Well-Defined Porphyrin Architectures From Self-Assembly and Dewetting - Molecular Self-Assembly: Advances and Applications - page 250

monodispersity in the distance between the lines, which amounted
to 660

±

30 nm.
Just like in the formation of the ring-shaped assemblies described
earlier, also in this case a combination of two forces, self-assembly
and dewetting, is responsible for the generation of the line patterns.
Careful analysis of the final surface structure revealed that the lines
run parallel to the receding drop edges of the solution. In this case,
dewetting is discontinuous and occurs according to the so-called
'coffee-stain mechanism' (Fig. 8.8C) [16]: during the evaporation
of the droplet, its contact line with the surface gets pinned one or
several times (Fig. 8.8A

−

B). This pinning can be caused by a surface
irregularity, or by the accumulation of solute material. As a result,
the droplet shrinks through discontinuous 'jumps', and after each of
these 'jumps' a thin film of solution is left behind, which subsequently
becomes subject to rapid dewetting. When the film reaches a certain
thinness, undulations can occur, which in the case of the porphyrin
trimers are believed to direct the molecules to self-assemble into
long columnar stacks. The periodicity of the undulations, which can
be considered ripple-like features in the film, is proposed to govern
the highly defined spatial distance observed between the porphyrin
lines (Fig. 8.8D). Although this line spacing varied depending on the
domain between the contact pinning lines, it always lies in between
500 nm and 1

µ

m.
When the size of the evaporating droplet was larger (e.g., 10
instead of 3

µ

L), similar periodic and highly monodisperse line
patterns were obtained, but both their properties and formation
mechanism were dramatically different. Instead of single molecule
thick columnar stacks, the lines now appeared to consist of bundles
of 10-12 stacks, which were much further apart than in the case of
the small droplets, i.e., 13

µ

m instead of 660 nm (Fig. 8.9A). In the
case of the larger droplets, evaporation requires a longer time. In
contrast to the case of the rapid dewetting shown in Fig. 8.8, the
molecules of now have the time to flow radially towards the drop

edge, where they are deposited on the surface and grow in lines that
are perpendicular to the drop edge (Fig. 8.9B).
The observation that only a small variation in one of the
parameters - in this case the size of the evaporating droplet - has such
a large influence on the mechanism of pattern formation indicates
that the process, in all its elegance, is very complex and dependent

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