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•An astigmatic HS-AFM was modified to image the formation process of surface nanobubble.•The HS-AFM can image nanobubbles at a scan rate of 100l/s (640μm/s) without damaging them.•The initial nanobubble formation began with unstable flat structures, which later stabilized into cap-shaped structures.•Migration of an unstable structure was captured.
The initial formation process of nanobubbles at solid–water interfaces remains unclear because of the limitations of current imaging techniques. To directly observe the formation process, an astigmatic high-speed atomic force microscope (AFM) was modified to enable imaging in the liquid environment. By using a customized cantilever holder, the resonance of small cantilevers was effectively enhanced in water. The proposed high-speed imaging technique yielded highly dynamic quasi-two-dimensional (2D) gas structures (thickness: 20–30 nm) initially at the graphite–water interface. The 2D structures were laterally mobile mainly within certain areas, but occasionally a gas structure might extensively migrate and settle in a new area. The 2D structures were often confined by substrate step edges in one lateral dimension. Eventually, all quasi-2D gas structures were transformed into cap-shaped nanobubbles of higher heights and reduced lateral dimensions. These nanobubbles were immobile and remained stable under continuous AFM imaging. This study demonstrated that nanobubbles could be stably imaged at a scan rate of 100 lines per second (640 μm/s).