Frictional Forces
Friction between two surfaces depends on the chemical and mechanical
interaction between the surfaces. Changes in chemical composition giving
rise to friction are measurable with the AFM. The technique for measuring
these forces is called lateral force, or frictional force microscopy.
As the probe moves over a surface in the AFM, changes in the chemical
composition of the surface can give rise to torsions of the cantilever
on which the probe is mounted. The torsion of the cantilever is then
proportional to the friction between the probe and the surface.
Figure 4: Illustration of the principle that allows lateral force or
frictional fore microscope images to be measured. The cantilever will
twist as the probe interacts with the surface while scanning.
In an AFM it is possible to simultaneously measure topography and frictional
force images. The topography image is derived from monitoring the vertical
forces on the cantilever and the friction image is acquired simultaneously
by monitoring the lateral motions of the cantilever. Below is a FFM image of
a sample illustrating changes in the friction.
Figure 5: (left) Two dimensional view of a composite material measured
with the AFM. Barely visible in this image are striation marks derived
from a change in chemical composition at the surface.
(right) The lateral force or frictional force image of the composite
material clearly shows changes in chemical composition at the surface
of the composite material. Both the topography and frictional force
image are measured simultaneously with the AFM.
|
