W. Richard Bowen and Nidal Hilal 4

10. FuTuRE PRosPECTs 279
In either case, as the system response is not simply related to the stress
and the strain in the fluid, there is at present no satisfactory way to quantitatively
determine the complex moduli. However, the promising results
of studies to date suggest that an AFM operating in dynamic mode is
suited to adaptation as a mechanical interferometer, i.e. a device that can
be used to determine the ‘time of flight’ of small amplitude shear waves
across a fluid-filled gap. This is analogous to the so-called virtual gap
rheometer (VGR), wherein the phase delays determined at two different
locations in the fluid can be used to determine the phase velocity of the
shear wave. By studying several frequencies, the frequency-dependent
behaviour of viscoelastic systems may be exploited to determine the gel
point of curing polymers.
Most importantly, the future prospects of all applications of AFM in
process engineering will be greatly enhanced by increased collaboration
between engineers, physical scientists, biological scientists, mathematicians
and instrument manufacturers. Among the most crucial of the multidisciplinary
themes that need to be addressed are the development of probes of
highly specified and reproducible physical properties; the development of
high-speed image acquisition so that dynamic processes may be followed;
the combination of AFM with other techniques in a single instrument,
especially techniques that allow chemical characterisation of surfaces and
the integration of AFM data acquisition and advanced mathematical
modelling software for data interpretation.