In
this article, we have studied the surface nanobubbles on polystyrene
(PS)/water interfaces using tapping mode atomic force microscopy (TM-AFM).
Detailed bubble coalescence phenomenon of differently sized surface
nanobubbles (with lateral size up to about ∼10 μm) was
obtained. The quantity of gas molecules, before and after coalescence,
was calculated. It was found that after coalescence the quantity of
gas molecules was increased by approximately 112.5%. The possible
reasons for this phenomenon were analyzed and discussed. Our analysis
shows that a reasonable explanation should be an influx of gas into
the bubble caused by the depinning of the contact line and the decrease
in the inner pressure during bubble coalescence. The factors affecting
the coalescence speed of surface bubbles were also discussed. It was
found that the coalescence speed of larger bubbles is usually slower
than that of the smaller ones. We also noticed that it is uncertain
whether a larger or smaller bubble will move first to merge into others.
This is due to the combined effects of the contact line and the surface
properties. Furthermore, the temporal evolution of surface bubbles
was studied. The three-phase contact line of bubbles kept the pinning
within the incubation time. This was consistent with the contact line
pinning theory, based on which the theoretical lifetime of the surface
bubbles in our experiments was calculated to be tb ≈ 6.9 h. This value is close to the experimental
results. Meanwhile, the faster gas diffusion from the oversized bubbles
after 12 h of incubation was observed and analyzed. Our results indicate
that a viable stability mechanism for surface nanobubbles would be
favored simultaneously by the contact line pinning, gas influx near
the contact line from an interfacial gas enrichment (IGE), a thin
“contaminant film” around the gas/liquid interface,
and even the electrostatic effect.