Astringency is one of the most complex oral sensations. This dry, puckering mouthfeel occurs when consuming wine, tea, or other foods containing polyphenols. The exact mechanism behind this dry ...mouthfeel is not completely understood. Here, we describe a systematic tribological approach to measure model and real saliva to understand the specific role of the salivary proteins (mucins and proline-rich proteins (PRPs)) on lubrication-based astringency. Our approach reveals that there are two routes towards lubrication losses, partly involving irreversible molecular mechanisms for which the order of reactivity matters. For human saliva, we find two lubrication mechanisms: (I) Using phenolic compounds, we find aggregation-induced lubrication losses due to hydrogen bond formation, which depend critically on phenol size: large polyphenols allow for aggregation-induced lubrication losses, but small phenols do not. (II) For metal salts combined with saliva, we observe aggregation without lubrication losses as a result of electrostatic interactions. We find that lubrication losses are caused by the specific removal of the salivary PRP layer, whereas mucin aggregation in the presence of PRPs does not lead to lubrication losses. Additionally, we show that the addition of solvents that are able to reduce protein-polyphenol hydrogen bonding (e.g. ethanol) can prevent lubrication losses. Lubrication losses can also be compensated by the addition of highly viscous fluids (glycerol) that can provide viscous lubrication.
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•Large polyphenols aggregate PRPs leading to salivary lubrication losses related to astringency perception.•Mucin aggregation does not cause salivary lubrication losses when PRPs remain unaffected.•Salivary lubrication losses can be prevented by the addition of hydrogen-bond donors or highly viscous fluids.
The present study investigates the tribological properties of nanolubricants dispersed with dodecylamine functionalized graphene (DAG) in commercial engine oil. Standard techniques were used to ...determine the morphology of the DAG and the worn surfaces. The tribological properties of the nanolubricants were measured using UMT-2 tribotester with an in-house attachment developed for measuring the friction of liquid lubricants. The nanolubricants were observed to reduce the coefficient of friction (COF) by a maximum ∼40% in comparison to base engine oil. Parameters including the concentration, load and sliding velocity were found to influence the variation of COF considerably. Characterization of the wear tracks by EDX and XPS suggests the formation of a tribo-film as the plausible mechanism which lowers the COF in case of the nanolubricants.
•Tribological properties of engine oil based nanolubricants (NLB) is investigated.•Dodecylamine (DAG) functionalized graphene are utilized as the dispersant in NLB.•Using DAG-NLB the COF of steel-steel sliding contact reduces in the range 20–40%.•Concentration of DAG, sliding velocity and load strongly affect the COF of NLB.•The reduction in COF is governed by the formation of plausible tribo-film mechanism.
As a new type of carbon material, carbon dots (CDs) with exceptional properties are regarded as a type of potential water-based lubricant additive. In this study, it is first demonstrated that CDs ...perform excellently when used as lubricant additives for water-lubricated amorphous carbon (a-C) contacts. The results show that the introduction of CDs into water at a concentration of 0.1 wt% could reduce friction and wear in a-C contacts by 33% and 80%, respectively. Additionally, the lubricating performance of CDs is highly sensitive to its adding concentration. The excellent tribological performance of CDs results from the minimized CDs sediment particles (deposited in situ on worn a-C surfaces) and the reduced friction and wear via nano-bearing and nano-filling mechanisms. With high CDs concentrations, however, micron and submicron-sized sediment particles form when friction occurs and cause undesirable abrasive wear. For water-lubricated a-C contacts, therefore, the CDs concentration is the crucial determinant of lubrication performance that has to be appropriately controlled.
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Aluminum alloy (AA6061)-based hybrid metal matrix composites (HMMCs) are manufactured using a dual stir casting method, with varying volume percentages of Bsub.4C (5%, 10%, and 15%) and Gr (10%, 15%, ...and 20%) incorporated. The resulting HMMC and reinforcement elements are uniformly dispersed within the main matrix, forming a mechanically mixed layer with interfacial reactions. This layer reduces wear loss and friction coefficient compared to AA6061, especially with higher amounts of Bsub.4C and Gr, as they demonstrate little aggregation of reinforced material. The presence of Gr particles enabled the impact of different wear parameters (applied load, sliding speed, and distance) to be combined. Micro-hardness studies demonstrate that the hardness of HMMC increases as the volume fraction of reinforced particles and sliding distance increase. The compression test revealed a 22% improvement over AA6061. As a result, adding reinforcing materials to the matrix contributes to inducing greater strength by increasing wear resistance with a Gr-imparted lubrication effect.
Ferrochromium, an important emerging abrasion component, is added to Cu metal matrix composites (Cu-MMCs) to enhance the high-energy braking properties of such composites. This study examined the ...microstructures, interfacial characteristics, and micro-tribology behaviors of two kinds of ferrochromium, and their effects on the braking performance of Cu-MMCs. The high-carbon ferrochromium (HCF) with higher hardness, consisting of (Cr,Fe)7C3 and (Cr,Fe)23C6-CrFe phases, formed a poor mechanical-diffusion mixed interface with the matrix and exhibited a low coefficient of friction (COF) in micro-friction tests. By cracking or crushing of HCFs, as well as third-body wear debris formed by HCFs, the COF in macro-friction tests was increased, helping Cu-MMCs maintain their braking performance at medium and high breaking energy densities (BEDs). The extra low-carbon ferrochromium (ELCF), consisting of a CrFe phase, generated a diffusion interface with the matrix, exhibited a greater COF, promoted the formation of a tribo-layer at high BED, and provided better wear resistance for Cu-MMCs. The wear mechanisms changed from plowing to severe plastic deformation-induced delamination and oxidation-induced delamination for Cu-MMCs with ELCF and HCF, respectively.
This study presents a surface, referred as ‘pocket-textured surface’, which combines traditional surface texture with the oil storage pocket that can improve the tribological performance of line ...contact under extremely starved lubrication conditions. The pocket-textured surface has up to 48% lower friction coefficient than the untextured surface with the same little lubricant supply. Tribological experiments were carried out to explore the tribological behavior of pocket-textured surfaces with different ratios of texture hole depth and oil pocket depth under different loads and sliding speeds. In addition, a high-speed camera observed the lubricant inflow and outflow, and Ansys simulation calculated the deformations and pressures of the pocket-textured surface.
•A textured surface with oil pocket was designed and fabricated.•The textured surface showed superior tribological behaviors under starved lubrication.•The textured surface could pre-wet contact area and amplify the effects of oil storage.
Commercial pure titanium (Cp-Ti) is widely used in many industrial and biomedical fields. However, depending on the conditions of use of Cp-Ti, it exhibits low wear and corrosion resistance, thus ...reducing its service life. Therefore, in this study, the cuticle of the Japanese jewel beetle (Chrysochroa rajah) was morphologically examined and duplex coatings were produced by dip coating technique on Cp-Ti to provide protection against wear and corrosion by imitating the layered structure of the Chrysochroa rajah cuticle. Duplex coated surfaces were obtained by dip coating technique with chitin as the first layer and TiO2, SiC and h-BN layers as the second layer, respectively. Electrochemical corrosion properties were investigated under Simulated Body Fluid (SBF) solution. In electrochemical corrosion tests, open circuit potential (OCP), electrochemical polarization and electrochemical impedance spectroscopy (EIS) analysis were performed. Wear tests were performed in dry and SBF solution, and tribocorrosion tests were performed in SBF solution. The chitin + h-BN duplex coated surface has the highest corrosion potential (Ecorr) of -14 mV and the lowest corrosion current density (icorr) of 1.060 × 10−9 A cm−2. In addition, chitin + h-BN duplex coated surface has the highest corrosion resistance among duplex coated surfaces with corrosion rate of 4.601 × 10−4 mpy. In the wear test results, the lowest wear rate was 0.12 ± 0.011 × 10−3 mm3/Nm in dry environment and 0.28 ± 0.017 × 10−3 mm3/Nm in SBF solution on chitin + TiO2 duplex coated surface. In addition, the highest surface hardness was obtained as 4 ± 0.28 GPa on the chitin + TiO2 duplex coated surface. As a result of the tribocorrosion tests performed in SBF solution, the highest resistance was obtained on the chitin + TiO2 duplex coated surface. The chitin layer exhibited low corrosion, wear and tribocorrosion performance. Due to the effect of different electrical charge transfer and different hardness of organic and ceramic powders used in coatings, the coated surfaces improved the electrochemical, abrasion and tribocorrosion properties of Cp-Ti.
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•Duplex coatings were obtained by imitating the morphological structure of the Japanese jewel beetle (Chrysochroa rajah).•The electrochemical and tribological behaviors of the obtained biomimetic surfaces were investigated.•Electrochemical and tribological properties of the coatings are associated with the ionization energies and hardness values of each layer.
Twenty-three commercial samples of facial moisturizers were tested using rheological and tribological measurements and further compared to sensorial attributes as rated by descriptive panels. The ...correlation between panel ratings and mechanical properties of these products were studied and reported herein. As the panel evaluation proceeds from initial appearance, pick-up and rub-out to residue attributes, the sample thickness decreases from the centimeter scale to the micrometer scale. It was found that with this decrease, the dominating mechanical properties shift from rheological to tribological. Appearance and pick-up attributes seem to be governed by bulk rheology, while rub-out attributes are in a transient regime where both rheology and tribology are relevant. Besides, residue attributes are actually represented by the tribology of films. It is interpreted as evidence that the relevant mechanical properties pertain to film thickness. This conclusion is confirmed by stepwise multivariate linear regression analysis, which allows for the top two predictors to emerge: instantaneous viscosity maximum from rheology and coefficient of friction at a slow sliding speed from tribology.
•Sensory attributes that are related to bulk properties are correlated with rheology.•Sensory attributes that are related to thin film properties are correlated with tribology.•A governing mechanical property shifts from rheology to tribology as the sample length scale decreases.
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