Scanning force microscopy (SFM) of topography, frictional force
and stiffness on thin gelatin films reveals an entire spectrum
of tribological behavior. Images display two distinct phases of
gelatin whose characteristics relate to the degree of crystallinity.
Dissimilar regimes of velocity- and and load-dependent friction
and wear on each phase indicate glassy, rubbery or melt behavior.
Of fundamental importance is the finding that energy transferred
to the film in the vicinity of the sliding SFM microasperity modifies
film response. Moderate frictional heating melts the highly-crystalline
phase, but reversibly induces rubbery behavior on the partially-amorphous
phase. More extreme frictional heating melts the latter and allows
the liberated molecules to reassociate irreversibly into the highly-crystalline
phase. This relatively slow process (minutes) is imaged in real
time on the submicron scale. Relaxation from rubbery to glassy
behavior upon termination of perturbative scanning is extremely
slow (hours) and also is characterized in frictional images. In
this case the imaging process itself hastens the relaxation, however,
apparently by providing energy to activate some relaxational processes.