Biomedical Engineering Reference
In-Depth Information
We always observed this coincidence in the conservative and dissipative interaction
curves. Since the kink appears at the position where most of the dextran monomers
unfold, it is very likely that a large amount of energy dissipation is caused by this
unfolding processes. However, the dissipation increases also shortly before the rup-
ture of the dextran strand from the tip. At this position, the majority of the dextran
monomers is already unfolded. Therefore, we conclude that other effects like hydro-
dynamic damping and/or the rupture of the molecule from the tip will contribute to
the energy dissipation.
2.8 CONCLUSION AND PERSPECTIVES
In summary, an overview over the basic principles of AFM driven in the contact as
well as in dynamic modes is presented. While the contact mode is very easy to apply
and the standard technique to measure force versus distance curves, the application of
the tapping-mode in air and liquids is an everyday tool in nanotechnology, enabling
the imaging of sample surfaces with very high resolution. In addition, it allows the
quantitative interpretation of the tip-sample interactions.
AFM is a nice example for the often observed incident that scientific progress is
frequently triggered by the development of new experimental techniques. The fric-
tion force microscope (Mate et al., 1987) is an instance since it opened a complete
new field of science: the analysis of friction and wear at the atomic-scale (Gnecco &
Meyer, 2006; Holscher et al., 2008). Many other recent achievements in the field of
nanotechnology are unthinkable without the help of AFM and other scanning probe
methods. However, despite these success stories, there is still room for improve-
ments. From my viewpoint, the following recent developments might be of interest
for the AFM enthusiast.
•
Scanning a sample surface with an AFM enables the experimentalist to scan
the surface topography with high resolution but additional effort has to be
spent to map other physical quantities. As described in this chapter, it is
possible to determine the tip-sample forces with high resolution at arbitrary
sample positions. This, however, has to be done after imaging the topogra-
phy. Of course, it is possible to do this on a fine grid, reconstructing again
the surface topography but this is quite time-consuming. A possible solu-
tion to this problem was given by Sahin et al., (2007) who used torsional
harmonic cantilevers to measure the time varying forces between tip and
substrate. In this way it is possible to determine the indentation forces on
soft samples like polymers during scanning.
•
It is often criticized that the AFM features atomic-scale resolution but no
chemical identification. Of course, it is often possible to distinguish between
different materials through friction force (Figure 2.5) or phase images (Fig-
ure 2.9). Even the chirality of molecules can be detected in this way (McK-
endry et al., 1998). Also, on the atomic scale, it is possible to distinguish
different atomic elements as nicely shown by Sugimoto et al. (2007). Pre-
vious knowledge about the sample, however, is necessary in these cases.
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