  Surface nanobubbles are stable gaseous phases in liquids that form on solid
substrates. While their existence has been confirmed, there are many open
questions related to their formation and dissolution processes along with their
structures and properties, which are difficult to investigate experimentally.
To address these issues, we carried out molecular dynamics simulations based on
atomistic force fields for systems comprised of water, air (N2 and O2), and a
Highly Oriented Pyrolytic Graphite (HOPG) substrate. Our results provide
insights into the formation/dissolution mechanisms of nanobubbles and estimates
for their density, contact angle, and surface tension. We found that the
formation of nanobubbles is driven by an initial nucleation process of air
molecules and the subsequent coalescence of the formed air clusters. The
clusters form favorably on the substrate, which provides an enhanced stability
to the clusters. In contrast, nanobubbles formed in the bulk either move
randomly to the substrate and spread or move to the water--air surface and pop
immediately. Moreover, nanobubbles consist of a condensed gaseous phase with a
surface tension smaller than that of an equivalent system under atmospheric
conditions, and contact angles larger than those in the equivalent nanodroplet
case. We anticipate that this study will provide useful insights into the
physics of nanobubbles and will stimulate further research in the field by
using all-atom simulations.
