Yang Haichang, Xing Yaowen, Zhang Fanfan, Gui Xiahui, Cao Yijun
Chinese National Engineering Research Center of Coal Preparation and Purification, China University of Mining and Technology, Xuzhou 221116, China.
School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, China.
Fundam Res. 2022 May 14;4(1):35-42. doi: 10.1016/j.fmre.2022.05.005. eCollection 2024 Jan.
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, 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, 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.
自从首次对固液界面纳米气泡（INBs）进行成像以来，它们的长期稳定性和大接触角一直困扰着科学家们。本研究旨在通过分子动力学模拟研究内部气体密度和外部气体单分子层对INB接触角和稳定性的影响。首先，在不同氮气密度下模拟了水滴的接触角。结果表明，接触角随氮气密度的增加而急剧增加，这主要是由固气界面张力的降低引起的。然而，当氮气密度达到2.57纳米时，在固体和水之间形成了一个中间气体单分子层（GML）。GML形成后，接触角随气体密度的增加而略有增加。当氮气密度达到11.38纳米时，接触角增加到180°，这表明当INBs太小时会转变为气体层。对于不同疏水性的基底，GML形成后的接触角总是大于140°，并且与基底疏水性弱相关。接触角随气体密度的增加代表了接触角从宏观气泡到纳米气泡的演变，而GML的形成可能对应于稳定的INBs。平均力势曲线表明，与裸基底相比，具有GML的基底可以在不存在势垒的情况下从更长距离吸引气体分子，这表明GML有作为气体收集板的潜力。进一步的研究表明，GML可以作为将气体分子输送到INBs的通道，这有利于INBs的稳定性。这项研究可能为INBs异常接触角和长期稳定性的潜在机制提供新的线索。
