Nanobubbles at Hydrophobic Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 161

to prevent drainage of the liquid film between the bubble and the surface, as pre-
dicted by the DLVO theory of colloidal interaction. However, if the silica surface
was made hydrophobic by silanation, the liquid film became unstable at thicknesses
of the order of 100 nm. This is notable as the silanation process was found to have
almost no effect on the surface charge of the silica surface and therefore one ex-
pects that the long-range forces would be unchanged. At the time the authors were
concerned that their data may have been affected by the existence of small particles
on the surfaces, though it now seems likely that the presence of nanobubbles may
have influenced their results.
A significant step in the development of research into nanobubbles came with
the arrival of the new millennium when images of nanobubbles obtained using
the Atomic Force Microscope (AFM) were published by two independent groups
working in China [5] and Japan [6]. It is noteworthy that both of these reports
demonstrated that the presence, or absence, of nanobubbles is strongly dependent
upon the history of the sample, though the wider community still often fails to grasp
the significance of this. In the work by Ishida et al. [6] they were able to demonstrate
that the range of the hydrophobic attractive force was extended when nanobub-
bles were present on the surface. Nanobubbles were found to have a diameter of
∼ 40 nm. Importantly they reported that nanobubbles were

found when a hydrophobic surface was immersed in water, whereas if the surface
was initially hydrophilic and rendered hydrophobic by chemical reaction without
exposure to air, no nanobubbles were found. Further evidence that the features in
the images were actually gas pockets was provided by employing degassed water, in
which no nanobubbles were found. The work by the Shanghai group [5], focused on
the production and imaging of nanobubbles and they presented some high quality
detailed images of nanobubbles on both mica surfaces and Highly Ordered Py-
rolytic Graphite (HOPG) surfaces (see Fig. 1). One presumes that the mica surface
bore a small amount of contamination that rendered it hydrophobic, at least in parts.
Indeed the authors report that the production of nanobubbles on mica was intermit-
tent. Importantly in this work the authors describe the first method of controllably
producing nanobubbles. This is achieved by a solvent exchange method which is
discussed in more detail below. Both of these reports state that the nanobubbles
observed were stable for hours.

650 nm and a height of ∼

C. Confirmation of the Existence of Nanobubbles

Surface force measurements have inferred the presence of nanobubbles on hy-
drophobic surfaces. Steps are reported in plots of force versus separation that are
not described by any conventional surface force. Whilst an estimate of the height
of nanobubbles can be obtained from such measurements very little other informa-
tion is revealed, therefore other techniques are required in order to demonstrate the
existence of nanobubbles and to investigate their properties.