W. Richard Bowen and Nidal Hilal 4
W. Richard Bowen and Nidal Hilal 4
W. Richard Bowen and Nidal Hilal 4
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9.6 MESOSCALE ExPERIMENTAL STUdIES 261<br />
the greatest effort in cavitation damage research has involved the study of<br />
bubble collapse <strong>and</strong> its consequences. Lord Rayleigh’s seminal analysis of<br />
the collapse of an isolated spherical void in an incompressible liquid [87]<br />
leads to the conclusion that, as the collapse nears completion, the pressure<br />
inside the liquid becomes indefinitely large. It is principally this ‘Rayleigh<br />
collapse’ mechanism (albeit extensively modified) which has led to the<br />
association of bubble collapse with cavitation damage [88].<br />
Little is known about the dynamics of cavities formed in thin films but,<br />
due to their inevitably close proximity to the film’s bounding surfaces, significant<br />
departures from spherical symmetry may be anticipated [89]. The<br />
asymmetry of cavity collapse leads to potentially damaging phenomena,<br />
such as liquid jets [90], but the issue of cavitation damage due to cavity<br />
growth has received relatively little attention, despite the possibly damaging<br />
consequences to capillaries <strong>and</strong> small blood vessels due to the intraluminal<br />
expansion of cavitation bubbles in the areas of laser angioplasty [91],<br />
electrohydraulic lithotripsy [92] <strong>and</strong> shock wave lithotripsy [93].<br />
9.6 MESoSCALE ExPERIMEnTAL STuDIES oF<br />
ThE TEnSILE BEhAvIouR oF ThIn FLuID FILMS<br />
As discussed, many processes of emerging scientific <strong>and</strong> technological<br />
interest involve the rheological behaviour of complex fluids in the mesoscale<br />
domain (ca. 0.1–10 �m), <strong>and</strong> in order to study the mesoscopic behaviour of<br />
fluids, particularly filament formation (i.e. extensional flow) <strong>and</strong> cavitationinduced<br />
failure <strong>and</strong> tack, it is necessary to satisfy some basic requirements.<br />
Notwithst<strong>and</strong>ing the fact that the creation of a liquid bridge between a colloid<br />
probe <strong>and</strong> a flat surface is a fairly straightforward process, manipulation<br />
of the desired quantity of fluid is not. Therefore in addition to recording<br />
the evolution of tensile forces, the deformation of the fluid should ideally be<br />
recorded. Although instruments such as the AFM suggest themselves for<br />
adaptation in terms of the former requirement, the latter task (i.e. recording<br />
the deformation of mesoscale filaments with sufficient temporal resolution)<br />
is non-trivial. But it is one which must be accomplished as a precursor to the<br />
development of a satisfactory mesoscale extensional flow technique.<br />
Extensional flows of complex fluids may be studied using several macroscale<br />
techniques such as the filament stretching rheometer (FSR) or the<br />
capillary break-up extensional rheometer (CaBER), whose illustrations are<br />
shown in Figures 9.6 <strong>and</strong> 9.7. In these techniques the characteristic dimensions<br />
<strong>and</strong> the quantity of liquid are known, whereas the interpretation of<br />
results from AFM-based microrheometry is restricted by the necessary<br />
assumptions about the flow field.<br />
As a colloid probe moves rapidly away from a surface, it is reasonable<br />
to assume that the fluid confined between the surface <strong>and</strong> the sphere may