Biomedical Engineering Reference
In-Depth Information
Figure 1.
AFM tapping mode images of nanobubbles on HOPG in water. Image size 10 µm
×
10 µm.
In the lower image it can be seen that the imaging process has led to the removal of bubbles in a
4µm
×
4 µm area. Image taken from reference [5].
The most direct evidence for the existence of nanobubbles is provided by Atomic
Force Microscope images [5-9]. These images are obtained in 'tapping mode' in
aqueous solution whereby the cantilever and tip are oscillated at a frequency gen-
erally in excess of
10 kHz and the amplitude of the oscillation is used to control
the feedback of the instrument. That is, the surface separation is adjusted to keep
the oscillation amplitude at a predetermined level and the adjustment in separation
required is mapped in two dimensions to produce a height image. This technique is
commonly employed to image soft surfaces, such as biological samples, in fluids.
Tapping mode imaging of nanobubbles is challenging and the images are influenced
by the imaging conditions [9], nonetheless careful analysis has revealed a great deal
about the morphology of nanobubbles. The other common mode of imaging using
the AFM is contact mode, which is known to be more suitable for hard surfaces. In
general nanobubbles are too soft to be revealed by contact mode imaging.
Additional evidence for nanobubbles has been provided by rapid cryofixation
and freeze fracture [10], which has been used to study the interface between water
and a silicon substrate in both the native state and following treatment with hex-
amethyldisilazane vapor, which renders the surface hydrophobic. By freezing the
sample at a very rapid rate, the ice is trapped in an amorphous state and any struc-
tures present are preserved without modification. It is then necessary to fracture the
sample and image the interface. This is done using techniques that are well estab-
lished for the imaging of biological samples. This study found that no structures
∼
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