an increase in flow competence (mostly driven by channel steepening) but also by a change in sediment source and in sediment dynamics: on the plateau upstream of the break in slope, rivers transport easily mobilised fine-grained sediment derived exclusively from soils. Downstream of the break in slope, mass wasting processes supply a wide range of grain sizes that rivers entrain selectively, depending on the competence of their flow. Our results also suggest that in this study site, hillslopes respond rapidly to an increase in the rate of base-level lowering compared to rivers.

Conditional Sampling in a Transitional Boundary Layer Under High Freestream Turbulence Conditions

2003 ◽  
Vol 125 (1) ◽  
pp. 28-37 ◽  
Author(s):  
Ralph J. Volino ◽  
Michael P. Schultz ◽  
Christopher M. Pratt
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Streamwise Velocity ◽  
Turbulent Shear ◽  
Conditional Sampling ◽  
Freestream Turbulence ◽  
Mean Velocity ◽  
Transitional Boundary Layer ◽  
Turbulent Shear Stress ◽  
Order Of Magnitude

Conditional sampling has been performed on data from a transitional boundary layer subject to high (initially 9%) freestream turbulence and strong (K=ν/U∞2dU∞/dx as high as 9×10−6) acceleration. Methods for separating the turbulent and nonturbulent zone data based on the instantaneous streamwise velocity and the turbulent shear stress were tested and found to agree. Mean velocity profiles were clearly different in the turbulent and nonturbulent zones, and skin friction coefficients were as much as 70% higher in the turbulent zone. The streamwise fluctuating velocity, in contrast, was only about 10% higher in the turbulent zone. Turbulent shear stress differed by an order of magnitude, and eddy viscosity was three to four times higher in the turbulent zone. Eddy transport in the nonturbulent zone was still significant, however, and the nonturbulent zone did not behave like a laminar boundary layer. Within each of the two zones there was considerable self-similarity from the beginning to the end of transition. This may prove useful for future modeling efforts.

The Dose Determines the Stimulation (and Poison): Development of A Chemical Hormesis Database

1997 ◽  
Vol 16 (6) ◽  
pp. 545-559 ◽  
Author(s):  
Edward J. Calabrese ◽  
Linda A. Baldwin
Keyword(s):  
A Priori ◽  
Dose Range ◽  
Evaluation Criteria ◽  
Metabolic Effects ◽  
Growth Responses ◽  
Wide Range ◽  
Order Of Magnitude ◽  
Real Phenomenon ◽  
Effect Level ◽  
Taxonomic Groups

A comprehensive effort was undertaken to identify articles demonstrating chemical hormesis. Nearly 4000 potentially relevant articles were retrieved from preliminary computer searches utilizing various keyword descriptors and extensive cross-referencing. A priori evaluation criteria were established including study design features (e.g., number of doses, dose range), statistical analysis, and reproducibility of results. Evidence of chemical hormesis was judged to have occurred in approximately 350 of the 4000 studies evaluated. Chemical hormesis was observed in a wide range of taxonomic groups and involved agents representing highly diverse chemical classes, many of potential environmental relevance. Numerous biologic endpoints were assessed, with growth responses the most prevalent, followed by metabolic effects, longevity, reproductive responses, and survival. Hormetic responses were generally observed to be of limited magnitude with the average low-dose maximum stimulation approximately 50% greater than controls. The hormetic dose-response range was generally limited to about one order of magnitude with the upper end of the hormetic curve approaching the estimated no-observed-effect level (NOEL) for the particular endpoint. Based on the evaluation criteria, high to moderate evidence of hormesis was observed in studies comprised of ≥ doses with <3 doses in the hormetic zone. The present analysis suggests that chem ical hormesis is a reproducible and generalizable biologic phenomenon. Over the last decade advances have been made providing mechanistic insight helpful in explaining the phenomenon of chemical hormesis in multiple biologic systems with various endpoints. The reason for the uncertainty surrounding the existence of hormesis as a “real phenomenon” is believed to be the result of its relatively infrequent observation in the literature due to experimental design considerations, especially with respect to the number of doses, range of doses, and endpoint selection.

The A39G FF domain folds on a volcano-shaped free energy surface via separate pathways

2021 ◽  
Vol 118 (46) ◽  
pp. e2115113118
Author(s):  
Ved P. Tiwari ◽  
Yuki Toyama ◽  
Debajyoti De ◽  
Lewis E. Kay ◽  
Pramodh Vallurupalli
Keyword(s):  
Free Energy ◽  
Chemical Exchange ◽  
Exchange Process ◽  
Conformational Dynamics ◽  
Chemical Exchange Saturation Transfer ◽  
Folding Kinetics ◽  
Free Energy Surface ◽  
Wide Range ◽  
Order Of Magnitude ◽  
Energy Surfaces

Conformational dynamics play critical roles in protein folding, misfolding, function, misfunction, and aggregation. While detecting and studying the different conformational states populated by protein molecules on their free energy surfaces (FESs) remain a challenge, NMR spectroscopy has emerged as an invaluable experimental tool to explore the FES of a protein, as conformational dynamics can be probed at atomic resolution over a wide range of timescales. Here, we use chemical exchange saturation transfer (CEST) to detect “invisible” minor states on the energy landscape of the A39G mutant FF domain that exhibited “two-state” folding kinetics in traditional experiments. Although CEST has mostly been limited to studies of processes with rates between ∼5 to 300 s−1 involving sparse states with populations as low as ∼1%, we show that the line broadening that is often associated with minor state dips in CEST profiles can be exploited to inform on additional conformers, with lifetimes an order of magnitude shorter and populations close to 10-fold smaller than what typically is characterized. Our analysis of CEST profiles that exploits the minor state linewidths of the 71-residue A39G FF domain establishes a folding mechanism that can be described in terms of a four-state exchange process between interconverting states spanning over two orders of magnitude in timescale from ∼100 to ∼15,000 μs. A similar folding scheme is established for the wild-type domain as well. The study shows that the folding of this small domain proceeds through a pair of sparse, partially structured intermediates via two discrete pathways on a volcano-shaped FES.

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