W. Richard Bowen and Nidal Hilal 4

9.6 MESOSCALE ExPERIMENTAL STUdIES 261
the greatest effort in cavitation damage research has involved the study of
bubble collapse and its consequences. Lord Rayleigh’s seminal analysis of
the collapse of an isolated spherical void in an incompressible liquid [87]
leads to the conclusion that, as the collapse nears completion, the pressure
inside the liquid becomes indefinitely large. It is principally this ‘Rayleigh
collapse’ mechanism (albeit extensively modified) which has led to the
association of bubble collapse with cavitation damage [88].
Little is known about the dynamics of cavities formed in thin films but,
due to their inevitably close proximity to the film’s bounding surfaces, significant
departures from spherical symmetry may be anticipated [89]. The
asymmetry of cavity collapse leads to potentially damaging phenomena,
such as liquid jets [90], but the issue of cavitation damage due to cavity
growth has received relatively little attention, despite the possibly damaging
consequences to capillaries and small blood vessels due to the intraluminal
expansion of cavitation bubbles in the areas of laser angioplasty [91],
electrohydraulic lithotripsy [92] and shock wave lithotripsy [93].
9.6 MESoSCALE ExPERIMEnTAL STuDIES oF
ThE TEnSILE BEhAvIouR oF ThIn FLuID FILMS
As discussed, many processes of emerging scientific and technological
interest involve the rheological behaviour of complex fluids in the mesoscale
domain (ca. 0.1–10 �m), and in order to study the mesoscopic behaviour of
fluids, particularly filament formation (i.e. extensional flow) and cavitationinduced
failure and tack, it is necessary to satisfy some basic requirements.
Notwithstanding the fact that the creation of a liquid bridge between a colloid
probe and a flat surface is a fairly straightforward process, manipulation
of the desired quantity of fluid is not. Therefore in addition to recording
the evolution of tensile forces, the deformation of the fluid should ideally be
recorded. Although instruments such as the AFM suggest themselves for
adaptation in terms of the former requirement, the latter task (i.e. recording
the deformation of mesoscale filaments with sufficient temporal resolution)
is non-trivial. But it is one which must be accomplished as a precursor to the
development of a satisfactory mesoscale extensional flow technique.
Extensional flows of complex fluids may be studied using several macroscale
techniques such as the filament stretching rheometer (FSR) or the
capillary break-up extensional rheometer (CaBER), whose illustrations are
shown in Figures 9.6 and 9.7. In these techniques the characteristic dimensions
and the quantity of liquid are known, whereas the interpretation of
results from AFM-based microrheometry is restricted by the necessary
assumptions about the flow field.
As a colloid probe moves rapidly away from a surface, it is reasonable
to assume that the fluid confined between the surface and the sphere may