Friction Reduction Achieved by Ultraviolet Illumination on TiO2 Surface.

Controlling friction by light field is a low-cost, low-energy, non-polluting method. By applying ultraviolet light on the surface of photosensitive materials, the properties of the friction pairs or lubricant can be influenced, thus achieving the purpose of reducing friction. In this study, TiO2, an inorganic photosensitive material, was selected to investigate the modulating effect of light fields on friction lubrication when using polyalphaolefin (PAO) base oil as a lubricant, and the modulation law of light fields on the friction lubrication behavior was investigated under different loads (1-8 N), different speeds (20-380 mm/s), and different viscosities (10.1-108.6 mPa·s) of PAO base oil. The experimental results showed that light treatment could reduce the friction coefficient of PAO4 base oil lubrication from 0.034 to 0.016, with a reduction of 52.9% under conditions of 3 N-load and 56.5 mm/s-speed, and the best regulation effect could be achieved under the mixed lubrication condition. After TiO2 was treated with ultraviolet light, due to its photocatalytic property, PAO molecules were oxidized and adsorbed on the TiO2 surface to form an adsorption layer, which avoided the direct contact of rough peaks and thus reduced the friction coefficient. This study combines photosensitivity, photocatalysis, and friction, presenting a method to reduce the friction coefficient by applying a light field without changing the friction pairs or lubricants, which provides a new direction for friction modulation and gives new ideas for practical applications.

Hydration layer structure modulates superlubrication by trivalent La3+ electrolytes.

Water-based lubricants provide lubrication of rubbing surfaces in many technical, biological, and physiological applications. The structure of hydrated ion layers adsorbed on solid surfaces that determine the lubricating properties of aqueous lubricants is thought to be invariable in hydration lubrication. However, we prove that the ion surface coverage dictates the roughness of the hydration layer and its lubricating properties, especially under subnanometer confinement. We characterize different hydration layer structures on surfaces lubricated by aqueous trivalent electrolytes. Two superlubrication regimes are observed with friction coefficients of 10-4 and 10-3, depending on the structure and thickness of the hydration layer. Each regime exhibits a distinct energy dissipation pathway and a different dependence to the hydration layer structure. Our analysis supports the idea of an intimate relationship between the dynamic structure of a boundary lubricant film and its tribological properties and offers a framework to study such relationship at the molecular level.

Significantly Affected Lubrication Behavior of Silicone Oil Lubricated Si3N4/Glass Contact after Cleaning with Different Solvents.

Conventional methyl silicone oils have poor lubricating properties in boundary lubrication regions, particularly for ceramic/oxide point contact lubrication. In this study, the residues of various organic solvents on the surfaces of Si3N4 spheres/glass disks were used to determine their effect on the lubricating properties of silicone oil 200. The minute ethanol residues significantly enhanced the antifriction and antiwear properties of silicone oil. Compared to the blank sample, the coefficient of friction (COF) and wear volume of silicone oil 200 with the residual ethanol friction pair were reduced by >40% and >98%, respectively. Being immiscible with silicone oil, the minute ethanol residues also removed impurities from the glass surface and maintained a clean interface, thus effectively blocking direct interactions between the friction pair interfaces. In addition, the residual ethanol reduced the atomic force microscope probe-to-glass surface adhesive force in the silicone oil 200 environment, thus allowing it to maintain low COF and wear rates over a broader range of speeds, loads, and times. In contrast to previous work, this study is the first to effectively regulate the lubrication properties of silicone oil using a residual organic solvent. The findings further verified that the adsorption of vapor molecules can significantly alter the surface forces between interfaces. Thus, adjusting the adhesion force through trace amounts of organic solvent residues may provide novel research inputs, thereby guiding the expansion and scope of silicone oil lubrication applications.

Enhanced dewaterability of sludge by Fe(II)-sludge biochar activate persulfate.

Sludge biochar supported Fe(II) (Fe(II)-SBC) was successfully prepared using waste activated sludge as peroxydisulfate (PDS) activator to condition sludge for deep dewatering. The experimental results showed that Fe(II)-SBC with FeO on it could effectively active PDS to produce SO4-⋅ and HO⋅. The radicals could destroy the structure of sludge cells and extracellular polymeric substance (EPS), transformed the hydrophilic and tightly bound EPS into soluble-EPS, degrade partial proteins and polysaccharides and released bound water. The negatively charged groups on sludge floc were dripped off by SO4-⋅/HO⋅ or neutralized with Fe2+, Fe3+, H+, or Fe(II)-SBC, leading to an increase in zeta potential to -2.24 mV and sludge destabilization. The residual Fe(II)-SBC served as a skeleton builder that decreased the compression coefficient of the sludge cake to 0.75. Under the combined functions, the CST and SRF were reduced by 70% and 82.7%, respectively, and Wc was reduced to 72.4%. The byproducts of Fe3+ and SO42- finally remained in sludge cake in the form of NaFeSi2O6 and CaSO4.

High-Temperature Superlubricity Realized with Chlorinated-Phenyl and Methyl-Terminated Silicone Oil and Hydrogen-Ion Running-in.

Ceramic friction pairs lubricated with chlorinated-phenyl and methyl-terminated silicone oil (CPSO) systems have potential applications in the aerospace industry. In this study, the effects of the running-in process and temperature on the lubricating performance of CPSO were investigated. The superlubricity of Si3N4/sapphire lubricated with CPSO was realized at >190 °C after H+-ion running-in. The mechanism of this high-temperature superlubricity was investigated by determining the stable adsorption configurations and adsorption energies of CPSO on different surfaces using density functional theory calculations. Compared with that on the Si3N4 surface, the adsorption capacity of CPSO on the hydroxylated SiO2 surface generated by H+-ion running-in increased, whereas the steric hindrance decreased. The viscosity-temperature curve of CPSO was measured, wherein the viscosity and pressure-viscosity coefficient of CPSO considerably decreased with increasing temperature, leading to high-temperature superlubricity in a wide speed/load range. This is the first paper to report oil-based superlubricity at temperatures of 190 °C, or even higher-temperature conditions. Furthermore, it provides guidance for the use of ceramic-CPSO systems in high-temperature conditions, including in the aerospace industry.

Homogeneous interfacial water structure favors realizing a low-friction coefficient state.

HYPOTHESIS: The use of water to reduce friction has always played a significant role in a wide range of areas ranging from biology to engineering. Many efforts have been made to extensively investigate the water behavior between two contacted surfaces, but its role in water-based friction remains incompletely understood. EXPERIMENTS: Herein, we utilize the sum-frequency generation (SFG) spectroscopy to identify interfacial water structures upon adjusting the wettability of titanium dioxide (TiO2) and silicon surfaces. And the corresponding wettability-tunable underwater friction is measured by atomic force microscopy (AFM). FINDINGS: It demonstrates that enhanced wettability induces higher friction on the TiO2 surface but lower friction on the silicon surface. Although the tribological properties of TiO2 show independence of surface forces in contrast to the case of silicon, both TiO2 and silicon surfaces covered with homogeneous water molecules correspond to a lower friction coefficient. This observation indicates that a homogeneous interfacial water structure, dominating over surface forces, is of the utmost importance for achieving low friction. Our results shed new light on the origins of friction in the presence of water and reveal the ubiquitous role of interfacial water structures on friction.

Imaging dynamic three-dimensional traction stresses.

Traction stress between contact objects is ubiquitous and crucial for various physical, biological, and engineering processes such as momentum transfer, tactile perception, and mechanical reliability. Newly developed techniques including electronic skin or traction force microscopy enable traction stress measurement. However, measuring the three-dimensional distribution during a dynamic process remains challenging. Here, we demonstrated a method based on stereo vision to measure three-dimensional traction stress with high spatial and temporal resolution. It showed the ability to image the two-stage adhesion failure of bionic microarrays and display the contribution of elastic resistance and adhesive traction to rolling friction at different contact regions. It also revealed the distributed sucking and sealing effect of the concavity pedal waves that propelled a snail crawling in the horizontal, vertical, and upside-down directions. We expected that the method would advance the understanding of various interfacial phenomena and greatly benefit related applications across physics, biology, and robotics.

Intermolecular hydrogen bond ruptured by graphite with different lamellar number.

Intermolecular hydrogen bonds are formed through the electrostatic attraction between the hydrogen nucleus on a strong polar bond and high electronegative atom with an unshared pair of electrons and a partial negative charge. It affects the physical and chemical properties of substances. Based on this, we presented a physical method to modulate intermolecular hydrogen bonds for not changing the physical-chemical properties of materials. The graphite and graphene are added into the glycerol, respectively, by being used as a viscosity reducer in this paper. The samples are characterized by Raman and 1H-nuclear magnetic resonance. Results show that intermolecular hydrogen bonds are adjusted by graphite or graphene. The rheology of glycerol is reduced to varying degrees. Transmission electron microscopes and computer simulation show that the spatial limiting action of graphite or graphene is the main cause of breaking the intermolecular hydrogen bond network structure. We hope this work reveals the potential interplay between nanomaterials and hydroxyl liquids, which will contribute to the field of solid-liquid coupling lubrication.

Light-Controlled Friction by Carboxylic Azobenzene Molecular Self-Assembly Layers.

Nowadays, reversible friction regulation has become the focus of scientists in terms of the flexible regulatory structure of photosensitive materials and theories since this facilitates rapid development in this field. Meanwhile, as an external stimulus, light possesses great potential and advantages in spatiotemporal control and remote triggering. In this work, we demonstrated two photo-isomerized organic molecular layers, tetra-carboxylic azobenzene (NN4A) and dicarboxylic azobenzene (NN2A), which were selected to construct template networks on the surface of the highly oriented pyrolytic graphite (HOPG) to study the friction properties, corresponding to the arrangement structure of self-assembled layers under light regulation. First of all, the morphology of the self-assembled layers were characterized by a scanning tunneling microscope (STM), then the nanotribological properties of the template networks were measured by atomic force microscope (AFM). Their friction coefficients are respectively changed by about 0.6 and 2.3 times under light control. The density functional theory (DFT) method was used to calculate the relationship between the force intensity and the friction characteristics of the self-assembled systems under light regulation. Herein, the use of external light stimulus plays a significant role in regulating the friction properties of the interface of the nanometer, hopefully serving as a fundamental basis for further light-controlling research for the future fabrication of advanced on-surface devices.

Surface wettability effect on aqueous lubrication: Van der Waals and hydration force competition induced adhesive friction.

HYPOTHESIS: Wettability effect has long been a concern in various aqueous lubrication systems including biological and industrial applications. The wettability may affect lubrication performance by changing interfacial viscosity or hydration force. The key point to reveal the mechanism is to design an ideal experimental system to exclude other bulk factors other than surface wettability. EXPERIMENTS: In this work, silicon surfaces with different treatments were used to study the single factor effect of wettability on aqueous lubrication. The normal and friction forces of these surfaces were quantified by atomic force microscopy (AFM) in water environment. The interfacial viscosity was evaluated according to the probe dynamic approaching process. Macroscale and microscale lubrication experiments of other materials were also conducted as verification and supplement. FINDINGS: A semi-quantitative relationship between friction and wettability was revealed and attributed to the competition between the attractive van der Waals interactions and wettability-dependent repulsive hydration interaction, which determined the strength of the adhesive interaction and dominated the sliding energy dissipation. The contribution of viscous effect of water was considered to be relatively minor. The findings provide an in-depth understanding of aqueous lubrication and outline important guidelines for tuning adhesion and friction.

Effects of Abrasive Particles on Liquid Superlubricity and Mechanisms for Their Removal.

Liquid superlubricity results in a near-frictionless lubrication state, which can greatly reduce friction and wear under aqueous conditions. However, during the running-in process, a large number of abrasive particles are generated, and because these may lead to a breakdown in superlubricity performance, they should be effectively removed. In this paper, the morphology, size, and composition of abrasive particles were verified using scanning electron microscopy with energy-dispersive X-ray spectroscopy, and their influence on liquid superlubricity was explored through friction tests. Subsequently, different solvents were used to remove the abrasive particles, and the optimal cleaning process was determined by macroscopic tribo-tests and microscopic analysis. Finally, droplet-spreading experiments and a force-curve analysis were carried out to understand the abrasive-particle removal mechanism by different solvents. We found that SiO2 was the main component in the abrasive particles, and micron-sized SiO2 particles resulted in random "wave peaks" in the coefficient of friction and, thus, the superlubricity. Absolute ethanol + ultrapure water was determined to be the optimal solvent for effectively removing abrasive particles from friction-pair surfaces and helped the lubricant in exhibiting an ultralow friction coefficient for long periods of time. We proposed a "wedge" and "wrap" model to explain the abrasive-particle removal mechanism of different solvents. The SiO2 removal mechanism outlined in this study can be applied under aqueous conditions to improve the stability and durability of liquid superlubricity in practical engineering applications.

Micro-Nano Hierarchical Structure Enhanced Strong Wet Friction Surface Inspired by Tree Frogs.

Superior wet attachment and friction performance without the need of special external or preloaded normal force, similar to the tree frog's toe pad, is highly essential for biomedical engineering, wearable flexible electronics, etc. Although various pillar surfaces are proposed to enhance wet adhesion or friction, their mechanisms remain on micropillar arrays to extrude interfacial liquid via an external force. Here, two-level micropillar arrays with nanocavities on top are discovered on the toe pads of a tree frog, and they exhibit strong boundary friction ≈20 times higher than dry and wet friction without the need of a special external or preloaded normal force. Microscale in situ observations show that the specific micro-nano hierarchical pillars in turn trigger three-level liquid adjusting phenomena, including two-level liquid self-splitting and liquid self-sucking effects. Under these effects, uniform nanometer-thick liquid bridges form spontaneously on all pillars to generate strong boundary friction, which can be ≈2 times higher than for single-level pillar surfaces and ≈3.5 times higher than for smooth surfaces. Finally, theoretical models of boundary friction in terms of self-splitting and self-sucking are built to reveal the importance of liquid behavior induced by micro-nano hierarchical structure.

Streptomyces lycii sp. nov., an endogenous actinomycete isolated from Lycium ruthenicum.

A novel endogenous actinobacteria strain, designated TRM 66187T, was isolated from Lycium ruthenicum sampled at Alar, Xinjiang, Northwest PR China, and characterized using a polyphasic taxonomic approach. Phylogenetic analysis based on 16S rRNA gene sequences affiliated strain TRM 66187T with the genus Streptomyces. The whole-cell sugar pattern of TRM 66187T consisted of galactose, glucose and ribose. The predominant menaquinones were MK-9(H4) and MK-9(H6). Major cellular fatty acids were iso-C14:0, iso-C15:0, anteiso-C15:0 and anteiso-C16:0. The detected polar lipids were diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylinositol, phosphatidylinositol mannoside and two unidentified polar lipids. The G+C content of strain TRM 66187T was 71.8 mol%. Results of phylogenetic analysis showed that strain TRM 66187T had 98.48% 16S rRNA gene sequence similarity to the closest described species Streptomyces qinglanensis DSM 42035T. The average nucleotide identity value between strain TRM 66187T and the closest related strain Streptomyces qinglanensis DSM 42035T was calculated to be 77.2%. The digital DNA-DNA hybridization value between them was 22.4%. Multilocus sequence analyses based on five housekeeping genes (atpD, gyrB, recA, rpoB and trpB) also indicated that strain TRM 66187T should be assigned to the genus Streptomyces. On the basis of evidence from this polyphasic study, strain TRM 66187T should be designated as representing a novel species of the genus Streptomyces, for which the name Streptomyces lycii sp. nov. is proposed. The type strain is TRM 66187T (=LMG 31493T=CCTCC AA 2018094T).

Streptomyces apocyni sp. nov., an endogenous actinomycete isolated from Apocynum venetum.

A novel actinomycete, designated strain TRM 66233T, was isolated from Apocynum venetum L. collected from the Xinjiang Uygur Autonomous Region of China and characterized using a polyphasic taxonomic approach. Phylogenetic analysis based on 16S rRNA gene sequences affiliated strain TRM 66233T with the genus Streptomyces. Strain TRM 66233T showed a high similarity value to Streptomyces bikiniensis NRRL B-1049T (98.07 %) based on the 16S rRNA gene phylogenetic tree. The whole-cell sugar pattern of TRM 66233T consisted of glucose, galactose, mannose and ribose. The predominant menaquinones were MK-9(H2), MK-9(H6), MK-9(H8) and MK-9(H10). The polar lipids were diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylinositol and four unidentified lipids. The major fatty acids were iso-C15 : 0, anteiso-C15 : 0, iso-C16 : 0, C16 : 0 and iso-C17 : 0. The G+C content of the DNA was 70.35 mol%. The DNA-DNA relatedness and average nucleotide identity values as well as evolutionary distances based on multilocus (atpD, gyrB, recA, rpoB and trpB) sequences between strain TRM 66233T and closely related type strains were significantly lower than the recommended threshold values. The whole-genome average nucleotide identity and digital DNA-DNA hybridization values between strain TRM 66233T and S. bikiniensis NRRL B-1049T were 78.86 and 23.2 %, respectively. On the basis of evidence from this polyphasic study, strain TRM 66233T should represent a novel species of the genus Streptomyces, for which the name Streptomyces apocyni sp. nov. is proposed. The type strain is TRM 66233T (=CCTCC AA 2019056T=LMG 31559T).

Negative friction coefficient in microscale graphite/mica layered heterojunctions.

The friction of a solid contact typically shows a positive dependence on normal load according to classic friction laws. A few exceptions were recently observed for nanoscale single-asperity contacts. Here, we report the experimental observation of negative friction coefficient in microscale monocrystalline heterojunctions at different temperatures. The results for the interface between graphite and muscovite mica heterojunction demonstrate a robust negative friction coefficient both in loading and unloading processes. Molecular dynamics simulations reveal that the underlying mechanism is a synergetic and nontrivial redistribution of water molecules at the interface, leading to larger density and more ordered structure of the confined subnanometer-thick water film. Our results are expected to be applicable to other hydrophilic van der Waals heterojunctions.

Macroscale Light-Controlled Lubrication Enabled by Introducing Diarylethene Molecules in a Nanoconfinement.

Reversible friction regulation is of long-standing great interest in the fields of both industry and scientific research, so some materials and theories have been developed aiming to solve this problem. Light-sensitive materials are promising because of the easy controllable switching of the properties and structures. Here, a reversible light-controlled macrolubrication was realized by regulating the performance of nanoscale light-sensitive molecules adsorbed on contact surfaces. In this work, symmetric diarylethene and asymmetric diarylethene had been designed and synthesized as functional materials. The friction forces were found to be obviously increased upon exposure to ultraviolet light and decayed to the initial value under visible light. In addition, the friction coefficient changed alternately with ultraviolet and visible illumination. According to the results of experiments and simulation of material properties, the behavior was suggested to be attributed to the difference in shear stiffness of the nanoscale diarylethene molecule adsorption layer triggered by two wavelength lights. This work not only provides a new lubrication regulation technology but also develops intelligent engineering materials.

Extreme-Pressure Superlubricity of Polymer Solution Enhanced with Hydrated Salt Ions.

The development of new routes or materials to realize superlubricity under high contact pressure can result in energy-saving and reduction of emissions. In this study, superlubricity (µ = 0.0017) under extreme pressure (717 MPa, more than twice the previously reported liquid superlubricity) between the frictional pair of Si3N4/sapphire was achieved by prerunning-in with a H3PO4 (HP) solution followed by lubrication with an aqueous solution consisting of poly(vinyl alcohol) (PVA) and sodium chloride (NaCl). Under the same test condition, the aqueous PVA lubricant did not show superlubricity. Results of X-ray photoelectron spectroscopy and Raman spectroscopy indicate the formation of a PVA-adsorbed film at the frictional interface after lubrication with PVA but not after lubrication with PVA/NaCl, indicating competitive adsorption between hydrated Na+ ions and PVA molecules. The hydrated Na+ ions adsorbed preferentially to the solid surfaces, causing the transformation of the shear interface from a polymer film/polymer film to a solid/polymer film. Meanwhile, the hydrated Na+ ions also produced hydration repulsion force and induced low shear stress between the solid surfaces. Furthermore, NaCl increased the viscosity of the polymer lubricant, enhanced the hydrodynamic effect between interfaces, and decreased direct contact between the friction pair, causing a further reduction in friction. Thus, the superlubricity of the PVA/NaCl mixture is attributed to the combination of hydration and hydrodynamic effects. This study provides a novel route and mechanism for achieving extreme-pressure superlubricity at the macroscale, through the synergistic lubricating effect of hydrated ions and a polymer solution, propelling the industrial application of superlubricity.

Tribo-Induced Near-Infrared Light Emission between Metal and Quartz.

Triboluminescence (TL) refers to the luminescence phenomenon at the material surface under the action of pressure or shear. This fascinating phenomenon can directly convert mechanical energy into light emission without the need for other auxiliary components; therefore, it attracts more and more researchers to conduct research in different wavelength ranges, such as X-ray, ultraviolet, visible light, and terahertz. However, there have been few reports on the study of the near-infrared (NIR) range, which is very important in the integrity of the triboluminescence research. In this research, we found that NIR light with a wavelength ranging from 800 to 1000 nm was generated by friction between solid metals and a quartz crystal. Analysis of the cross section of the quartz disk after friction revealed that the TL phenomenon had a strong relationship with the doping of metal grains into the silica. Density functional theory (DFT) and X-ray photoelectron spectroscopy were also conducted to further identify the results. We infer that such light emission arises from the implantation of metal grains into the surface of the quartz, which forms a metal-insulator junction with amorphous silica. Moreover, electron transition between the metal and the insulator, followed by a transition at the center of the defects, causes near-infrared light emission. Our research reveals the infrared luminescence behavior from a different perspective, the transfer of materials, and perhaps deepens the understanding of the near-infrared emission mechanism.

On Lubrication States after a Running-In Process in Aqueous Lubrication.

Recently, many studies have reported the ultralow friction coefficient of sliding friction between rigid solid surfaces in aqueous lubrication. A running-in process that goes through high-friction and friction-decreasing regions to a stable ultralow friction region is often required. However, the role of the friction-decreasing region is often ascribed to tribofilm formation in which complexity hindered the quantitative description of the running-in process and the prediction of its subsequent lubrication state. In this work, the frictional energy (Ef) dissipated in the running-in process of a poly(oligo(ethylene glycol) methyl ether acrylate) aqueous lubrication was related to the wear of solid surfaces under different conditions and lubrication states. Experimental results indicated that the high-friction region was in a boundary lubrication state, contributed to most of the wear, and significantly reduced the contact pressure, whereas the friction-decreasing region was in a mixed lubrication state, contributed only to the slight and slow removal of materials, and slightly reduced the contact pressure. Therefore, by establishing relationships among the wear scar diameter, Ef, and the Stribeck curve of the tribological system, the subsequent lubrication state after a running-in process under various working loads and sliding speeds could be quantitatively predicted. The running-in experiments with different aqueous lubrication systems showed good agreement with the prediction of this method. This investigation provides an effective method for the wear and lubrication state prediction after a running-in process, further proving the importance of the Stribeck curve for a lubrication system. This study may also have important implications for the strategy design of the running-in process in various industrial applications.

Big data analysis for evaluating bioinvasion risk.

BACKGROUND: Global maritime trade plays an important role in the modern transportation industry. It brings significant economic profit along with bioinvasion risk. Species translocate and establish in a non-native area through ballast water and biofouling. Aiming at aquatic bioinvasion issue, people proposed various suggestions for bioinvasion management. Nonetheless, these suggestions only focus on the chance of a port been affected but ignore the port's ability to further spread the invaded species. RESULTS: To tackle the issues of the existing work, we propose a biosecurity triggering mechanism, where the bioinvasion risk of a port is estimated according to both the invaded risk of a port and its power of being a stepping-stone. To compute the invaded risk, we utilize the automatic identification system data, the ballast water data and marine environmental data. According to the invaded risk of ports, we construct a species invasion network (SIN). The incoming bioinvasion risk is derived from invaded risk data while the invasion risk spreading capability of each port is evaluated by s-core decomposition of SIN. CONCLUSIONS: We illustrate 100 ports in the world that have the highest bioinvasion risk when the invaded risk and stepping-stone bioinvasion risk are equally treated. There are two bioinvasion risk intensive regions, namely the Western Europe (including the Western European margin and the Mediterranean) and the Asia-Pacific, which are just the region with a high growth rate of non-indigenous species and the area that has been identified as a source for many of non-indigenous species discovered elsewhere (especially the Asian clam, which is assumed to be the most invasive species worldwide).