In this study, the uniform dense polydopamine (PDA) coating was deposited on hyper-cross-linked polystyrene nanospheres (HPSs) through the oxidative polymerization of dopamine with polyethyleneimine (PEI), then which underwent acidification and subsequent anion exchange with LiNTf₂ to obtain HPSs@PDA electrolyte (HPSs@PDA-NTf₂). So, the multi-element-doped carbon nanospheres (F,N,S-PCNs) were synthesized through the carbonization of HPSs@PDA-NTf₂, demonstrating exceptional tribological performance. Compared to 500SN, the mean COF of nanolubricant (500SN + 2.0 wt.% F,N,S-PCNs) decreased from 0.181 to 0.110, and the wear volume reduced by 90.3%. The load-carrying capacity of F,N,S-PCNs as lubricant additives is increased from 150N (500SN) to 450N. The F,N,S-PCNs can infiltrate the contact area and adsorb on the friction pair surface, forming a physical adsorption film that prevents the direct contact of surface. Additionally, the active elements (F,N,S) in F,N,S-PCNs undergo tribochemical reactions with the friction pair under mechanical force and thermal effects to form a chemical protective film. This dual effect significantly enhances the boundary lubrication performance of the lubricating oil. This study presents a novel approach for synthesizing multi-element co-doped carbon nanospheres, significantly enhancing the effectiveness of oil-based lubrication technology in the field.

Stimulus-responsive polymers have steadily grown in significance over the past few decades, with extensive research dedicated to the intelligent design of friction materials inspired by natural processes. In this study, we introduce a hydrogel system, CS-MXene@P(AAc-CaAc-co-HEMA-Br)@PSPMA (M-PAAc@PSPMA), that adeptly modulates its modulus across temperature variations, subsequently influencing the interface friction coefficient. The integration of CS-MXene as a photothermal agent facilitates interface temperature modulation under near-infrared (NIR) light irradiation. By manipulating the temperature, the modulus of the P(AAc-CaAc-co-HEMA-Br) hydrogel can be effectively regulated. Moreover, the poly(3-sulfopropyl methacrylate potassium) (PSPMA) polyelectrolyte brush further refines the lubricating attributes of the system. Under ambient conditions, the hydrogel is characterized by a low modulus, heightened flexibility, diminished strength, and a friction coefficient of approximately 0.24. In contrast, under NIR irradiation, the modulus, hardness, and strength of the hydrogel increased, and the friction coefficient decreased to approximately 0.1. This innovative hydrogel system offers advanced friction control by modulating its modulus, setting a precedent for the future development of intelligent lubricant hydrogels, interface detection, and regulated transmission.

Water friction in nanoconfinement is of great importance in water lubrication and membrane-based applications, yet remains fraught with doubts despite great efforts. Our molecular dynamics simulations demonstrate that the first water layer adjacent to the surface plays an important role in interfacial friction. Applying a uniform strain to the surface (changing the lattice constant) can induce a significant change in friction, and is quite different scenarios for the hydrophilic and hydrophobic cases. Specifically, in the hydrophilic case, there is a maximum friction when the lattice constant approaches the preferential oxygen-oxygen distance of the first water layer (a constant value), and the further it deviates the smaller the friction. The maximum friction corresponds to the most ordered first water layer. While in the hydrophobic case, the friction increases monotonically with the increasing lattice constant, which hardly changes the first water layer structure but only increases the difficulty of water molecular jump (meaning jump from one equilibrium position to another). Starting from the molecular jump in the first water layer, a theoretical dependence of the friction on the molecular activation barrier and the shear velocity is established, which provides a reasonable explanation for the friction behavior. Moreover, the water transport behavior in nanochannels supports the finding of the friction dependence on the lattice constant, suggesting great potential for improving and controlling water transport. Our results not only provide a novel understanding of nanoconfined water friction, but are instructive for friction control and water transport.

Bacterial infection and tissue damage caused by friction are two major threats to patients’ health in medical catheter implantation. Hydrogels with antibacterial and lubrication effects are competitive candidates for catheter coating materials. Photothermal therapy (PTT) is a highly efficient bactericidal method. Here, a composite hydrogel containing MXene nanosheets and hydrophilic 3-sulfopropyl methacrylate potassium salt (SPMK) is reported, which is synthesized through the one-pot method and heat-initiated polymerization. The hydrogel shows excellent antibacterial performance against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) in 3 min in the air or 20 min in the water environment under near-infrared light (NIR; 808 nm) irradiation. The friction coefficient of the hydrogel is about 0.11, which is 48% lower than that without SPMK. The rapid photothermal sterilization is attributed to the outstanding antibacterial ability and thermal effect of photoactivated MXene. The ultra-low friction is the result of the hydration lubrication mechanism. This study provides a potential strategy for the surface coatings of biomedical catheters, which enables rapid sterilization and extremely low interface resistance between catheters and biological tissues.

Herein, we have prepared SiO₂ particles uploaded MXene nanosheets via in-situ hydrolysis of tetraetholothosilicate. Due to the large number of groups at the edges of MXene, SiO₂ grows at the edges first, forming MXene@SiO₂ composites with a unique core-rim structure. The tribological properties of MXene@SiO₂ as lubricating additive in 500 SN are evaluated by SRV-5. The results show that MXene@SiO₂ can reduce the friction coefficient of 500 SN from 0.572 to 0.108, the wear volume is reduced by 73.7%, and the load capacity is increased to 800 N. The superior lubricity of MXene@SiO₂ is attributed to the synergistic effect of MXene and SiO₂. The rolling friction caused by SiO₂ not only improves the bearing capacity but also increases the interlayer distance of MXene, avoiding accumulation and making it more prone to interlayer slip. MXene@SiO₂ is adsorbed on the friction interface to form a physical adsorption film and isolate the friction pair. In addition, the high temperature and high load induce the tribochemical reaction and form a chemical protection film during in the friction process. Ultimately, the presence of these protective films results in MXene@SiO₂ having good lubricating properties.

Cartilage is well lubricated over a lifetime and this phenomenon is attributed to both of the surface hydration lubrication and the matrix load-bearing capacity. Lubricious hydrogels with a layered structure are designed to mimic cartilage as potential replacements. While many studies have concentrated on improving surface hydration to reduce friction, few have experimentally detected the relationship between load-bearing capacity of hydrogels and their interface friction behavior. In this work, a bilayer hydrogel, serving as a cartilage prototype consisted of a top thick hydrated polymer brush layer and a bottom hydrogel matrix with tunable modulus was designed to investigate this relationship. The coefficient of friction (COF, μ) is defined as the sum of interfacial component (μ_Int) and deformation/hysteresis component (μ_Hyst). The presence of the top hydration layer effectively dissipates contact stress and reduces the interface interaction (μ_Int), leading to a stable and low COF. The contribution of mechanical deformation (μ_Hyst) during the sliding shearing process to COF can be significantly reduced by increasing the local mechanical modulus, thereby enhancing the load-bearing capacity. These results show that the strategy of coupling surface hydration layer with a high load-bearing matrix can indeed enhance the lubrication performance of hydrogel cartilage prototypes, and implies a promising routine for designing robust soft matter lubrication system and friction-control devices.

The polyionic liquid poly-PEGMA-r-METAC (PPM) with quaternary ammonium has been synthesized and evaluated as additive in aqueous lubricating fluids. The rheological behavior of aqueous lubricating fluids with PPM has been characterized to confirm PPM’s function as a viscosity modifier. The tribological behavior of aqueous lubricating fluids with PPM has been investigated on SRV-V and MTM testing machines. It was found that PPM has excellent viscosity-increasing, lubricating, and anti-wear properties as an additive for aqueous, which can be attributed to the ability of PPM to form the protective film and boundary tribofilm generated from complex tribochemical reaction on rubbing surface. The obtained PPM with dual functions of anti-corrosion additives and viscosity index improver can play an important role in diverse lubrication regimes.

Surface wettability plays a significant role in reducing solid–liquid frictional resistance, especially the superhydrophilic/hydrophilic interface because of its excellent thermodynamic stability. In this work, poly(acrylic acid)-poly(acrylamide) (PAA–PAM) hydrogel coatings with different thicknesses were prepared in situ by polydopamine (PDA)-UV assisted surface catalytically initiated radical polymerization. Fluid drag reduction performance of hydrogel surface was measured using a rotational rheometer by the plate–plate mode. The experimental results showed that the average drag reduction of hydrogel surface could reach up to about 56% in Couette flow, which was mainly due to the interfacial polymerization phenomenon that enhanced the ability of hydration layer to delay the momentum dissipation between fluid layers and the diffusion behavior of surface. The proposed drag reduction mechanism of hydrogel surface was expected to shed new light on hydrogel–liquid interface interaction and provide a new way for the development of steady-state drag reduction methods.

This study presents a nitrogen-doped microporous carbon nanospheres (N@MCNs) prepared by a facile polymerization–carbonization process using low-cost styrene. The N element in situ introduces polystyrene (PS) nanospheres via emulsion polymerization of styrene with cyanuric chloride as crosslinking agent, and then carbonization obtains N@MCNs. The as-prepared carbon nanospheres possess the complete spherical structure and adjustable nitrogen amount by controlling the relative proportion of tetrachloromethane and cyanuric chloride. The friction performance of N@MCNs as lubricating oil additives was surveyed utilizing the friction experiment of ball-disc structure. The results showed that N@MCNs exhibit superb reduction performance of friction and wear. When the addition of N@MCNs was 0.06 wt%, the friction coefficient of PAO-10 decreased from 0.188 to 0.105, and the wear volume reduced by 94.4%. The width and depth of wear marks of N@MCNs decreased by 49.2% and 94.5%, respectively. The carrying capacity of load was rocketed from 100 to 400 N concurrently. Through the analysis of the lubrication mechanism, the result manifested that the prepared N@MCNs enter clearance of the friction pair, transform the sliding friction into the mixed friction of sliding and rolling, and repair the contact surface through the repair effect. Furthermore, the tribochemical reaction between nanoparticles and friction pairs forms a protective film containing nitride and metal oxides, which can avert direct contact with the matrix and improve the tribological properties. This experiment showed that nitrogen-doped polystyrene-based carbon nanospheres prepared by in-situ doping are the promising materials for wear resistance and reducing friction. This preparing method can be ulteriorly expanded to multi-element co-permeable materials. Nitrogen and boron co-doped carbon nanospheres (B,N@MCNs) were prepared by mixed carbonization of N-enriched PS and boric acid, and exhibited high load carrying capacity and good tribological properties.

MXene possesses great potential in enriching the functionalities of hydrogels due to its unique metallic conductivity, high aspect ratio, near-infrared light (NIR light) responsiveness, and wide tunability, however, the poor compatibility of MXene with hydrogels limits further applications. In this work, we report a uniformly dispersed MXene-functionalized poly-N-isopropylacrylamide (PNIPAM)/poly-2-acrylamido-2-methyl-1-propanesulfonic acid (PAMPS) double network hydrogel (M–DN hydrogel) that can achieve switchable friction regulation by using the NIR light. The dispersity of MXene in hydrogels was significantly improved by incorporating the chitosan (CS) polymer. This M–DN hydrogel showed much low coefficient of friction (COF) at 25 °C due to the presence of hydration layer on hydrogel surface. After illuminating with the NIR light, M–DN hydrogel with good photothermal effect rapidly raised the temperature to above the lower critical solution temperature (LCST), which led to an obvious increase of surface COF owing to the destruction of the hydration layer. In addition, M–DN friction control hydrogel showed good recyclability and controllability by tuning "on-off" of the NIR light. This work highlights the construction of functional MXene hydrogels for intelligent lubrication, which provides insight for interface sensing, controlled transmission, and flexible robotic arms.