Civil Engineering Reference
In-Depth Information
Figure 8.17
Comparison of helical structures found in electrospun APC i bers and in climbing plants
(
Passil ora edulis
). (a) to (c) are pictures taken from the plant and (d) to (h) are SEM pictures of the APC
i bers. If the i ber or tendril is connected by both ends [(a), (b), (e), (h)] a helix is formed as the tension is
released, if they are connected by only one end [(c), (d), (f )] a spiral is formed. Reversal in the rotation of
the helices is observed in both systems and is marked by circles. Reproduced by permission of the Royal
Society of Chemistry.
Figure 8.18
Comparison of complex shapes found on the tendrils of the plant
Passil ora edulis
(top
photos) and on the electrospun i bers of APC (black and white SEM images). Reproduced by permission
of the Royal Society of Chemistry.
h e origin of the helical structures in the electrospun i bers is the release of the ten-
sion that is introduced by the electrospinning process in order to obtain suspended
i bers between two electrodes [105]. h e release of this tension, which can be achieved
by bringing the electrodes closer together, causes the i bers to rotate spontaneously
forming a structure with two helices with opposite handedness connected by a small
straight segment denominated perversion. However, if the i ber is only connected in
one end, the other extremity is free to rotate and the i ber assumes the shape of a spiral.
Even though the mechanisms that cause the rotation in these systems are dif er-
ent, there is a physical model that describes the formation of helical structures with
perversions in both cases despite the dif erence in order of scale and it is known as the
Calugareanu theorem [106]. h is model takes into account the fact that in a i lament
with intrinsic curvature the total curvature must remain constant. As the tension is
removed from the i lament, and the helix starts to form, the formation of two helices
