Friction is ubiquitous and plays a key role in the functionality of many biological and engineering systems, from articular cartilage to machinery. While friction facilitates motion, it also causes wear and energy loss in moving parts. Lubricants (particularly liquid lubricants) are essential to reduce the negative effects of friction, and their properties (e.g., rheology and compatibility with friction materials) significantly influence lubrication performance and related mechanisms. The tribological phenomena between friction surfaces separated by a nanoconfined liquid film are governed by both external load and surface forces involved. Despite significant progress over the past few decades, the molecular and interfacial interaction mechanisms driving liquid-lubricated friction are not yet fully understood, and a comprehensive correlation between surface forces and tribological behaviors in nanoconfined liquids has not been fully established. In this review, we first summarize the latest understanding of fundamental concepts in surface forces, nano-rheology, and tribology in nanoconfined liquids. Representative tribological phenomena in nanoconfined liquids are analyzed and correlated with surface forces and liquid properties involved in specific cases. Additionally, advanced nanomechanical technologies (e.g., surface forces apparatus (SFA) and atomic force microscopy (AFM)), which show great potential in the field of tribology, are introduced. The advantages and current limitations of these technologies are also discussed. Moreover, key findings from recent tribological studies involving different liquids (both aqueous solutions and nonpolar liquids) are reviewed, and the underlying mechanisms of lubrication performance are analyzed from the perspective of surface forces. The future directions of tribology in nanoconfined liquids are discussed, providing insights and inspirations for developing effective lubrication strategies. This review enhances the understanding of nanotribology and correlates tribological phenomena with surface forces and rheology in nanoconfined liquids, offering new insights for developing advanced lubricants and wear-resistance materials.

The unexpected scaling phenomena have resulted in significant damages to the oil and gas industries, leading to issues such as heat exchanger failures and pipeline clogging. It is of practical and fundamental importance to understand the scaling mechanisms and develop efficient anti-scaling strategies. However, the underlying surface interaction mechanisms of scalants (e.g., calcite) with various substrates are still not fully understood. In this work, the colloidal probe atomic force microscopy (AFM) technique has been applied to directly quantify the surface forces between calcite particles and different metallic substrates, including carbon steel (CR1018), low alloy steel (4140), stainless steel (SS304) and tungsten carbide, under different water chemistries (i.e., salinity and pH). Measured force profiles revealed that the attractive van der Waals (VDW) interaction contributed to the attachment of the calcium carbonate particles on substrate surfaces, while the repulsive electric double layer (EDL) interactions could inhibit the attachment behaviors. High salinity and acidic pH conditions of aqueous solutions could weaken the EDL repulsion and promote the attachment behavior. The adhesion of calcite particles with CR1018 and 4140 substrates was much stronger than that with SS304 and tungsten carbide substrates. The bulk scaling tests in aqueous solutions from an industrial oil production process showed that much more severe scaling behaviors of calcite was detected on CR1018 and 4140 than those on SS304 and tungsten carbide, which agreed with surface force measurement results. Besides, high salinity and acidic pH can significantly enhance the scaling phenomena. This work provides fundamental insights into the scaling mechanisms of calcite at the nanoscale with practical implications for the selection of suitable anti-scaling materials in petroleum industries.

Understanding the friction behavior of hydrogels is critical for the long-term stability of hydrogel-related bioengineering applications. Instead of maintaining a constant sliding velocity, the actual motion of bio-components (e.g., articular cartilage and cornea) often changes abruptly. Therefore, it is important to study the frictional properties of hydrogels serving under various sliding velocities. In this work, an unexpected low friction regime (friction coefficient μ < 10^-4 at 1.05×10^-3 rad/s) was observed when the polyacrylamide hydrogel was rotated against a glass substrate under alternative sliding velocity cycles. Interestingly, compared with the friction coefficients under constant sliding velocities, the measured μ decreased significantly when the sliding velocity changed abruptly from high speeds (e.g., 105 rad/s) to low speeds (e.g., 1.05×10^-3 rad/s). In addition, μ exhibited a downswing trend at low speeds after experiencing more alternative sliding velocity cycles: the measured μ at 1.05 rad/s decreased from 2×10^-2 to 3×10^-3 after 10 friction cycles. It is found that the combined effect of hydration film and polymer network deformation determines the lubrication and drag reduction of hydrogels when the sliding velocity changes abruptly. The observed extremely low friction during alternative sliding velocity cycles can be applied to reduce friction at contacted interfaces. This work provides new insights into the fundamental understanding of the lubrication behaviors and mechanisms of hydrogels, with useful implications for the hydration lubrication related engineering applications such as artificial cartilage.