Contact angle and stability of interfacial nanobubble supported by gas monolayer

Since solid-liquid interfacial nanobubbles (INBs) were first imaged, their long-term stability and large contact angle have been perplexing scientists. This study aimed to investigate the influence of internal gas density and external gas monolayers on the contact angle and stability of INB using molecular dynamics simulations. First, the contact angle of a water droplet was simulated at different nitrogen densities. The results showed that the contact angle increased sharply with an increase in nitrogen density, which was mainly caused by the decrease in solid-gas interfacial tension. However, when the nitrogen density reached 2.57 nm^-3, an intervening gas monolayer (GML) was formed between the solid and water. After the formation of GML, the contact angle slightly increased with increasing gas density. The contact angle increased to 180° when the nitrogen density reached 11.38 nm^-3, indicating that INBs transformed into a gas layer when they were too small. For substrates with different hydrophobicities, the contact angle after the formation of GML was always larger than 140° and it was weakly correlated with substrate hydrophobicity. The increase in contact angle with gas density represents the evolution of contact angle from macro- to nano-bubble, while the formation of GML may correspond to stable INBs. The potential of mean force curves demonstrated that the substrate with GML could attract gas molecules from a longer distance without the existence of a potential barrier compared with the bare substrate, indicating the potential of GML to act as a gas-collecting panel. Further research indicated that GML can function as a channel to transport gas molecules to INBs, which favors stability of INBs. This research may shed new light on the mechanisms underlying abnormal contact angle and long-term stability of INBs.