Long-Term Stability of Surface Nanobubbles in Undersaturated Aqueous Solution.

Surface nanobubbles should not be stable for more than a few milliseconds; however they have been shown to persist for days. Pinning of the three-phase contact line of surface nanobubbles has been proposed to explain the discrepancy between the theoretical and experimental results. According to this model, two factors stabilize surface nanobubbles, namely solution oversaturation and surface pinning. Hereby, we investigate experimentally the impact of the solution saturation on the stability of nanobubbles. For this purpose, surface nanobubbles have been nucleated on hydrophobic surfaces by two methods, and then characterized by Atomic Force Microscopy (AFM). Thereafter, the surrounding liquid has been exchanged multiple times with partially degassed water. Two degassing techniques are presented. Both sets of experiments lead to the conclusion that surface nanobubbles are stable in undersaturated conditions for hours. We compare the measured lifetime of nanobubbles to calculations for pinned nanobubbles in undersaturated conditions. The stability of surface nanobubbles in undersaturated solutions observed here is incommensurate with the pinning mechanism as the origin of the long-term stability of surface nanobubbles.