Wetting morphologies on surfaces nanopatterned with chemical stripes

Here we investigate the wetting of simple, volatile liquids on model chemical nanopatterns created using Local Oxidation Nanolithography. This technique makes use of a biased, metallic AFM tip to locally oxidize the methyl-terminations of a self-assembled monolayer (octadecylthrichlorosilane) into carboxylic acid termination[1]. With this method we have realized parallel, 50 to 500 nm wide, wettable stripes (carboxylic) embedded into a non-wettable (methyl) surface. Several organic (polar, non-polar), volatile liquids have been condensed onto the nanopatterned surface and the resulting wetting morphologies have been studied in-situ by using an environmental AFM. Initially the liquid only condenses on the wettable stripes to form a thin liquid film. Close to saturation the liquid morphology becomes drop-like. Eventually, when more and more liquid is condensed on the stripes, the liquid drops may ``spill over'' into the non-wettable spacer so that neighboring lines merge and undergo a ``morphological wetting transition''. For all of these regimes we show that long-range forces are relevant to nanoliquid ``shape''. Results will be compared with those of Density Functional Theory.[1] R. Maoz, S. Cohen, and J. Sagiv, Adv. Mater. \textbf{11}, 55 (1999)