The behavior of water droplets sliding down an inclined surface has significant implications in both daily life and various industrial processes. While hydrophobicity and the sliding behavior of a water drop are crucial phenomena, the distinction between them remains unclear. As water droplets slide down a slope, they experience both static and dynamic friction forces. This article compiles and reviews various studies on the behavior of water droplets on inclined surfaces, with a particular emphasis on the frictional interactions between the droplets and the surfaces. The relationship between the initial states and shapes of sliding water droplets and the fine irregularities of the inclined surfaces is explored, including how these irregularities influence the droplet’s sliding angle. Further, the characteristics of droplets in a stability state of sliding are examined, focusing on their terminal velocities and their correlation with dynamic friction forces. The main objective of this article is to provide a novel understanding of the behavior and potential control mechanisms of droplets sliding down slopes.

Liquid transfer caused by the breakage of a liquid meniscus bridge is introduced as an example of phenomena related to surface tension/surface force. Calculational results obtained from a simple model of the breakage of a liquid meniscus bridge agree fairly well with experimental results. The surface roughness, however, significantly affects the liquid transfer between two solid surfaces.

Liquids confined in nanometer-level thicknesses (nanoconfined liquids) are known to exhibit properties different from those of the bulk liquid, such as a significant increase in viscosity. Such properties should be important not only for micromachines such as MEMS/NEMS, but also for the boundary lubrication of general machines since the macroscopic rough surface can form nanometer level gap at the protruded region. Therefore, the understanding of the properties of nano-confined liquids at the molecular level has significant importance since it can contribute for designing lubricant with required properties under nano-confinement. Authors have been working on structural evaluation of nano-confined liquids using synchrotron radiation X-rays,and succeeded to obtain X-ray diffraction of nano-confined liquids after the much effort to reduce the background scattering. In this paper, we introduced the studies which afforded new insights on the structure and properties of nano-confined liquids based on the X-ray diffraction measurements.

A glass sphere is coated with self-assembled monolayer (SAM) to be a probe for surface force measurement, so that the surface energy of the probe is adjusted with keeping its surface morphologies. Some specific probes with different surface energy are useful to characterize surface force of sample. It is demonstrated that surface force between two samples is indirectly estimated by surface forces of each sample measured by two probes with different surface energy. The glass sphere of probe is also covered with protein molecules by contact printing, as it is previously coated with APTES-SAM as an intermediate layer. Surface force measured by the protein-coated probe allow us to quantitatively evaluate adhesiveness of protein to a sample surface.

The thin-film structure and tribological properties of the dilute solution of a water-soluble amphiphilic polymer, bis-isobutyl polyethylene glycol (PEG)-14/amodimethicone (BIPA) copolymer, confined between mica surfaces were investigated using the surface forces apparatus (SFA). The results showed that BIPA molecules in the solution formed an adsorbed layer on each mica surface and friction between the adsorbed layers was extremely low; friction coefficient of the order of 10^-5. This extremely low friction is originated from the fluid-like slippage of monomolecular layer of water confined between the adsorbed layers that constitute a flat and continuous slip plane. BIPA polymer was applied as an additive in hair cosmetics to improve the lubrication properties of human hair in wet condition. Damaged hair tends to tangle easily during shampooing, which is a common and stubborn hair trouble, mainly caused by the stiction between wet hair fibers. BIPA polymer completely suppressed stiction; the shampoo formulation including BIPA exhibited excellent detangling performance during washing.

Soft matter such as rubber is a common material for various products in our daily life to ensure the slip resistance and sealing performance. In addition, most of biological materials such as human skin also belong to soft matter. The friction behavior of soft matter is sensitive to the wettability, for example, slightly wet skin has higher friction than dry one. Such a high friction effect is achieved when there are both water and air between two substrates, which corresponds to nonuniform wettability. Nonuniform wettability can promote expansion of real contact between soft matter and a counterpart, resulting in increase of adhesion term of friction. It is theoretically explained that this high friction effect cannot be achieved for hard materials such as metals. As one of the technical attempts, high-friction rubber can be achieved by adding activated carbon which can supply air into the interface in water. In a process of designing products relating to soft matter contact/friction, it is desirable to ensure or avoid the high friction effect caused by the nonuniform wettability depending on purposes of usage.

Lubricating oils have been used in various machines and are essential for the normal operation of the machines. Still, they degrade with operation time, and various lubricating oil analyses are performed in the field. Notably, monitoring solid particles in lubricating oil is important because solid particles significantly impact the machine's life. Thus, analyses of solid particles in lubricating oil have been widely used in the field, whereas these analyses still leave room for improvement in terms of cost and convenience. Therefore, we developed a Lubricating oil Image Analysis (LIA), which uses the color information of the lubricating oil itself. This method makes it possible to indirectly monitor the residual rate of oxidation inhibitors by RGB values and is expected to be used in the field for lubricating oil analysis easily and at a low cost. In this study, we proposed on-site measurement technology for solid particles in lubricating oil by binarization as a new diagnostic technology for LIA and investigated its usefulness. Consequently, the binarized area ratio made it possible to diagnose the degree of contamination of lubricating oil. Additionally, the particle diameter distribution by binarization has high sensitivity over a relatively large particle diameter range. Furthermore, it was shown that particle measurement using LIA can quantitatively evaluate sludge in simulated oxidized oil with different degrees of oxidation.

Rolling contact fatigue, RCF, of steel under lubrication with alkyl diphenyl ether, ADE, and poly alpha olefin, PAO, in three environmental gas conditions was studied. RCF tests were conducted at 150°C with ball and disk configuration, and hydrogen concentration in steel, chemical changes of oils and rolling surfaces were analyzed after the tests. In order to confirm basic oil properties, such as viscosity-pressure coefficient of oil under contact condition, film thicknesses under pure rolling contact were also measured by interferometry method with using ball on disk type tribo tester. ADE had greater RCF life in nitrogen than PAO with the same viscosity. This was caused by ADE’s higher viscosity-pressure coefficient, carbon-rich surface film formation and higher chemical stability to decompose and to generate hydrogen on nascent steel surface. On the other hand, in air, ADE also showed greater RCF life than PAO. This was ascribed to higher anti-oxidation stability of ADE.