The nature of solid–liquid interfaces is of great significance in lubrication. Remarkable advances have been made in lubrication based on hydration effects. However, a detailed molecular-level understanding is still lacking. Here, we investigated water molecule behaviors at the TiO₂–aqueous interfaces by the sum-frequency generation vibrational spectroscopy (SFG-VS) and atomic force microscope (AFM) to elucidate the fundamental role of solid–liquid interfaces in lubrication. Combined contributions of water structures and hydration effects were revealed, where water structures played the dominant role in lubrication for TiO₂ surfaces of varying hydrophilicity, while hydration effects dominated with the increasing of ion concentrations. Superior lubrication is observed on the initial TiO₂ surfaces with strongly H-bonded water molecules compared to the hydrophilic TiO₂ surfaces with more disordered water. The stable ordered water arrangement with strong hydrogen bonds and the shear plane occurring between the ordered water layer and subsequent water layer may play a significant role in achieving lower friction. More adsorbed hydrated molecules with the increasing ionic concentration perturb ordered water but lead to the enhancement of hydration effects, which is the main reason for the improved lubrication for both TiO₂. This work provides more insights into the detailed molecular-level understanding of the mechanism of hydration lubrication.

Superlubricity control is of great interest in both industry and scientific research, and several methods have been proposed to achieve this goal. In this work, ultraviolet (UV) light was introduced into titanium dioxide (TiO₂) and silicon nitride (Si₃N₄) tribosystems to accomplish photoinduced superlubricity. The friction coefficients (COFs) between Si₃N₄ balls and TiO₂ plates in the mixtures of sulfuric acid (H₂SO₄) solution and glycerol solution were obviously reduced, and the system entered the superlubricity region (COF < 0.01) after UV illumination at a speed of 56 mm/s. However, the COF was much larger without UV treatment than that with UV treatment. The formation of silica (SiO₂) layers on the surfaces of Si₃N₄ balls and the elastohydrodynamic effects were determined to be fundamental to the low friction in this experiment, and the enhancement of the combination between the TiO₂ surface and the hydroxy group of glycerol by UV illumination was the key to the photoinduced superlubricity in this system. These findings showed one method for achieving superlubricity by introducing a light field that could be further applied to special working conditions.

Marine propellers are important propulsion devices for both surface ships and underwater vehicles. Increasingly severe environmental problems have required further performance enhancement for propellers. Nowadays, traditional methods to improve propeller performances through geometrical and structural optimizations have been extensively investigated, while the underlying mechanisms of the effects of surface and interface properties on marine propellers are still far from being fully understood. This paper presented a comprehensive review of recent advances in the effects of surface and interface properties, such as surface roughness and surface wettability, on marine propellers with an emphasis on the significant improvements in both hydrodynamic and cavitation performances, hoping to arouse more in-depth investigations in the field of surface/interface science and technologies on marine propellers, and also promote the state-of-the-art technologies, such as superlubricity technology, into practical applications.

In water-based boundary lubrication regime, the contact gaps (or boundary lubricated film thickness) and surface pressure distribution must be determined to really understand the boundary lubricated contact mechanism. However, the accurate determination of these parameters is limited. In this study, a mechanical model based on boundary lubricated contact involving surface force effects is developed. The surface force distribution characteristics, normal force vs. central film thickness curve, and macroscale water-based boundary lubricated contact are investigated numerically. The results show that hydration directly affects surface force interaction. The accurate boundary lubricated film thickness and surface pressure distribution can be obtained using this model in point contact. Furthermore, the mechanism of macroscale water-based liquid boundary lubricated contact is investigated, in which a water-based boundary lubricated film is formed under appropriate operating conditions based on surface force effects during running-in. This study can reveal the water-base boundary lubricated contact behavior and the carrying capacity of the surface force effect, and provides important design guidance for the surface force effect to achieve liquid superlubricity in water-based boundary lubricated contacts.

Friction drag is a nonnegligible matter when relative motion happens between solid and liquid phase, which brings many inconveniences in ship navigation, fluid transportation, microfluid devices, etc. Thereby various methods have been developed focusing on friction drag reduction. In this article, a review of several widely studied drag reduction methods is given, specially, their advantages and limitations in practical applications are discussed. Besides, a comparison of different methods is made and the development prospect of drag reduction is concluded.

Fluid viscosity is ubiquitous property and is of practical importance in intelligent fluids, industrial lubrication, and pipeline fluid transportation. Recently, there has been a surging interest in viscosity regulation. Here, we have developed a group of photorheological fluids by utilizing azobenzene polymers with a light-induced microstructure transformation. In this work, a photosensitive polymer with 4,4'-bis-hydroxyazobenzene as the main chain was designed and synthesized as a pivotal functional material. The sufficiently large structural difference under ultraviolet and near-infrared light makes it possible to regulate the viscosity of a polyethylene glycol solution. The viscosity of the photosensitive rheological fluids under ultraviolet light radiation is found to be up to 45.1% higher than that under near-infrared light radiation. To explore this intelligent lubricating technology, the friction regulation of ceramic sliding bearings was investigated utilizing photosensitive rheological fluids. Reversible friction regulation with a ratio of up to 3.77 has been achieved by the alternative irradiation of near-infrared and ultraviolet light, which can be attributed to the differences in mechanical properties and molecular structures under ultraviolet and near-infrared light according to both simulations and experiments. Such photorheological fluids will have promising applications in controllable lubrication, intelligent rheological fluids, and photosensitive dampers.

Sum-frequency generation (SFG) vibrational spectroscopy is a second-order nonlinear optical spectroscopy technique. Owing to its interfacial selectivity, SFG vibrational spectroscopy can provide interfacial molecular information, such as molecular orientations and order, which can be obtained directly, or molecular density, which can be acquired indirectly. Interfacial molecular behaviors are considered the basic factors for determining the tribological properties of surfaces. Therefore, owing to its ability to detect the molecular behavior in buried interfaces in situ and in real time, SFG vibrational spectroscopy has become one of the most appealing technologies for characterizing mechanisms at friction interfaces. This paper briefly introduces the development of SFG vibrational spectroscopy and the essential theoretical background, focusing on its application in friction and lubrication interfaces, including film-based, complex oil-based, and water-based lubricating systems. Real-time detection using SFG promotes the nondestructive investigation of molecular structures of friction interfaces in situ with submonolayer interface sensitivity, enabling the investigation of friction mechanisms. This review provides guidance on using SFG to conduct friction analysis, thereby widening the applicability of SFG vibrational spectroscopy.