Nanobubbles at Hydrophobic Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 170

produce nanobubbles has revealed the presence of surface nanobubbles using X-ray
scattering [36].
Some of the presented evidence for nanobubbles has been questioned. Tyrell and
Attard [37, 38] reported AFM images depicting very high density coverage of fea-
tures at a hydrophobic surface. These were interpreted to be nanobubbles. However
the features presented in the images are consistent with a polymeric layer formed by
reaction of the dichlorodimethylsilane (used to make the surface hydrophobic) with
water vapor [39]. It is worth noting that the images of nanobubbles presented in
this work have a morphology that differs from the many other reports of nanobub-
bles in that the contact line is highly convoluted and it is unfortunate that many cite
this work when referring to images of nanobubbles when the prior investigations by
both Chinese [5] and Japanese [6] groups report images that are true representations
of nanobubbles.
During the imaging of nanobubbles using AFM it is necessary that the tip come
into contact with the nanobubble. This brings the tip in close proximity to the sub-
strate and it has been suggested that this process actually nucleates nanobubbles [29,
33]. That is, the belief is that nanobubbles are not present prior to the commence-
ment of the imaging process but rather arise as an artifact of the imaging process.
This idea has arisen from the force measurement community where the existence
of nanobubbles has been associated with the measurement of the hydrophobic at-
traction between two hydrophobic surfaces. It is known that upon separating two
hydrophobic surfaces in water a vapor phase can be produced [40]. However, AFM
tips are hydrophilic and as such there is no reason to believe that proximity to an-
other surface can nucleate bubbles. Further, one would expect that if the tip were
nucleating bubbles that the distribution and size of bubbles would increase with
continued imaging, whereas in most cases the image remains very stable for many
hours [5, 6, 9] and in other cases the tip is seen to sweep the surface free of nanobub-
bles (see Fig. 1, lower frame) [5]. More evidence that the tip is not responsible
for the nucleation of nanobubbles comes from investigations using the solvent ex-
change technique to produce nanobubbles. This technique causes supersaturation
of gas in the aqueous phase and on occasion this causes the nucleation of micron
sized bubbles, in areas adjacent to the presence of nanobubbles, which can be seen
with standard optical microscopy [9].
G. The Stability of Nanobubbles
The stability of nanobubbles remains an open question. It has been variously argued
that the stability arises from surface roughness [41], line tension [42], a dynamic
equilibrium between gaseous depletion layer and the nanobubble [43] and the pres-
ence of insoluble material at the surface of the nanobubble that arrests dissolution
[20]. The latter is an appealing explanation as one can imagine the presence of
such material is unavoidable however it has been argued that this effect alone is
insufficient to explain the extended stability of nanobubbles observed [44]. More