Experimental studies have demonstrated that the gas phase contact angle (CA) of a surface nanobubble (SNB) is much smaller than that of a macroscopic gas bubble. This reduced CA plays a crucial role in prolonging the lifetime of SNBs by lowering the bubble pressure and preventing gas molecules from dissolving in the surrounding liquids. Despite extensive efforts to explain the anomalously small CA, a consensus about the underlying reasons is yet to be reached. In this study, we conducted experimental investigations to explore the influence of gas molecules adsorbed at the solid–liquid interface on the CA of SNBs created through the solvent exchange (SE) method and temperature difference (TD). Interestingly, no significant change is observed in the CA of SNBs on highly oriented pyrolytic graphite (HOPG) surfaces. Even for nanobubbles on micro/nano pancakes, the CA only exhibited a slight reduction compared to SNBs on bare HOPG surfaces. These findings suggest that gas adsorption at the immersed solid surface may not be the primary factor contributing to the small CA of the SNBs. Furthermore, the CA of SNBs formed on polystyrene (PS) and octadecyltrichlorosilane (OTS) substrates was also investigated, and a considerable increase in CA was observed. In addition, the effects of other factors including impurity, electric double layer (EDL) line tension, and pinning force upon the CA of SNBs were discussed, and a comprehensive model about multiple factors affecting the CA of SNBs was proposed, which is helpful for understanding the abnormally small CA and the stability of SNBs.

中文翻译：

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全面了解表面纳米气泡的异常小接触角

实验研究表明，表面纳米气泡（SNB）的气相接触角（CA）远小于宏观气泡的气相接触角。这种减少的 CA 通过降低气泡压力并防止气体分子溶解在周围液体中，在延长 SNB 的寿命方面发挥着至关重要的作用。尽管人们做出了大量努力来解释异常小的CA，但其根本原因尚未达成共识。在本研究中，我们进行了实验研究，探讨固液界面吸附的气体分子对通过溶剂交换（SE）方法和温差（TD）制备的SNB的CA的影响。有趣的是，在高取向热解石墨（HOPG）表面上的 SNB 的 CA 没有观察到显着变化。即使对于微/纳米煎饼上的纳米气泡，与裸 HOPG 表面上的 SNB 相比，CA 也仅表现出轻微的降低。这些发现表明，浸入固体表面的气体吸附可能不是导致 SNB 的 CA 小的主要因素。此外，还研究了在聚苯乙烯（PS）和十八烷基三氯硅烷（OTS）基材上形成的SNB的CA，并观察到CA的显着增加。此外，还讨论了杂质、双电层（EDL）线张力和钉扎力等其他因素对SNBs CA的影响，并提出了影响SNBs CA的多种因素的综合模型，有助于了解异常小的 CA 和 SNB 的稳定性。