title: Spontaneous formation of nanopatterns in velocity-dependent dip-coated organic films: from dragonflies to stripes.
abstract: We present an experimental study of the micro- and mesoscopic structure of thin films of medium length n-alkane molecules on the native oxide layer of a silicon surface, prepared by dip-coating in a n-C32H66/n-heptane solution. Electron micrographs reveal two distinct 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 lengths of a few hundred micrometers and a few micrometer lateral separation. For a constant v, the stripes' quality and separation increase with the solution concentration. Grazing incidence X-ray diffraction and atomic force microscopy show that both patterns are 4.2 nm thick monolayers of fully extended, surface-normal-aligned alkane molecules. With increasing v, the surface coverage first decreases then increases for v > v(cr) ∼ 0.15 mm/s. The critical v(cr) marks a transition between the evaporation regime, where the solvent's meniscus remains at the bulk's surface, and the entrainment (Landau-Levich-Deryaguin) regime, where the solution is partially dragged by the substrate, covering the withdrawn substrate by a homogeneous film. The dragonflies are single crystals with habits determined by dendritic growth in prominent 2D crystalline directions of randomly seeded nuclei assumed to be quasi-hexagonal. The stripes' strong crystalline texture and the well-defined separation are due to an anisotropic 2D crystallization in narrow liquid fingers, which result from a Marangoni flow driven hydrodynamic instability in the evaporating dip-coated films, akin to the tears of wine phenomenology.
authors.affiliation: Facultad de Física, Pontificia Universidad Católica de Chile , 7820436 Santiago, Chile.
authors.forename: Tomas P
authors.initials: TP
authors.lastname: Corrales
authors.forename: Mengjun
authors.initials: M
authors.lastname: Bai
authors.forename: Valeria
authors.initials: V
authors.lastname: del Campo
authors.forename: Pia
authors.initials: P
authors.lastname: Homm
authors.forename: Piero
authors.initials: P
authors.lastname: Ferrari
authors.forename: Armand
authors.initials: A
authors.lastname: Diama
authors.forename: Christian
authors.initials: C
authors.lastname: Wagner
authors.forename: Haskell
authors.initials: H
authors.lastname: Taub
authors.forename: Klaus
authors.initials: K
authors.lastname: Knorr
authors.forename: Moshe
authors.initials: M
authors.lastname: Deutsch
authors.forename: Maria Jose
authors.initials: MJ
authors.lastname: Retamal
authors.forename: Ulrich G
authors.initials: UG
authors.lastname: Volkmann
authors.forename: Patrick
authors.initials: P
authors.lastname: Huber
keywords: 2D crystallization; Marangoni flows; X-ray diffraction; atomic force microscopy; n-alkane; scanning electron microscopy; silicon