Interfacial Dynamics and Adhesion Behaviors of Water and Oil Droplets in Confined Geometry

To simulate the interfacial behaviors in real heterogeneous systems, the point contact condition is constructed to study the classical immiscible displacement problem in this work. Specifically, the interfacial dynamics during the water droplet passing through the oil capillary bridge formed under the point contact condition is investigated. Emphasis is put on the influences of the wettabilities and the relative separation motion of the solid surfaces on the dynamic behavior of the droplets. The observations suggested that the capillary pressure had negligible effect on the movement of the water droplet when it was passing though the oil capillary bridge. The wettability and the relative separation of the disk and ball would influence the final adhesion behaviors of the water droplet after the droplet passed through the oil capillary bridge. Surface tension and adhesion energy were used to interpret these observations

Interfacial Dynamics and Adhesion Behaviors of Water and Oil Droplets in Confined Geometry

To simulate the interfacial behaviors in real heterogeneous systems, the point contact condition is constructed to study the classical immiscible displacement problem in this work. Specifically, the interfacial dynamics during the water droplet passing through the oil capillary bridge formed under the point contact condition is investigated. Emphasis is put on the influences of the wettabilities and the relative separation motion of the solid surfaces on the dynamic behavior of the droplets. The observations suggested that the capillary pressure had negligible effect on the movement of the water droplet when it was passing though the oil capillary bridge. The wettability and the relative separation of the disk and ball would influence the final adhesion behaviors of the water droplet after the droplet passed through the oil capillary bridge. Surface tension and adhesion energy were used to interpret these observations

Improvement of Load Bearing Capacity of Nanoscale Superlow Friction by Synthesized Fluorinated Surfactant Micelles

Although surfactant micelles usually exhibit superlow friction at the nanoscale due to the formation of the hydration layer, the load-bearing capacity (LBC) is limited. In this study, the friction behaviors of two different surfactant micelles (fluorinated and hydrocarbon surfactants, denoted as F-surfactant and H-surfactant) were compared, with the results showing that both can achieve superlow friction (μ = 0.001–0.002) when the self-assembled micelle layers on the two surfaces were not ruptured. Although the two different surfactant micelles have the similar friction behaviors, the LBC of superlow friction for the F-surfactant is 2.5 times larger than that for the H-surfactant. The mechanisms of the superlow friction and the reasons for different LBC were investigated using an atomic force microscopy. The superlow friction can be attributed to the formation of hydration layer on the surfactant headgroups, whereas the higher LBC for F-surfactant originates from the fatness of its carbon chain, which produces the larger hydrophobic attraction and meanwhile increases the stiffness of the micelle layer

AFM Studies on Liquid Superlubricity between Silica Surfaces Achieved with Surfactant Micelles

By using atomic force microscopy (AFM), we showed that the liquid superlubricity with a superlow friction coefficient of 0.0007 can be achieved between two silica surfaces lubricated by hexadecyl­trimethyl­ammonium bromide (C₁₆TAB) solution. There exists a critical load that the lubrication state translates from superlow friction to high friction reversibly. To analyze the superlow friction mechanism and the factors influencing the critical load, we used AFM to measure the structure of adsorbed C₁₆TAB molecules and the normal force between two silica surfaces. Experimental results indicate that the C₁₆TAB molecules are firmly adsorbed on the two silica surfaces by electrostatic interaction, forming cylinder-like micelles. Meanwhile, the positively charged headgroups exposed to solution produce the hydration and double layer repulsion to bear the applied load. By controlling the concentration of C₁₆TAB solution, it is confirmed that the critical load of superlow friction is determined by the maximal normal force produced by the hydration layer. Finally, the superlow friction mechanism was proposed that the adsorbed micellar layer forms the hydration layer, making the two friction surfaces be in the repulsive region and meanwhile providing excellent fluidity without adhesion between micelles

Black Phosphorus: Degradation Favors Lubrication

Due to its innate instability, the degradation of black phosphorus (BP) with oxygen and moisture was considered the obstacle for its application in ambient conditions. Here, a friction force reduced by about 50% at the degraded area of the BP nanosheets was expressly observed using atomic force microscopy due to the produced phosphorus oxides during degradation. Energy-dispersive spectrometer mapping analyses corroborated the localized concentration of oxygen on the degraded BP flake surface where friction reduction was observed. Water absorption was discovered to be essential for the degraded characteristic as well as the friction reduction behavior of BP sheets. The combination of water molecules as well as the resulting chemical groups (P–OH bonds) that are formed on the oxidized surface may account for the friction reduction of degraded BP flakes. It is indicated that, besides its layered structure, the ambient degradation of BP significantly favors its lubrication behavior