  Spherical-cap-shaped interfacial nanobubbles (NBs) forming on hydrophobic
surfaces in aqueous solutions have extensively been studied both from a
fundamental point of view and due to their relevance for various practical
applications. In this study, the nucleation mechanism of spontaneously
generated NBs at solid-liquid interfaces of immersed nanostructured hydrophobic
surfaces is studied. Depending on the size and density of the surface
nanostructures, NBs with different size and density were reproducibly and
deterministically obtained. A two-step process can explain the NB nucleation,
based on the crevice model, i.e., entrapped air pockets in surface cavities
which grow by diffusion. The results show direct evidence for the spontaneous
formation of NBs on a surface at its immersion. Next, the influence of size and
shape of the nanostructures on the nucleated NBs are revealed. In particular,
on non-circular nanopits we obtain NBs with a non-circular footprint,
demonstrating the strong pinning forces at the three-phase contact line.
