Thesis-PDF - IAP/TU Wien

- Force curves provide detailed information on viscoelastic properties of the
material. They record the response of the material to forces applied by the
cantilever tip.
Force curves
Force curves are an excellent tool to investigate the viscoelastic properties of a material
at the nanoscale. The main elements of a force curve are shown in Fig. 3.10.
A force curve is made by pressing and releasing the cantilever against and from the
surface. The material responds through small deformations to the exerted forces.
For a hard material the deformation stays small, the lowering of the cantilever
base corresponds to the bending of the cantilever tip in the opposite direction (as
in Figs. 3.11, 3.13). Soft materials show higher compliance and materials with
adhesive properties bend the cantilever towards the surface upon retraction (Figs.
3.14, 3.15).
Figure 3.10: A selection of information that can be gained from a force
curve graph. The red curve corresponds to the lowering of the cantilever, the
blue to its retraction. Values on the abscissa correspond to the height of the
cantilever base, ordinate values correspond to the measured displacement of
the cantilever tip. Image adapted from [69].
The information of a force curve can also be represented in a force-distance
graph, showing the deformation of the contacted surface area as a function of the
applied cantilever force. The forces also can be pulling forces, if the cantilever
is held back during retraction e.g. on adhesive surfaces and while performing
molecular recognition. For a single protein that unfolds while one end is attached
to the surface and the other is being pulled by the cantilever, this graph is called
a force extension profile. The steplike character of such a curve is characteristic of
how the protein unfolds under a pulling load. A force extension profile of a single
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