Mechanism of Friction Enhancement Induced by Nanoscale Liquid Film: A Brief Review

Abstract

Surficial water adsorption and interfacial water condensation as natural phenomena play an essential role in the contact adhesion and friction performances of the solid interface. As the characteristic dimensions downscale to nanometers, the structure and dynamics of the water film at an interface differ significantly from those of its bulk counterpart. In particular, a specific wetting condition termed as the tacky regime has recently sparked great interest in the community, where transient high friction and contact instabilities are observed at the interface that is subjected to the wet-to-dry transition. Unveiling the influence of nanoscale water film on the friction enhancement in the tacky regime will provide theoretical guidance for the friction regulation in the wetting condition. In this article, special emphasis is placed on the development of experimental techniques which allow the visualization of the contact interface (e.g., contact surface deformation, real contact area) and characterization of water film structures (e.g., film thickness, molecular configuration). Building upon the accumulation of recent research activities, we provide an overview of significant advances in understanding the critical mechanisms for friction enhancement, such as vertical capillary force, interfacial shear strength, and ice-like water. Some common design strategies are further given to regulate the friction behavior by tuning the distribution of the water film, surface roughness, and elastic modulus. Finally, we end this review article with a summary of the research status and outlook on areas for future research directions.