Nanobubbles exist at solid–liquid
interfaces between pure
water and hydrophobic surfaces with very high stability, lasting in
certain cases up to several days. Not only semispherical but also
other shapes, such as micropancakes, are known to exist at such interfaces.
However, doubt has been raised as to whether or not the nanobubbles
are gas-phase entities. In this study, surface nanobubbles at a pure
water–highly ordered pyrolytic graphite (HOPG) interface were
investigated by peak force quantitative nanomechanics (PF-QNM). Multiple
isolated nanobubbles generated by the solvent-exchange method were
present on the terraced areas, avoiding the steps of the HOPG surface.
Adjacent nanobubbles coalesced and formed metastable nanobubbles.
Coalescence was enhanced by the PF-QNM measurement. We determined
that nanobubbles can exist for a long time because of nanoscale contact
angle hysteresis at the water–HOPG interface. Moreover, the
hydrophilic steps of HOPG were avoided during coalescence, providing
evidence that the nanobubbles are truly gas phase
