W. Richard Bowen and Nidal Hilal 4

262 9. APPLICATION OF ATOMIC FORCE MICROSCOPy
FIguRE 9.6 Formation of macroscopic viscoelastic liquid filaments formed in a filament
stretching rheometer. The fluid is constrained between the ends of two cylinders, which are
pulled apart at a specified rate. The generated tensile force and the filament profile are used
to calculate the extensional stress. The cylinder shown is 10 mm in diameter.
FIguRE 9.7 Images showing a typical CaBER experiment on a biopolymer solution.
The surfaces are separated at a predetermined distance, thereafter the temporal evolution of
the surface profile is used to determine rheological properties. Cylinder diameter is 2 mm.
be extended. If the initial minimum separation distance is sub-micron
and the retraction event is sufficiently rapid, then the stresses generated
within the liquid may approach or exceed the tensile strength of the liquid.
However, in AFM studies the fluid is not directly observed, therefore
fluid draining, viscoelastic filamentatious behaviour or cavitational failure
is not readily substantiated. It is, therefore, useful to examine those
means by which the deforming liquid can be observed and documented.
The separation distances considered in most AFM force studies are near
to, or beyond, the limits of conventional optical microscopy. Although it
is not possible to observe nanofilaments through an optical microscope,