Spontaneous Formation of Nanopatterns in Velocity-Dependent Dip-Coated Organic Films: From Dragonflies to Stripes

We present the structure of thin, n-alkane films on the oxide layer of a silicon surface, prepared by dip-coating in a n-C$_{\rm 32}$H$_{\rm 66}$/n-heptane solution. Electron micrographs reveal two adsorption morphologies depending on the substrate withdrawal speed v. For small v, dragonfly-shaped molecular islands are observed. For a large v, stripes parallel to the withdrawal direction are observed. These have a few hundred micrometer lengths and a few-micrometer lateral separation. With increasing v, the surface coverage first decreases, then increases for $v > v_{cr} \sim 0.15$~mm/s. The critical $v_{cr}$ marks a transition between the evaporation regime and the entrainment regime. The stripes' strong crystalline texture and the well defined separation are due to an anisotropic 2D crystallization in narrow liquid fingers, which presumably results from a Marangoni-flow-driven hydrodynamic instability in the evaporating dip-coated films.