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Item type:Publication, Access status: without files , Spontaneous formation of nanopatterns in velocity-dependent dip-coated organic films : from dragonflies to stripes(American Chemical Society (ACS), 2014); ; ; ; 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 > vcr ∼ 0.15 mm/s. The critical vcr marks a transition between the evaporation regime, where the solvents meniscus remains at the bulks 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entrainment (Landau-Levich-Deryaguin) regime, where the solution is partially dragged by the substrate, covering the withdrawn substrate by a homogeneous film.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.Publicationtype: Journal Article44 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Faraday instability in a surface-frozen liquid(American Physical Society (APS), 2005-04-08); ; ; ; Faraday surface instability measurements of the critical acceleration, ac, and wavenumber, kc, for standing surface waves on a tetracosanol (C₂4H₅0) melt exhibit abrupt changes at Ts=54degC above the bulk freezing temperature. The measured variations of ac and kc vs. temperature and driving frequency are accounted for quantitatively by a hydrodynamic model, revealing a change from a free-slip surface flow, generic for a free liquid surface (T>Ts), to a surface-pinned, no-slip flow, characteristic of a flow near a wetted solid wall (T < Ts). The change at Ts is traced to the onset of surface freezing, where the steep velocity gradient in the surface-pinned flow significantly increases the viscous dissipation near the surface.Publicationtype: Journal Article21