| 1. |
- Müser, M. H., et al.
(författare)
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Meeting the Contact-Mechanics Challenge
- 2017
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Ingår i: Tribology letters. - : Springer New York LLC. - 1023-8883 .- 1573-2711. ; 65:4
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Tidskriftsartikel (refereegranskat)abstract
- This paper summarizes the submissions to a recently announced contact-mechanics modeling challenge. The task was to solve a typical, albeit mathematically fully defined problem on the adhesion between nominally flat surfaces. The surface topography of the rough, rigid substrate, the elastic properties of the indenter, as well as the short-range adhesion between indenter and substrate, were specified so that diverse quantities of interest, e.g., the distribution of interfacial stresses at a given load or the mean gap as a function of load, could be computed and compared to a reference solution. Many different solution strategies were pursued, ranging from traditional asperity-based models via Persson theory and brute-force computational approaches, to real-laboratory experiments and all-atom molecular dynamics simulations of a model, in which the original assignment was scaled down to the atomistic scale. While each submission contained satisfying answers for at least a subset of the posed questions, efficiency, versatility, and accuracy differed between methods, the more precise methods being, in general, computationally more complex. The aim of this paper is to provide both theorists and experimentalists with benchmarks to decide which method is the most appropriate for a particular application and to gauge the errors associated with each one..
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| 2. |
- Bennett, A. I., et al.
(författare)
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Contact Measurements of Randomly Rough Surfaces
- 2017
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Ingår i: Tribology letters. - : Springer Science and Business Media, LLC. - 1023-8883 .- 1573-2711. ; 65:4
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Tidskriftsartikel (refereegranskat)abstract
- This manuscript presents an experimental effort to directly measure contact areas and the details behind these scaled experiments on a randomly rough model surface used in the “Contact Mechanics Challenge” (2017). For these experiments, the randomly rough surface model was scaled up by a factor of 1000× to give a 100 mm square sample that was 3D printed from opaque polymethylmethacrylate (PMMA). This sample was loaded against various optically smooth and transparent samples of PDMS that were approximately 15 mm thick and had a range in elastic modulus from 14 kPa to 2.1 MPa. During loading, a digital camera recorded contact locations by imaging the scattering of light that occurs off of the PMMA rough surface when it was in contact with the PDMS substrate. This method of illuminating contact areas is called frustrated total internal reflection and is performed by creating a condition of total internal reflection within the unperturbed PDMS samples. Contact or deformation of the surface results in light being diffusely transmitted from the PDMS and detected by the camera. For these experiments, a range of reduced pressure (nominal pressure/elastic modulus) from below 0.001 to over 1.0 was examined, and the resulting relative contact area (real area of contact/apparent area of contact) was found to increase from below 0.1% to over 60% at the highest pressures. The experimental uncertainties associated with experiments are discussed, and the results are compared to the numerical results from the simulation solution to the “Contact Mechanics Challenge.” The simulation results and experimental results of the relative contact areas as a function of reduced pressure are in agreement (within experimental uncertainties).
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| 3. |
- Bennett, A. I., et al.
(författare)
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Deformation Measurements of Randomly Rough Surfaces
- 2017
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Ingår i: Tribology letters. - : Springer Science and Business Media, LLC. - 1023-8883 .- 1573-2711. ; 65:4
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Tidskriftsartikel (refereegranskat)abstract
- Measurements of surface deformations as part of the “Contact Mechanics Challenge” were collected using digital image correlation (DIC). For these experiments, a scaled version (1000×) of the periodic and random roughness surface provided for the “Contact Mechanics Challenge” was used. A 100 mm × 100 mm scale replica of the surface, approximately 10 mm thick, was 3D-printed using an opaque polymethylmethacrylate and pressed into contact against flat, transparent polydimethylsiloxane (PDMS) sheets with dead weight loads. Four different formulations of PDMS were used, and the resulting elastic moduli ranged from 64 kPa to 2.1 MPa. The DIC technique was used in situ to measure the deformation of the PDMS surface at each load increment from 22.5 to 450 N. Surface deformations in and out of contact were measured across the entire apparent area of contact and overlaid with the measurements of contact area to provide a complete description of the surface profile during loading. A direct comparison between these experiments and the simulations regarding the gap within the contact at a reduced pressure of 0.164 agrees to within ±10% when normalized to the maximum gap.
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| 4. |
- Chalifour, B., et al.
(författare)
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Drought alters the spatial distribution, grazing patterns, and radula morphology of a fungal-farming salt marsh snail
- 2019
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Ingår i: Marine Ecology Progress Series. - : Inter-Research. - 0171-8630 .- 1616-1599. ; 620, s. 1-13
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Tidskriftsartikel (refereegranskat)abstract
- Climate change is altering consumer− plant interactions in ecosystems worldwide. How consumers alter their spatial distribution, grazing activities, and functional morphology in response to climate stress can determine whether their effects on plants intensify or relax. Few studies have considered multiple consumer response metrics to elucidate the mechanisms underpinning the resulting changes in consumer− plant interactions. Here, we tested how drought stress influences the interaction between the dominant consumer, the fungal-farming periwinkle snail Littoraria irrorata, and a foundational plant, cordgrass Spartina alterniflora, in a southeastern US salt marsh. In a 4 mo field experiment, we maintained moderate snail densities in mesh control chambers and clear plastic climate chambers that simulated drought by elevating temperatures and drying soils. Monitoring revealed that snails more often congregated on cordgrass stems than leaves in climate chambers than in controls. Image analyses indicated that this behavioral shift corresponded to snails inflicting shorter, but more numerous, fungal-infested scars on cordgrass leaves, and causing less plant damage in climate chambers than controls. Coincident with their net reduction in grazing, snails maintained longer radulae, whose central teeth were blunter and lateral teeth were sharper, in climate chambers compared to controls. These results suggest that under drought, snail radulae may experience less frictional wear and that, at intermediate densities, snail−cordgrass interactions relax. Together with prior research showing that at high densities, snails can denude cordgrass during drought, we conclude that con-Saltmarsh snails (top left) stressed by drought conditions show reduced radula wear and shifts in tooth morphology (e.g. sharper lateral teeth; bottom right), suggesting that fewer resources are invested in maintaining the grazing apparatus. Climate change is expected to increase drought stress. © The authors 2019.
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| 5. |
- McGhee, A. J., et al.
(författare)
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Contact and Deformation of Randomly Rough Surfaces with Varying Root-Mean-Square Gradient
- 2017
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Ingår i: Tribology letters. - : Springer New York LLC. - 1023-8883 .- 1573-2711. ; 65:4
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Tidskriftsartikel (refereegranskat)abstract
- The “Contact Mechanics Challenge” posed to the tribology community by Müser and Dapp in 2015 detailed a 100 µm × 100 µm randomly rough surface with a root-mean-square gradient of unity, g ¯ = 1. Many surfaces, both natural and synthetic, can be described as randomly rough, but rarely with a root-mean-square gradient as steep as g ¯ = 1. The selection of such a challenging surface parameter was intentional, but potentially limiting for broad comparisons across existing models and theories which may be limited by small-slope approximations. In this manuscript, the root-mean-square gradients (g ¯) of the “Contact Mechanics Challenge” surface were produced on 1000 × scaled models such that there were three different surfaces for study with g¯=0.2,0.5, and 1. In situ measurements of the real area of contact and contact area distributions were performed using frustrated total internal reflectance along with surface deformation measurements performed using digital image correlation. These optical in situ experiments used the scaled 3D-printed rough surfaces that were loaded into contact with smooth, flat, and elastic samples that were made from unfilled PDMS: (10:1) E* = 2.1 MPa Δγ = 4 mJ/m2; (20:1) E* = 0.75 MPa Δγ = 3 mJ/m2; (30:1) E* = 0.24 MPa Δγ = 2 mJ/m2. All of the loading was performed using a uniaxial load frame under force control. A Green’s function molecular dynamics simulation assuming the small-slope approximation was compared to all experimental data. These measurements reveal that decreasing root-mean-square gradient noticeably increases real area of contact area under conditions of “equal” applied load, but variations in the root-mean-square gradient did not significantly alter the contact patch geometry under conditions of nearly equal real area of contact. Including g ¯ in the reduced pressure (p= P/ (E∗ g ¯)) reduced the root-mean-square error between the simulation (g ¯ = 1) and all experimental data for the relative area of contact as a function of reduced pressure over the entire range of surfaces, materials, and loads tested.
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| 6. |
- Harris, Kurt L., et al.
(författare)
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Ancestral Sequence Reconstruction of a Cytochrome P450 Family Involved in Chemical Defense Reveals the Functional Evolution of a Promiscuous, Xenobiotic-Metabolizing Enzyme in Vertebrates
- 2022
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Ingår i: Molecular biology and evolution. - : NLM (Medline). - 0737-4038 .- 1537-1719. ; 39:6
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Tidskriftsartikel (refereegranskat)abstract
- The cytochrome P450 family 1 enzymes (CYP1s) are a diverse family of hemoprotein monooxygenases, which metabolize many xenobiotics including numerous environmental carcinogens. However, their historical function and evolution remain largely unstudied. Here we investigate CYP1 evolution via the reconstruction and characterization of the vertebrate CYP1 ancestors. Younger ancestors and extant forms generally demonstrated higher activity toward typical CYP1 xenobiotic and steroid substrates than older ancestors, suggesting significant diversification away from the original CYP1 function. Caffeine metabolism appears to be a recently evolved trait of the CYP1A subfamily, observed in the mammalian CYP1A lineage, and may parallel the recent evolution of caffeine synthesis in multiple separate plant species. Likewise, the aryl hydrocarbon receptor agonist, 6-formylindolo[3,2-b]carbazole (FICZ) was metabolized to a greater extent by certain younger ancestors and extant forms, suggesting that activity toward FICZ increased in specific CYP1 evolutionary branches, a process that may have occurred in parallel to the exploitation of land where UV-exposure was higher than in aquatic environments. As observed with previous reconstructions of P450 enzymes, thermostability correlated with evolutionary age; the oldest ancestor was up to 35 °C more thermostable than the extant forms, with a 10T50 (temperature at which 50% of the hemoprotein remains intact after 10 min) of 71 °C. This robustness may have facilitated evolutionary diversification of the CYP1s by buffering the destabilizing effects of mutations that conferred novel functions, a phenomenon which may also be useful in exploiting the catalytic versatility of these ancestral enzymes for commercial application as biocatalysts.
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| 7. |
- Harris, Kathryn L, et al.
(författare)
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Wear Debris Mobility, Aligned Surface Roughness, and the Low Wear Behavior of Filled Polytetrafluoroethylene
- 2015
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Ingår i: Tribology letters. - : Springer New York LLC. - 1023-8883 .- 1573-2711. ; 60:1
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Tidskriftsartikel (refereegranskat)abstract
- PTFE/α-alumina composites are well known to exhibit very low wear rates compared to unfilled PTFE and various other PTFE-matrix composites. The improved wear life of these composites is attributed in part to the formation of a uniform protective transfer film on the metal countersurface. It is postulated that the retention of transferred material and the recirculation of third bodies between the transfer film and running surface of the polymer composite are necessary for the maintenance of low wear within this tribological system. The accumulation of these third bodies was observed in reciprocating sliding tests on countersamples prescribed with aligned roughness. Wear performance of the polymer composite was tested as a function of the between the sliding direction and the aligned roughness of the countersample, ranging from parallel to perpendicular to the sliding direction. The wear rate of roughness oriented with the sliding direction was 300 times higher than roughness perpendicular to the sliding direction, revealing the importance of surface morphology and third body retention.
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| 8. |
- Holmberg, Krister, et al.
(författare)
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Poly(ethylene glycol) grafting as a way to prevent protein adsorption and bacterial adherence
- 1998
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Ingår i: Mittal Festschrift on Adhesion Science and Technology. - : VSP International Science Publishers. ; , s. 443-460
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Bokkapitel (refereegranskat)abstract
- Grafting of poly(ethylene glycol) (PEG) is an effective way of preventing adsorption of proteins and bacteria to hydrophobic surfaces. The paper discusses and compares two different routes of attaching PEG chains to surfaces: adsorption of block copolymers of ethylene oxide and propylene oxide (EO-PO block copolymers) and grafting via use of an anchoring polymer, poly(ethylene imine) (PEI). An overriding goal is to achieve a dense packing of PEG chains. The best effect in terms of protein and bacteria rejection is obtained by adsorbing a preformed copolymer of PEG grafted to PEI on a negatively charged surface. Using PEGs of molecular weight 1,500 g/mol or higher, protein adsorption is reduced to a few percent of the amount adsorbed at an untreated surface. The block copolymer adsorption route is less effective, mainly due to protein-induced desorption of the hydrophilizing agent. Bacterial adherence is also minimal when the PEI-PEG route is used. Branched PEGs are slightly less effective than linear PEGs of the same molecular weight. The difference in performance between linear and branched PEGs is discussed in terms of difference in entropy change when the hydrophilic surface-bound layer is compressed by an approaching protein. Branched PEGs, having smaller exclusion volumes and less freedom of motion, will lose less entropy on compression. The effects excerted on protein adsorption by PEG attached to a surface parallel its effect on particle mobility in electrophoresis. Similar molecular properties seem to be responsible for both protein and bacteria rejection and reduction of electrokinetic effects.
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| 9. |
- Rowe, K. G., et al.
(författare)
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Lessons from the lollipop : Biotribology, tribocorrosion, and irregular surfaces
- 2014
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Ingår i: Tribology letters. - : Springer New York LLC. - 1023-8883 .- 1573-2711. ; 56:2, s. 273-280
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Tidskriftsartikel (refereegranskat)abstract
- Biotribology and tribocorrosion are often not included in numerical or computational modeling efforts to predict wear because of the apparent complexity in the geometry, the variability in removal rates, and the challenge associated with mixing time-dependent removal processes such as corrosion with cyclic material removal from wear. The lollipop is an accessible bio-tribocorrosion problem that is well known but underexplored scientifically as a tribocorrosion process. Stress-assisted dissolution was found to be the dominant tribocorrosion process driving material removal in this system. A model of material removal was described and approached by lumping the intrinsically time-dependent process with a mechanically driven process into a single cyclic volumetric material removal rate. This required the collection of self-reported wear data from 58 participants that were used in conjunction with statistical analysis of actual lollipop cross-sectional information. Thousands of repeated numerical simulations of material removal and shape evolution were conducted using a simple Monte Carlo process that varied the input parameters and geometries to match the measured variability. The resulting computations were analyzed to calculate both the average number of licks required to reach the Tootsie Roll® center of a Tootsie Roll® pop, as well as the expected variation thereof.
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| 10. |
- Sidebottom, M. A., et al.
(författare)
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Ultralow wear Perfluoroalkoxy (PFA) and alumina composites
- 2016
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Ingår i: Wear. - : Elsevier Ltd. - 0043-1648 .- 1873-2577. ; 362-363, s. 179-185
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Tidskriftsartikel (refereegranskat)abstract
- Fluoropolymers have unique mechanical, chemical, and tribological properties (low friction coefficients) but their use as solid lubricants is inhibited by high wear rates (1-5×10-4 mm3/Nm). The addition of certain types of α-alumina has been shown to reduce the wear rate of PTFE by over three orders of magnitude, but due to its extremely high molecular weight PTFE cannot be screw injection molded. However, PFA, a perfluorinated copolymer of tetrafluoroethylene (TFE) and a perfluorinated alkylvinyl ether (PAVE), can be. Teflon® PFA 340 samples with various weight fractions of α-alumina (0%, 5%, 7.5%, 10%) were injection molded, and samples from each mold were wear tested against stainless steel (P=6.3 MPa, v=50.8 mm/s). Experiments showed that the friction behavior of the PFA 340-α alumina composite was very close to that of both unfilled PFA 340 and PTFE-α alumina composites. The wear rate of unfilled PFA 340 was 1.4×10-4 mm3/Nm, and dropped to 4.0×10-8 mm3/Nm for the PFA-α alumina composites. Just as in the case of PTFE-α alumina composites, these PFA composites generated brown-colored tribofilms on both the polymer and metal surfaces, which were indicative of tribochemical changes. ATR-IR and FTIR spectra of each surface showed evidence for the generation of perfluorinated carboxylate salts and waters of hydration. This spectral similarity between PTFE and PFA 340 samples shows that the same tribological mechanism found in PTFE-α alumina composites is responsible for ultralow wear in PFA-α alumina composites as well.
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