Polyether-etherketone (PEEK) is a corrosion-resistant material that has been widely used in aqueous lubrication. However, its anti-wear performance must be improved for its application in the industry. In this study, to improve the anti-wear performance of PEEK for aqueous boundary lubrication, PEEK/MoS₂ composites were prepared by ball-milling and spark plasma sintering processes. A competitive MoS₂ mechanism between the low shear strength property and the role of promoting wear debris generation influences the anti-wear performance of PEEK/MoS₂ composites. Experiments demonstrated that the coefficients of friction (COF) and wear rate of PEEK composite with 0.25 wt% MoS₂ were significantly reduced 68% and 94%, respectively. Furthermore, this was the first time that a PEEK composite could achieve a COF of less than 0.05 in aqueous boundary lubrication. Its anti-wear performance was verified to be better than that of PEEK/carbon fiber (CF) and Thordon composites. The PEEK/MoS₂ composite may be a potential material for underwater equipment because of its outstanding anti-wear performance in aqueous boundary lubrication.

Surfactants tend to adsorb on the surface/interface mostly in a directional manner. The alkyl chain orientation and conformation order for molecular monolayers of sodium dodecylbenzenesulfonate (SDBS) at low concentrations are studied by using the sum frequency generation vibrational spectroscopy (SFG-VS). The molecular arrangement of the surfactants adsorbed at the solid/liquid interface is further investigated. It is found that the arrangement of the SDBS at the interface becomes relatively ordered with increasing bulk concentration. Meanwhile, the orientation angle reduces gradually, and the molecules tend to be upright state. In addition, the effect of friction on the conformation order and orientation angle are also analyzed. The intensity of the SDBS vibrational contraction peak becomes lower after friction, which indicates that the anion has a reorientation process at the interface. The arrangement of molecules becomes more disordered due to friction. The orientation angle increases slightly, which indicates the monolayer has an inclined trend relative to the lateral direction on the interface. A modified adsorption model considering friction effect is proposed. This work may provide a reference for the further study of adsorption mechanism and application of surfactants.

Bio-inspired reversible adhesion has significant potential in many fields requiring flexible grasping and manipulation, such as precision manufacturing, flexible electronics, and intelligent robotics. Despite extensive efforts for adhesive synthesis with a high adhesion strength at the interface, an effective strategy to actively tune the adhesion capacity between a strong attachment and an easy detachment spanning a wide range of scales has been lagged. Herein, we report a novel soft-hard-soft sandwiched composite design to achieve a stable, repeatable, and reversible strong adhesion with an easily scalable performance for a large area ranging from ~1.5 to 150 cm² and a high load ranging from ~20 to 700 N. Theoretical studies indicate that this design can enhance the uniform loading for attachment by restraining the lateral shrinkage in the natural state, while facilitate a flexible peeling for detachment by causing stress concentration in the bending state, yielding an adhesion switching ratio of ~54 and a switching time of less than ~0.2 s. This design is further integrated into versatile grippers, climbing robots, and human climbing grippers, demonstrating its robust scalability for a reversible strong adhesion. This biomimetic design bridges microscopic interfacial interactions with macroscopic controllable applications, providing a universal and feasible paradigm for adhesion design and control.

With the development of surface and interface science and technology, methods for the online modulation of interfacial performance by external stimuli are in high demand. Switching between ultra-low and high friction states is a particular goal owing to its applicability to the development of precision machines and nano/micro-electromechanical systems. In this study, reversible switching between superlubricity and high friction is realized by controlling the electric potential of a gold surface in aqueous salt solution sliding against a SiO₂ microsphere. Applying positive potential results creates an ice-like water layer with high hydrogen bonding and adhesion at the interface, leading to nonlinear high friction. However, applying negative potential results in free water on the gold surface and negligible adhesion at the interface, causing linear ultra-low friction (friction coefficient of about 0.004, superlubricity state). A quantitative description of how the external load and interfacial adhesion affected friction force was developed, which agrees well with the experimental results. Thus, this work quantitatively reveals the mechanism of potential-controlled switching between superlubricity and high-friction states. Controlling the interfacial behavior via the electric potential could inspire novel design strategies for nano/micro-electromechanical and nano/micro-fluidic systems.

Over the past twenty years, thin film lubrication (TFL) theory has been used to characterize the molecular behaviors in lubrication films thinner than 100 nm, effectively bridging the gap between elastohydrodynamic lubrication and boundary lubrication. Unfortunately, to date, the TFL molecular model proposed in 1996 has not been directly proven by experimental detection. Herein, a method based on surface-enhanced Raman spectroscopy was developed to show both the packing and orienting of liquid molecules in the TFL regime. By trapping liquid crystal molecules between a structured silver surface and a glass surface, molecular ordering states dominated by shear effect and surface effect were successfully distinguished. A nanosandwich structure consisting of an adsorbed layer, an ordered-molecule layer, and a fluid layer was demonstrated. Molecule imaging in TFL was achieved. Our results illustrate the molecular behaviors and lubrication mechanism in nanoconfined films and facilitate the lubrication design of nanoelectromechanical and microelectromechanical systems.