Nanobubbles at Hydrophobic Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 167

A remarkably different novel morphology for a nano-scale gaseous state at a hy-
drophobic surface has also been reported [16]. This differs markedly from the spher-
ical caps that are generally reported and consists of a gaseous layer < 5 nm thick that
extends in the lateral direction for microns with constant height. This shape has led
to these gaseous states being dubbed nanopancakes. Imaged on HOPG, the shape of
these nanopancakes is clearly determined by the atomic layer steps in the underly-
ing suHas thbstrate. The Van der Waals force across such nanopancakes should be
attractive and contribute to the instability of such structures. It remains a mystery
as to how this morphology may arise, but it is known that it can be present along
with the formation of nanobubbles and it has only been observed on crystalline
surfaces, such as HOPG, MoS2 and talc [21]. Recent studies show that micropan-
cakes can be made up of discrete layers and that the micropancakes grow slowly
on the timescale of hours [22]. Even more mysteriously, composite structures are
seen where a nanobubble is present and free to move around on top of a nanopan-
cake. This strongly suggests that a thin film remains between the two structures.
Perhaps very small graphitic particles that arise from the substrate are operating
to stabilize such films by the same mechanism as emulsions are stabilized by par-
tially hydrophobic particles. This discovery poses several significant challenges to
our understanding of stable gas states at hydrophobic surfaces and clearly warrants
further investigation.
Another interesting feature of nanobubbles is their role as possible nuclei for
macroscopic bubbles. Interestingly they have been found that they do not act as
nucleation sites when shock waves are used to induce cavitation [23]. However
they do nucleate vapor bubbles upon superheating. Their possible role as nuclei for
macroscopic bubbles has important implications and warrants further investigation
[17].
E. How to Produce Nanobubbles
One of the earliest publications reporting images of nanobubbles also included
a clear description of a method by which nanobubbles could be produced on a
hydrophobic surface [5] and several subsequent papers have adopted this method-
ology. Despite this many investigations of nanobubbles have been conducted with-
out any effort to control the conditions that may influence nanobuble produc-
tion, though degassing has commonly been employed to prevent the formation of
nanobubbles or remove nanobubbles already present [6, 24]. This is the cause of
much of the confusion in the literature, indeed many studies reporting null findings
for nanobubbles did not employ conditions under which one might expect to find
them.
The solvent exchange technique [5] for producing nanobubbles operates by in-
ducing a supersaturation of gas at the interface. Common atmospheric gases such
as nitrogen and oxygen are present in water in millimolar quantities at atmospheric
pressure and room temperature. In many other solvents even greater quantities of