The ocular lubrication, where the eyelid constantly slides on the curved corneal surface, is considered as one of primary lubrication systems in bio-tribology. Under reliable lubrication conditions, sensitive ocular tissues remain intact from fatigue damage during spontaneous blink cycles. The tear film, evenly filled between cornea and conjunctiva, is a biological fluid with dynamic adjustment ability, which provides superior lubrication with the friction coefficient of below 0.01. However, the lubrication failure may result in a variety of uncomfortable symptoms such as inflammatory reactions, tissue damage and neurological abnormalities. Therefore, it is essential to clarify the fundamental mechanism of ocular lubrication, which helps to alleviate and even recover from various ocular symptoms. This review firstly demonstrates that the ocular components, containing lipids and mucins, contribute to maintaining the lubrication stability of tear film. Furthermore, the ocular lubrication state in various physiological environments and the physical effect on tear film dynamics are further discussed. As typical applications, the therapeutic agents of dry eye syndrome and contact lens with superior lubrication effects are introduced and their lubrication mechanisms are clarified. Finally, this review summarizes a series of the latest research inspired by ocular lubrication. Overall, this work will provide a valuable guidance on the theoretical research and extensive applications in the field of biological lubrication.

Layered double hydroxides (LDHs) have the potential to be superlubricated materials due to their strong adsorption effect and weak internal interaction. However, obtaining stable superlubricity during the ultrafast time (< 10 s) is still a challenge. Here, we demonstrated macroscale superlubricity based on LDHs of multiple metal ions at high surface roughness, achieving superlow friction coefficients (0.006) and ultrafast wearing-in time (< 7 s), which mainly originated from tribochemical reactions and the formation of nanostructured adsorption layers. Through cross-sectional analysis and density functional theory, we revealed the properties of the protective tribofilm to achieve ultrafast superlubricity. LDHs strongly adsorbed on the surface of the bearing steel, and the sliding interface transformed into a heterogeneous interface between the polytetrafluoroethylene and LDH, leading to macroscale superlubricity. These findings demonstrate that tribochemical treatment of surfaces produces tribofilm that effectively reduces wearing-in time and promotes ultralow friction.

Polyalkylene glycol (PAG) aqueous solutions have recently been demonstrated to exhibit an ultralow friction coefficient (COF, μ < 0.01). However, the prolonged running-in period and low bearing capacity have limited its widespread application. In this study, we determined that the running-in period can be decreased by more than 75% when the pH value of the lubricant is controlled at 3 by introducing various acid solutions. Additionally, less time was required to realize stable superlubricity with inorganic acid at lower pH values. This was mainly attributed to the acceleration effect of hydrogen ions around the contact region. In case of PAG aqueous solution with organic acid, the wear loss between sliding solid surfaces was reduced, and thus the bearing pressure during the superlubricity period was significantly improved from approximately 30 to 160 MPa. Furthermore, the organic acid molecules were considered to form strong hydrogen bonds with PAG macromolecules and solid surfaces. This in turn strengthened the structure of the adsorption layers. The unique effect of different acids in aqueous polymer lubrication can potentially significantly aid in advancing the study of polymer tribology and broadening industrial applications.

It is difficult to achieve macroscale superlubricity under high contact pressures and high normal loads. Layered double hydroxide (LDH) nanoadditives were introduced into an ionic liquid alcohol solution (IL_(as)) with contact pressures up to 1.044 GPa, which resulted in a friction coefficient (COF) of 0.004 and a robust superlubricity state lasting for 2 h. Compared with the LDH particles (LDH-Ps) with ca. 90-nm widths and 18-nm thickness, micron-scale LDH nanosheet (LDH-N) additives with ca. 1.5-µm width and 6-nm thickness increased the load-bearing capacity by approximately three times during superlubricity. The lubricant film thickness and the ultrathin longitudinal dimension of the LDH-N additives did not influence the continuity of the fluid film on the contact surface. These improvements resulted from the protective adsorption layer and ion distribution formed on the contact interface, as revealed by detailed surface analyses and simulation studies. In particular, the sliding energy barrier and Bader charge calculation revealed that weak shear sliding between the nanosheet and the solid surface formed easily and the anions in the liquid adsorbed on the solid surface exhibited electrostatic repulsion forces, which generated stable tribological properties synergistically. This research provides a novel method for obtaining macroscale superlubricity for practical industrial applications.