Biomedical Engineering Reference
In-Depth Information
challenging is substantial evidence that nanobubbles of radius 50 nm exist in solu-
tion for up to two weeks [45]. Whilst these solutions are significantly supersaturated
they are not sufficiently so to explain such longevity and their presence in large
concentrations and the concomitantly large surface area reduces the possibility of
stabilization by impurities.
H. Uses for Nanobubbles
Nanobubbles are inherently interesting and are also proving to be useful. As we
have seen we can already control the conditions under which they are produced and
a recent report has described a means by which the size and location of nanobubbles
can be controlled [46], so it appears that we will be able to manipulate nanobub-
bles to our advantage. It has been shown that nanobubbles can be used to prevent
surface fouling by proteins [47] and are also effective at removing proteins from an
already fouled surface [47, 48]. By combining these technologies it should be pos-
sible to controllably pattern proteins on a surface, where the nanobubbles are used
as masks. The presence of a gas phase at the interface is crucial in superhydropho-
bicity and can lead to hydrodynamic boundary slip [49]. Thus the manipulation of
nanobubbles could be used to actively change the contact angle of a surface or the
flow properties of a fluid. Such manipulations are likely to be employed in microflu-
idic devices either as switching or mixing technologies. The nanobubbles for these
applications could be produced on demand using electrolysis.
Recently nanobubbles have found application in medicine for drug delivery and
the treatment of cancers often in concert with ultrasound [50]. The ultrasound is
used to create nanobubbles or act on pre-existing nanobubbles to promote cellular
uptake of drugs. Nanobubbles have been produced from short laser pulses imping-
ing on metal nanoparticles resulting in the photothermal production of nanobubbles
[51] with the aim of targeting cancer cells [52-54]. Nanobubbles have even been
implicated in claims of homeopathic action taking place at extreme dilutions [55].
Further, there are circumstances where the presence of nanobubbles is unwanted.
Their presence on sensors can cause significant measurement errors. Examples in-
clude magnetoelastic gravimetric sensors and normal operation of the quartz crystal
microbalance [56, 57]. Nanobubbles are also thought to be the source of defects in
electroplating and electropolymerised films [58, 59].
I. Conclusion
The investigation of nanobubbles is still in its infancy and many challenges remain.
An understanding of the very high contact angle produced by nanobubbles is impor-
tant as it is critical to their stability. The newly reported nanopancakes pose several
challenges. Why are they stable? What is the internal pressure? How can we rec-
oncile the rather flat interface with the high interfacial curvature at the edge of the
pancake? How can a nanopancake and a nanobubble interact but remain as separate
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