Introduction
In atomic force microscopy (AFM), a
sharp tip is scanned over a sample while
maintaining a constant interatomic
force between atoms on the end of
the tip and atoms on the surface of
interest (1). Early implementation
of AFM employed the contact mode
of operation in which the repulsive
force experienced by the tip was
measured by recording the cantilever
deflection. Under ambient conditions,
these repulsive forces range from a
few tens to hundreds of nanonewtons.
The forces between the AFM tip and
the sample cause pressures large
enough to induce distortion, remove
portions of the sample from the
substrate, and even damage the tip.
By scanning in a liquid, capillary forces
are reduced, which decreases the
forces between the tip and sample by
orders of magnitude. Other forces such
as contact pressures, however, can
still lead to dulling of the AFM tip and
deformation of soft biological samples.