W. Richard Bowen and Nidal Hilal 4
W. Richard Bowen and Nidal Hilal 4
W. Richard Bowen and Nidal Hilal 4
- No tags were found...
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
268 9. APPLICATION OF ATOMIC FORCE MICROSCOPy<br />
Therefore when the initial surface separation distance h is large, i.e. the<br />
initial characteristic length of the fluid sample is �1 mm, the attainment<br />
of moderate extensional rates (�100 s �1 ) requires an initial separation<br />
velocity � 100 mm s �1 , which is readily achieved using linear drive systems.<br />
However, the filament length must increase exponentially as a function<br />
of time in order to maintain a constant rate of extension, a situation<br />
which most drive systems are unable to accommodate. Consequently,<br />
many tensile studies involving liquids are restricted to low rates of extension<br />
(�10 s �1 ) or small total strains. In this respect, the recreation of elongational<br />
flows at the mesoscale using piezoceramic technology presents<br />
one possible method by which high extensional strain rates <strong>and</strong> large total<br />
strains may be achieved. Furthermore, the difficulties encountered when<br />
attempting to measure rapidly changing, small tensile forces, which are<br />
generated within low viscosity, liquid filaments may also be addressed by<br />
utilising the precise force measuring capabilities of the AFM.<br />
The determination of tensile forces in flows which contain a dominant<br />
elongational component is readily achieved using an AFM force sensor;<br />
however, the ability to precisely determine elongational stress at large<br />
strains is only possible when the shape <strong>and</strong> size of the liquid layer is<br />
clearly understood. In this respect, it is necessary to document the evolution<br />
of a microfilament to ascertain the minimum diameter such that the<br />
maximum tensile stress may be calculated. This is particularly important<br />
as the transient evolution of the profile of Newtonian <strong>and</strong> non-Newtonian<br />
liquid bridges may vary significantly such that a representation of the<br />
tensile stress through terms such as F/R 2 may not be valid at large separations.<br />
This is because the critical dimension (i.e. the filament diameter) is<br />
not simply related to separation distance <strong>and</strong> is itself dependent upon the<br />
viscoelastic properties of the material.<br />
The ability of many fluid mechanical systems to perform the desired<br />
function is critically dependent upon the tensile properties of the material,<br />
<strong>and</strong> the inability of the fluid to sustain large tensions is often beneficial,<br />
e.g. in coating <strong>and</strong> lubricating flows. Characterisation of the shear, extensional<br />
<strong>and</strong> cavitational behaviour is clearly important, <strong>and</strong> the ability of<br />
an AFM to recreate high-rate deformations which are comparable to those<br />
encountered in industrial processes is of particular interest. Moreover, the<br />
ability to experimentally recreate film splitting, fibrillation <strong>and</strong> cavitational<br />
cohesive failure in one dynamic process is desirable. However, observation<br />
of rapid microscale phenomena is not straightforward, especially so<br />
when cavitational mechanisms are encountered, as the observation of the<br />
growth <strong>and</strong> collapse of vapourous cavities requires microsecond temporal<br />
resolution. Future work will extend the capabilities of the AFM–optical<br />
microscopy system to further investigate the presence <strong>and</strong> influence of cavitational<br />
effects prior to the apparent cohesive failure of lubricant layers. As<br />
such, it is necessary to evolve the design of the experiment <strong>and</strong> in the first