Superstability of Surface Nanobubbles

Shock wave induced cavitation experiments and atomic force microscopy measurements of flat polyamide and hydrophobized silicon surfaces immersed in water are performed. It is shown that surface nanobubbles, present on these surfaces, do not act as nucleation sites for cavitation bubbles, in contrast to the expectation. This implies that surface nanobubbles are not just stable under ambient conditions but also under enormous reduction of the liquid pressure down to $-6\mathrm{MPa}$. We denote this feature as superstability.

Figure 1

Pressure signal from the shock wave generator recorded inside of the protective flask with the fiber optic probe hydrophone close to the surface of the chip. The line depicts the low pass filtered signal averaged over five recordings. Triggering the shock wave generator corresponds to time $t=0$.

Figure 2

Cavitation activity (left), and corresponding nanobubble density (right) imaged by AFM (topography images) for various probes. The length scales given in (A) also refer to (B), (C), and (D). (A) and (B): polyamide substrates; (B) after ethanol-water exchange (see text). (C) and (D): hydrophobized silicon substrates; (D) after ethanol-water exchange. Scanning velocities for the cases (A)–(D) are 46, 20, 10, and $40\mu \mathrm{m}/\mathrm{s}$, respectively. There is hardly any cavitation though the substrates are densely covered by surface nanobubbles. Note that the cavitation bubbles in (A) emerge exclusively from microscopic cracks in the surface, whereas the whole substrate is covered by nanobubbles. The cavitation bubbles in (C) presumably originate from surface contaminations. In (D) it is shown that nanobubbles are still stably present after the cavitation experiments.