to a certain extent the particle-particle attraction. Normally, the solution is deposited on to a plain
silicon substrate that is covered by the native oxide layer only [34]. However, one may locally
change the wetting behaviour of the solvent by further oxidising the substrate [38]. By adding
excess thiol one can also vary the properties of the solvent [40].
Two different procedures are employed for the deposition of the solution on to the substrate: spin-
coating or a meniscus technique [61, 62]. The choice is important as it strongly inﬂuences the
evaporation rate and, as a result, the pattern formation process. When using spin-coating, one ﬁnds
that directly after deposition, evaporation competes with dewetting until all the solvent has evapo-
rated. The resulting deposits of nanoparticles are imaged by atomic force microscopy (AFM). For
spin-coated ﬁlms, the evaporation rate is high and structuring is normally ﬁnished before the spin-
coater is stopped. Conversely, the solvent evaporation rate is strongly decreased when employing
the meniscus technique [61], i.e., by depositing a drop of solution on a Teﬂon ring that is wetted by
the solvent. This allows for a better control of the process and enables the use of contrast-enhanced
microscopy to observe the dewetting process in situ [40]. All pattern formation is conﬁned to the
region of the receding contact line of toluene, silicon and air. With both techniques one may ﬁnd
mono-modal or bi-modal polygonal networks [34], labyrinthine spinodal structures, or branched
patterns (see Fig. 1). The meniscus technique allows for the study of branched structures in a
more controlled manner. The work in Ref. [40] indicates that ﬁngering strongly depends on the
interaction strength of the particles, i.e., on the chain length of the thiol molecules coating the gold
cores. For short chains (C 5 and C8) no formation of branched structures is observed. At similar
concentrations, well-developed branched structures are formed for longer chains (C 10 and C12).
For even longer chains (C 14), however, one again ﬁnds less branching. It also depends on the
amount of excess thiol in the solvent (for details see Ref. [40]).
When following the evolution of the branched patterns in situ (see the complementary video
material of Ref. [40]), one clearly observes that different processes occur on different lenght
scales. First, a macroscopic dewetting front recedes, leaving behind a seemingly dry substrate.
The macroscopic front can be transversely unstable resulting in large-scale ( > 100µm) strongly
anisotropic ﬁngered structures. For fronts that move relatively quickly these macroscopic struc-
tures cover all the available substrate. However, when at a later stage the macroscopic front be-
comes slower, those ﬁngers become scarce and ‘macroscopic ﬁngering’ ﬁnally ceases. At this
stage it is possible to appreciate that the seemingly dry region left behind by the front is not at all
dry, but covered by an ultrathin ‘postcursor’ ﬁlm that is itself unstable. The thickness of this ﬁlm
6