W. Richard Bowen and Nidal Hilal 4

246 9. APPLICATION OF ATOMIC FORCE MICROSCOPy
microscopy to differentiate between local mechanical properties is well
known, e.g. phase contrast maps obtained using tapping mode [2, 3] may
be used to identify the distribution of components in composite materials.
Alternatively, ‘adhesive’ forces may be determined by monitoring the
deflection of a calibrated cantilever during contact and withdrawal of a
probe from the surface of a ‘tacky’ material. During this process, the contact
time and applied force may be varied, and by performing multiple
tests at different locations, a representative ‘adhesive force map’ may be
constructed. The atomic force microscope (AFM) is clearly a powerful tool
for the investigation of forces which govern the mechanics of processes
occurring at or below the microscale, and the exceptional ability of atomic
force microscopy to determine forces associated with the microscale deformation
and flow of fluids is presently discussed.
An understanding of the rheology of complex fluids is of fundamental
importance in many practical engineering and biomedical applications.
Traditional rheometrical techniques, e.g. cone and plate rheometers, require
reasonably large volumes (i.e. several millilitres) of test fluid, which is often
undesirable as limited volumes of fluid may be available, e.g. in studies of
biological fluids. Many processes also involve the deformation of fluids in
geometric confinement, which are not clearly described by the macroscale
or bulk rheology alone, e.g. thin film lubrication, and in such instances, it
is desirable to characterise the fluid mechanics under ostensibly similar
conditions. Additionally, in such situations in situ measurement of fluid
rheology is impaired by either the confined space or the extreme process
environments.
The rheological behaviour of thin liquid films is an important aspect
of lubrication [4] and printing [5] – processes which often involve mesoscale
(0.1�10 �m) thickness films undergoing rapid deformation between
separating surfaces. In the case of fluid mechanical machinery, they are
usually solid surfaces whereas in biomechanics they may be flexible
surfaces such as biological membranes. The ‘cracking’ of knuckle joints has
been attributed to cavitation within mesoscale lubricating films of synovial
fluid [6] whereas surface damage in microelectromechanical systems
devices has been attributed to the cavitation of lubricant films [7]. The
ability of liquids to sustain tension is an important factor in the survival
of plants in which the cohesion–tension (C–T) theory has been proposed
to explain water transport [8]. The C–T theory assumes that water, when
confined in small tubes with wettable walls such as xylem elements, can
sustain a tension ranging from 3 to 30 MPa. The liquid forms a continuous
system in the water-saturated cell walls from the evaporating surfaces of
the leaves to the absorbing surfaces of the roots. During evaporation, the
reduction in water potential at the surfaces causes movement of water out
of the xylem, with water loss producing tension in the xylem sap that is
transmitted throughout the continuous water columns to the roots.