Nanobubbles at Hydrophobic Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 163

were visible when a hydrophilic surface was employed or if the hydrophobic sur-
face was used in conjunction with degassed water. In contrast, voids were found on
hydrophobic substrates in contact with gassed water that are commensurate in size
with the images of nanobubbles obtained by AFM studies. These findings parallel
the imaging studies of Ishida et al. [6]. Whilst this study is important in that evi-
dence for nanobubbles was found using an alternative technique, it was unable to
provide additional morphological information.
Both AFM images and the cryofixation and freeze fracture techniques provide
evidence for the existence of structures at the interface but they do not provide
chemical information on the make-up of these structures. It was possible, for ex-
ample, that these structures are formed by an organic contaminant rather than being
gas-filled bubbles—though a range of evidence strongly suggested that they were
indeed bubbles such as observation of coalescence [11] and production in situ via
photocatalysis [12]. In an elegant experiment, Zhang, Khan and Ducker [13] have
demonstrated that nanobubbles do indeed consist of gas. They used CO 2 saturated
water to produce nanobubbles by the solvent exchange method [5]. CO 2 has an in-
frared spectrum that varies considerably between the gaseous and aqueous states
due to the rotational fine structure that is visible in the gaseous state and therefore
it can be used to directly reveal the phase state of the material in the nanobubbles.
Furthermore, from the rotational fine structure the pressure of the CO 2 gas within
the nanobubbles could be approximately determined and was found to be consistent
with that expected from the nanobubbles.
D. Nanobubble Characteristics
The size, shape, contact angle, surface tension and internal pressure within a
nanobubble are of interest not only in describing nanobubbles but also in address-
ing the issue of nanobubble stability. When looking at AFM images it should be
remembered that most are presented with a considerable vertical exaggeration. The
scale in the z dimension can be 100 times smaller than the scale in the x and y
dimensions. This can easily give rise to an incorrect perception of the shape of
nanobubbles. A sectional profile through a nanobubble can be very useful in gain-
ing an understanding of their true shape. However, due to imaging artifacts caused
by the deformability of nanobubbles a section taken through a standard tapping
mode image can be misleading. In water the tip of the cantilever penetrates the
nanobubble and leads to an uncertainty in the height. The degree of penetration is
dependent upon the imaging conditions, in particular the amount of energy that the
tip loses when contacting the surface (amplitude setpoint). Figure 2 illustrates the
influence of setpoint on the image in surfactant solution. As the setpoint amplitude
is reduced (a to g) the nanobubbles appear smaller due to the tip deforming the in-
terface to a greater extent. Images h-j, show that the nanobubble recovers when the
setpoint amplitude is increased.