Nanobubbles at Hydrophobic Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 169

ever, we have seen that the morphology of nanobubbles is such that the interfacial
curvature only gives rise to a moderate internal pressure and nanobubbles can be
maintained under these conditions for long periods with a moderate supersaturation
of gas.
2. Surface Investigations

A number of surface sensitive experiments have been conducted on hydrophobic
surfaces in water that should reveal the presence of nanobubbles. These can be
categorized as ellipsometric studies and scattering studies.

2.1. Optical Methods
Nanobubbles present at a hydrophobic interface should provide excellent contrast
in ellipsometric studies, as the refractive index of the gas phase will be near unity
and as such it will be very different to both the aqueous phase ( n = 1 . 333) and solid

substrate ( n 1 . 35). However, at least two investigations that have sought to find

vapor or air layers at the interface between hydrophobic surfaces and water have
reported that no such layers are found [29, 30]. The sensitivity limits of these stud-
ies demand that a reasonable surface coverage of nanobubbles exist if they are to
be revealed. It has been estimated that in each 10 µm

≈

10 µm area more than 1000
nanobubbles would need to be present in order for them to be detected ellipsomet-
rically [29]. In neither case were procedures such as solvent exchange purposely
adopted to produce nanobubbles, so it is unsurprising that nanobubbles were not
found in these studies, particularly as this was not their aim. A repetition of these
investigations using the solvent exchange technique or electrochemical methods to
produce nanobubbles is likely. Further evidence for the existence of interfacial air
bubbles produced by solvent exchange was obtained from surface plasmon reso-
nance (SPR) measurements [13]. After the formation of nanobubbles, the resonant
angle shifts to a lower angle, demonstrating the displacement of water (refractive
index

×

∼

1 . 33 ) by a material of lower refractive index.
2.2. Scattering at the Interface
There is much interest in the structure of water adjacent to hydrophobic interfaces
and as such numerous scattering studies aimed at the interface have been conducted.
Given the large mass density and electron density difference between a gas phase
and either the hydrophobic substrate or water one might expect that nanobubbles
would be readily revealed by such investigations, yet several scattering studies re-
port no evidence for nanobubbles. Neutron reflectivity studies have reported results
that have been interpreted as consistent with the precursor of nanobubbles [31, 32]
but later investigations found no evidence for the existence of nanobubbles [33].
These latter experiments are supported by X-ray scattering studies [34, 35] that re-
veal evidence for a lower density of water adjacent to a hydrophobic surface but
no evidence of nanobubbles. None of these investigations employed conditions that
would be expected to produce nanobubbles, so it is unsurprising that evidence of
nanobubbles was not found. A recent investigation employing solvent exchange to