Nanobubbles at Hydrophobic Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 168

gas are dissolved. A solvent miscible with water and with a greater solubility of
atmospheric gases is chosen. Ethanol is most commonly used as it is available in
very high purity. The substrate is immersed in ethanol and then the ethanol is dis-
placed by water. Ethanol preferentially wets hydrophobic substrates and therefore
the ethanol at the surface is not easily displaced, but as it is miscible with water it
will mix with the aqueous phase. In doing so, the gases that were dissolved in the
ethanol will be 'precipitated' at the interface, producing nanobubbles as the solubil-
ity of gas in the aqueous phase is exceeded. It is unclear at this stage if the solvent
exchange method also leads to supersaturation of the bulk aqueous phase or even
if it produces (if only momentarily) nanobubbles in the bulk aqueous phase. The
generality of this technique has been demonstrated by using the same approach to
precipitate liquid organic materials at hydrophobic interfaces [25].
Electrochemical methods have also been used for the generation of nanobub-
bles [26]. Using HOPG as a conducting hydrophobic substrate, a battery is used to
complete an electrochemical cell such that hydrogen gas is formed at the surface
of the HOPG by electrolysis of water. After only a small amount of current is al-
lowed to flow the local concentration of gas is such that the solubility is exceeded
and nanobubbles are produced. Interestingly nanobubbles are more easily produced
at a neagative electrode. That is, the production of hydrogen nanobubbles is more
effective than the production of oxygen nanobubbles [27]. This method was not ini-
tially employed as extensively as the solvent exchange method due to the necessity
of having a conducting hydrophobic substrate. However, it is now attracting more
interest due to the improved control it offers over the conditions of nanobubble pro-
duction. A little used method for the production of nanobubbles makes use of the
photocatalytic properties of titania. Under exposure to UV light in water/methanol
solution hydrogen nanobubbles are produced [12].
Whilst these methods are well established and both are dependent upon the nu-
cleation of a gas phase nothing is currently known about the nucleation process.
Surprisingly it appears that nanobubbles do not act as nucleation sites for the pro-
duction of macroscopic bubbles [23].
F. Why Nanobubbles Were Thought not to Exist
The existence of nanobubbles remains a controversial topic despite mounting evi-
dence for their existence. The controversy was clearly demonstrated when in 2007
the prestigious journal Nature ran an article in the research highlights section which
headlined 'Physics: No nanobubbles' [28]. Here we will address the theoretical and
experimental work that has been cited as evidence that nanobubbles do not exist.
We address these arguments and demonstrate that none of this work is able to rule
out the existence of nanobubbles.
1. Bubble Lifetime
As described in the introduction, the existence of nanobubbles is thought to be
precluded by their rapid dissolution under a high internal Laplace pressure. How-