Nanobubbles at Hydrophobic Surfaces - Drops and Bubbles in Contact with Solid Surfaces - page 166

contact angle aids in our understanding of the stability of nanobubbles it is not clear
as to why the contact angle should be so high. It has been suggested that the line
tension at the three-phase line is significant and for such small volumes can have
a significant effect on the contact angle [7]. An alternative explanation is that very
small scale roughness can lead to pinning of the interface and result in a contact
angle that is far from the equilibrium value [16]—though one report of nanobub-
bles on HOPG surfaces reports nanobubbles with more conventional contact angles
of 119 ◦ and attributes the high contact angles measured to contamination. We feel
this explanation is unlikely as many different groups have measured similarly high
contact angles on surfaces that are more robust that HOPG. Nam and Jua study
the periodic nucleation of bubbles upon superheating and found behavior consis-
tent with the presence of nanobubbles with very high contact angles (

170 ◦ ) [17].
It is worth noting that nano-scale liquid droplets do not have contact angles that
differ from the macroscopic value. This suggests that the nanobubble contact angle
anomaly is associated with the low density of molecules in the gaseous phase and
their confinement in the z direction. The height of a nanobubble is of the order of
the mean free path, so in the z direction a molecule can travel across a nanobubble
without colliding with any others [18].
The size range of stable surface nanobubbles remains unclear. Nanobubbles have
not been observed with heights greater than 100 nm and base diameters of greater
than 1000 nm [19]. It currently cannot be determined what the lower limit is to the
size of nanobubbles due to difficulties associated with imaging soft materials using
the AFM. Very small bubbles may exist but if they are penetrated by the tip such
that the tip strikes the substrate they may be very difficult to discern.
The internal pressure in a nanobubble is not only controlled by the curvature of
the interface but also the surface energy of that interface. One can imagine that the
high energy air-water interface could readily adsorb contaminants from solution,
particularly as it ages. This will result in a lower surface tension and a reduced
Laplace pressure. Zhang et al . [9] have been able to estimate the surface tension
of a nanobubble in a non-ionic surfactant solution by evaluating the deformation of
the nanobubble interface under the pressure of the AFM tip. A surface tension of
43 mN/m was obtained which is commensurate with literature values for macro-
scopic interfaces with the same concentration of this surfactant. Consequently the
Laplace pressure would be reduced to

∼

60% of the Laplace pressure present for
a clean water interface and the same bubble geometry. Unfortunately this method
cannot be applied to nanobubbles in the absence of surfactant as the tip of the can-
tilever penetrates the nanobubble. The difference in the interaction between tip and
nanobubbles in water and surfactant solutions suggests that nanobubbles in water
have an interface that is not significantly contaminated [9], however a small amount
of insoluble contaminant at the nanobubble surface may strongly influence the sta-
bility of nanobubbles [20]. The development of a general method to measure the
surface tension of nanobubbles would be of great utility.

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