Paper Technology International 2020 - Journal - Page 82

PAPERTECHNOLOGYINTERNATIONAL
2. Mechanical properties in AFM measurements
In AFM measurements it is possible to record mechanical
properties in operation modes, where the tip touches the sample
surface. In these operation modes force vs. distance curves are
generated at every measuring point on the sample surface. Via
recording force-distance curves it is possible to detect mechanical
properties of surfaces.
In figure 2, the blue line represents the characteristics during
the approach of the tip towards the surface and the red line that
during the retraction of the tip. When the tip approaches the surface,
a “snap-into-contact” point occurs at a certain distance near the
surface (d0). Here the attractive force gradient is greater than the
spring constant of the cantilever and the tip snaps into the surface.
Then the set PeakForce is applied to the surface, which results in a
deformation of the sample. When the PeakForce setpoint has been
reached, the tip is pulled back from the surface. An adhesive force
now occurs between the tip and the surface. Further mechanical
properties of the surface can be read from this curve. The elastic
properties can be described by contact models such as Hertz model,
Derjaguin-Muller-Toporov (DMT) model, or Johnson-Kendall-Roberts
(JKR) model (Derjaguin et al. 1975b; Hertz 1881; Johnson et al.
1971). The respective modulus is fitted within the green line in
figure 2. The dissipation can be calculated from the integrated area
between approach and withdraw curve.
2.1 Recording static force-distance curves
Scanning bending test
One method to determine mechanical properties of single
cellulosic fibres is to record static force-distance curves along the
fibre axis. This method can be called as “scanning bending test”. In
the scanning bending test, the sharp tip is exchanged with a colloidal
probe to provide a defined contact area. It is schematically shown in
figure 3a). This method allows to detect the mechanical properties
on every local spot on a single fibre. Furthermore, the scanning
bending test circumvents the problem of assuming homogenised
fibre dimensions as in the common bending test, which is usually
performed in the centre of the fibre only. Additionally, the local
variations within the fibre diameters can be included in the scanning
bending test. The local fibre diameter in every bending point of
Figure 2: Schematics of a force-distance curve with the
mechanical properties which can be determined.
the fibre can be analysed via confocal laser scanning microscopy
(CLSM). With the scanning bending test, it is possible to detect
mechanical properties from the force-distance curves such as
adhesion or dissipation, but it is also possible to calculate the
contact stress and the strain. Therefore, a whole mechanical image
of a fibre along its longitudinal axis can be displayed (Auernhammer
et al. 2021b).
In figure 3b) a schematic force-bending curve is shown. The
transformation between force-distance curve to a force-bending
curve is necessary to extract the bending ability of the local fibre
spot. Here, the contact point of the probe and sample is detected
and set to zero. Then, the bending distance can be read out.
Furthermore, it is possible to extract several bending abilities for
different forces to detect a potentially nonlinear local response of the
fibre as schematically shown in figure 3b). The resulting contact area
of a colloidal probe and a curved cylinder (fibre) is elliptical shaped
as shown in figure 3c).
Figure 3:
a) Schematics of the section-wise
scanning process along the longitudinal
direction of the fibre. At every section,
the scanning trajectory was realigned to
the fibre axis.
b) Force-bending curve with
visualization of multiple virtual setpoint
forces F.
c) Schematics of the contact mechanics
between the colloidal probe and the
fibre before contact and in contact.
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