W. Richard Bowen and Nidal Hilal 4

260 9. APPLICATION OF ATOMIC FORCE MICROSCOPy
Berard et al. (1993) [83] showed that the free energy of a bridging cavity
is lower than that of liquid water when the surfaces are separated as far as
micrometres and claim that the fact that such cavities are not observed as
the two surfaces approach contact from far apart indicates that the liquid
between them is metastable, i.e. there is some barrier preventing cavitation.
The theory for the long-ranged hydrophobic attraction relies upon
this notion of induced cavitation – the force between two colloids in a
near-critical or a near-spinodal fluid is attractive and long-ranged – and
the connection between the spinodal attractions in the bulk and measured
long-range attractions between hydrophobic surfaces is the observed
cavitation [84].
Computer simulations by Berard et al. [83] on a Lennard–Jones liquid
confined between hard walls showed cavitation at small separations, and
that there was indeed a spinodal separation. Approaching this separation
it was found that the attractions were much stronger than the van der
Waals attraction and longer ranged. Qualitatively, a separation-induced
spinodal can account for the measured hydrophobic attractions.
In the case of cavitation which results from the development of fluid
mechanical stresses (as tension), Joseph [78] has pointed out that the concept
of ‘negative pressure’ is not particularly useful; it is more pertinent to
consider the state of stress experienced by a liquid. In doing so, it is convenient
to decompose the stress into a deviator and mean normal stress,
the most positive value of principal stresses being the maximum tension.
In order to facilitate a comparison of a liquid’s cavitation threshold stress
with the principal stresses at each point within the liquid, it is necessary
to know the flow field. In terms of studying cavitation inception within
mesoscale liquid films, this requirement imposes stringent experimental
demands as it requires a comparison of the cavitation threshold at each
point in a liquid sample with the principal stresses there. For liquids in
motion, cavitation criteria must be based not on the pressure, but on the
stress, and a cavitation bubble will open in the direction of maximum tension
in principal coordinates. An important point which emerges is that
a liquid can cavitate as a result of experiencing a shear deformation, the
resulting cavity being pulled open by tension in the direction defined by
principal stresses.
Of the few experimental techniques capable of working at (or below)
the mesoscale, the various ‘force microscopes’, such as the SFA, have been
the most successful, but instances of their use in studies of thin fluid films
are comparatively rare. Notable exceptions are provided by the work of
Israelachvili and co-workers [85] who observed the growth and disappearance
of vapour cavities in liquid films between separating mica surfaces in
SFA experiments. The growth of a cavity was claimed to represent a ‘new’
cavitation damage mechanism, insofar as surface damage occurred during
cavity inception [86]. This is an interesting finding given that by far