How Nanobubbles Nucleate at a Graphite/Water Interface?
Ing-Shouh Hwang1*, Chung-Kai Fang1, Hsien-Chen Ko1, Chih-Wen Yang1, Yi-Hsien Lu1
1Institute of Physics, Academia Sinica, Taipei, Taiwan
* presenting author:Ing-Shouh Hwang,
Nanobubbles at solid/water interface, often called surface nanobubbles or interfacial nanobubbles (INBs), have attracted much attention because of their potential implications for various interfacial phenomena and technical applications [1,2]. To date, most studies have focused on why INBs exhibit high stability and why they adopt a rather flat morphology. Although several models have been proposed to explain the high stability, no consensus has been reached. In addition, the nature of INBs remains highly debated. Another fundamental but rarely addressed issue is the mechanism by which INBs nucleate at hydrophobic/water interfaces. Here, we used advanced atomic force microscopy (AFM), frequency-modulation and PeakForce modes, to investigate the initial formation of gas-containing structures at the interface between water and highly ordered pyrolytic graphite (HOPG). In the very initial stage, a fluid phase first appeared as a circular wetting layer ~0.3 nm in thickness and was later transformed into a cap-shaped nanostructure (an INB). Two-dimensional gas-containing ordered domains [3,4] were nucleated and grew over time outside or at the perimeter of the fluid regions, eventually confining growth of the fluid regions to the vertical direction. We determined that INBs and fluid layers have very similar mechanical properties, suggesting low interfacial tension with water and a liquid-like nature, explaining their high stability and their roles in boundary slip and bubble nucleation. Our observations suggest that the ordered domains may be the interfacial hydrophilic gas hydrates and/or the long-sought chemical surface heterogeneities responsible for contact line pinning [2,5]. The gradual nucleation and growth of hydrophilic ordered domains renders the original homogeneous hydrophobic/water interface more heterogeneous over time. Scenarios are proposed to explain the evolution of the interfacial structures of gases.

[1] V.S.J. Craig, Soft Matter 7, 40 (2011).
[2] D. Lohse. & X. Zhang, Rev. Mod. Phys. 37, 981 (2015).
[3] Y.-H. Lu, C.-W. Yang, and I.-S. Hwang, Langmuir 28, 12691 (2012).
[4] Y.-H. Lu, C.-W. Yang , C.-K. Fang, H.-C. Ko, I.-S. Hwang, Sci. Rep. 4, 7189 (2014)
[5] C.-K. Fang, H.-C. Ko, C.-W. Yang , Y.-H. Lu, I.-S. Hwang, Sci. Rep. 6, 24651 (2016).

Keywords: water, hydrophobic/water interface, nanobubbles, atomic force microscopy