This study introduces a streamline theory to reveal the micro-engagement dynamics of bio-inspired spines on rough surfaces. It first defines two key phenomena: the "bypass effect" and the "trap effect." Simulations demonstrate that the bypass effect reduces engagement effectiveness on convex asperities, while the trap effect significantly enhances engagement capacity on concave valleys. Experiments confirm these findings, revealing that the gain from the trap effect (252.2%) far outweighs the loss from the bypass effect (9.7%). Furthermore, analysis on fractal surfaces found that stable self-locking engagement occurs almost exclusively in concave regions (80%~90%) due to the trap effect. This research provides a theoretical foundation for bio-inspired gripping technology and functional surface design, with implications for future applications in effective grippers, robotics, and space exploration.

Underwater surface haptics based on friction modulation provide new concepts for immersive extended reality haptic technology. A previous study proposed a modulation method by using a voltage-controlled technique in saline solutions, demonstrating that finger friction was actively adjusted by external voltages. This work further investigated the voltage-controlled finger friction on 304 stainless steel plates with two surface roughness in sodium dodecyl sulfate (SDS) solutions with three concentrations. By changing the roughness, the effects of both increasing friction and reducing friction were observed. This can be attributed to the competitive mechanisms between the adsorption/desorption of the surfactant and the formation/dissolution of the metal oxide film. This study improves the understanding of the mechanisms of finger friction based on the voltage-controlled technique and verifies the feasibility of underwater friction modulation.

The energy crisis and environmental pollution are worsening. Therefore, water-based hydraulic fluids, i.e., aqueous ethylene glycol-based, fire-retardant hydraulic fluid concentrates (HFCs), are becoming increasingly common. However, seawater intrusion inevitably occurs under marine conditions, generating hazards, such as corrosion and friction, within hydraulic system components, pipelines, and materials. Moreover, HFCs have several drawbacks, including low viscosity, inadequate lubrication, and high corrosivity. Therefore, the tribological characteristics and corrosivity of HFCs must be improved and reduced, respectively. This can be achieved using additives. Herein, we summarize the fundamental characteristics of HFCs and their modifications for use in the marine environment, focusing on the optimal water–ethylene glycol proportion and its influence on the physicochemical, lubricating, and tribological properties of this HFC under varying conditions. We discuss the latest progress on the effects of seawater on the tribological corrosion of HFCs and the reduction in corrosivity in the presence of different additives. Finally, we highlight challenges and propose future research to improve performance in the marine environment.

Lubrication recovery has been observed in tribology research. This refers to the recovery of oil lubrication on a metal after failure or semi-failure by waiting for a period. Past studies attributed this recovery to metal oxidation. However, this article presented an alternative mechanism. We proposed that the accumulation of carbon-containing tribofilms from hydrocarbon oils was crucial for lubrication recovery. Changes in surface states, such as increased contact areas, also play a significant role. For studying wear mechanism maps, it is essential to consider both working conditions and surface states. Two new wear mechanism maps based on initial surfaces with or without pregenerated tribofilm were developed. These findings highlight the mechanochemical effects of base oils on lubrication and anti-wear performance.

Soft grippers are key manipulation tools for robotics and end effectors for securely grasping objects of various shapes and sizes on demand. However, critical challenges, including self-adaptive grasping to curved surfaces and monitoring the contact state, remain. Here, a gecko-inspired curved flexible surface adaptive gripper (CSAG), which consists of a variable-bending pneumatic actuator, a triboelectric sensor (T-sensor), and a gecko-inspired microwedge adhesive, is proposed. The contact-sensitive triboelectric sensor can sense the critical contact state of objects to trigger a variable-bending pneumatic actuator with sufficient shear loading for the geckoinspired microwedge adhesive. A set of experiments are implemented to verify that the proposed soft gripper can adaptively grasp diverse curved objects, including quail eggs, cans, shuttlecocks, expanding objects with varying volumes (such as balloons, the range of diameter variation is 20–115 mm), and spherical acrylic cylinders (20–40 mm) at low pressures (20–25 kPa) with a maximum weight of 37 g. Additionally, the tracking and grasping of a moving ball is demonstrated via a mean-shift algorithm based on image recognition coupled with coordination tracking of a robotic arm. The soft gripper provides a new paradigm to achieve switchable grasping of curved flexible surfaces, which broadens future applications for versatile unstructured human‒robot‒environment interactions, such as adaptive robots and medical devices.

Polyimide (PI) coatings are highly valued for their easy processing and exceptional mechanical qualities, which enable them to be applied in a variety of approaches. By integrating MXene, a two-dimensional material renowned for its low shear strength and excellent load-bearing capacity, into the PI matrix and subsequently using it as a coating on GCr15 surfaces, MXene/PI composites were produced. The tribological performance of these composites improved with increasing MXene content and then decreased. In the dry state and lubricated with poly-α-olefin-8 (PAO8), the 0.25% MXene/PI composite demonstrated optimal tribological performance and achieved superlubricity, with a coefficient of friction (COF) of only 0.002 and a wear rate of only 1.92×10⁻⁷ mm³/(N·m). This improvement can be attributed to the even distribution of MXene in the PI matrix, particularly during frictional processes, as evidenced by scanning electron microscopy (SEM), dynamic mechanical analysis (DMA), and transmission electron microscopy (TEM) analyses. Furthermore, the interaction between MXene and PI was confirmed through X-ray photoelectron spectroscopy (XPS) analysis. These results not only establish the groundwork for developing high-performance PI coatings but also provide valuable insights for designing composite materials with superlubricity properties for engineering applications.

Humans rely on their fingers to sense and interact with external environment. Understanding the tribological behavior between finger skin and object surface is crucial for various fields, including tactile perception, product appearance design, and electronic skin research. Quantitatively describing finger frictional behavior is always challenging, given the complex structure of the finger. In this study, the texture and sliding direction dependence of finger skin friction was quantified based on explicit mathematic models. The proposed double-layer model of finger skin effectively described the nonlinear elastic response of skin and predicted the scaling-law of effective elastic modulus with contact radius. Additionally, the skin friction model on textured surface considering adhesion and deformation factors was established. It revealed that adhesive term dominated finger friction behavior in daily life, and suggested that object texture size mainly influenced friction-induced vibrations rather than the average friction force. Combined with digital image correlation (DIC) technique, the effect of sliding direction on finger friction was analyzed. It was found that the anisotropy in finger friction was governed by the finger’s ratchet pawl structure, which also contributes to enhanced stick-slip vibrations in the distal sliding direction. The proposed friction models can offer valuable insights into the underlying mechanism of skin friction under various operating conditions, and can provide quantitative guidance for effectively encoding friction into haptics.

Most studies of liquid lubricants were carried out at temperatures below 200 °C. However, the service temperature of lubricants for aerospace and aeroengine has reached above 300 °C. In order to investigate the friction mechanism and provide data for high temperature lubrication, the friction and wear properties of chlorophenyl silicone oil (CPSO)-lubricated M50 steel and Si₃N₄ friction pairs were investigated herein. Ball-on-disk experimental results show that the lubrication performance of CPSO varies significantly with temperature. Below 150 °C, coefficient of friction (COF) remains at 0.13–0.15 after the short running-in stage (600 s), while the COF in the running-in stage is 0.2–0.3. At 200 °C and above, the running-in time is much longer (1,200 s), and the initial instantaneous maximum COF can reach 0.5. Under this condition, the COF gradually decreases and finally stabilizes at around 0.16–0.17 afterwards. This phenomenon is mainly due to the different thickness of boundary adsorption film. More importantly, the wear rate of M50 steel increases significantly with the temperature, while the wear rate barely changes at temperatures above 200 °C. The anti-wear mechanism is explained as tribochemical reactions are more likely to occur between CPSO and steel surface with the increased temperature, generating the FeCl₂ protective film on the metal surface. Accordingly, FeCl₂ tribochemical film improves the lubrication and anti-wear capacity of the system. At high temperatures (200–350 °C), FeCl₂ film becomes thicker, and the contact region pressure becomes lower due to the larger wear scar size, so the wear rate growth of M50 steel is much smaller compared with that of low temperatures (22–150 °C). The main findings in this study demonstrate that CPSO lubricant has good anti-wear and lubrication capacity, which is capable of working under temperatures up to 350 °C.