W. Richard Bowen and Nidal Hilal 4

9.5 CAvITATION ANd AdHESIvE FAILURE OF THIN FILMS 259
The accuracy of the model proposed by Sader (1998) was evaluated
using several fluids including air, water, acetone, carbontetrachloride
and butanol, which presented wide viscosity and density ranges. Results
were presented for both precision fabricated and standard cantilevers,
and in both cases the theoretical and experimental results for the Q factor
and resonant frequency as determined from the thermal resonance
spectra were in close agreement [72] as were the viscosity and density
results [70], which were found to be in accordance with known bulk
measurements.
The resonance methods described provide a basis for the in situ determination
of both viscosity and density of inelastic liquids. However, the
use of resonance data to elucidate viscoelastic parameters is extremely
complicated. In this respect, the use of a modified Langevin model which
incorporates a complex drag coefficient in an attempt to overcome the
limitations of the simplified SHO model has also been studied [73–75].
9.5 CAvITATIon AnD ADhESIvE FAILuRE
oF ThIn FILMS
Due to the high deformation rates which typify many mesoscale phenomena,
such as film splitting, filamentation and cavitation processes,
significant viscoelastic effects may be anticipated. One such effect is a
delay in the cavitation of viscoelastic liquids in micrometre-sized gaps,
due to the development of normal stresses [76]. Others claimed that
viscoelastic effects include a displacement of the point of cavitation from
the centre of contact (where film thickness is a minimum) and enhanced
film thicknesses [77]. Little is known about the influence of viscoelasticity
in sub-micron liquid film cavitation, but the initial film thickness is a
crucial factor: for sufficiently thin films, even ostensibly low rates of
surface separation may provoke the high rates of fluid deformation necessary
to generate enough tension (through viscous forces) to result in
cavitation [78].
It is important to realise that in ultrathin films of water, cavitation may
occur spontaneously, due to the antipathy between the liquid and hydrophobic
surfaces between which it is confined [79]. Spontaneous cavitation
was first observed experimentally by Christenson and Claesson (1988)
[79]. Theory predicts that vaporous cavities will only form in pure liquids
as a result of large tensions, some 1300–1400 bar in the case of water [80],
although a somewhat higher figure (ca. 1900 bar) results from an interpretation
of the thermodynamic properties of stretched water known
as the stability limit conjecture [81]. Experiments involving very small
quantities of pure water have produced tensions close to this homogeneous
nucleation limit [82], but they are not commonly observed.