Nanoscale aggregation phenomena at the contact line of air-drying pure water droplets on silicon revealed by atomic force microscopy

In the present work, silicon wafer surfaces were studied during a pure water dewetting process in ambient conditions by intermittent-contact atomic force microscopy (AFM). With an acid-free surface cleaning, large network structures of tens of microns in extension but only a few nanometers in height were observed, being stable for days. Fractal-like assemblies have been previously reported in a variety of different scenarios, for example, when an aqueous solution of carbon-based species, especially carbon nanotubes, is left to evaporate on a solid substrate, provided that no complete wetting was produced. Chemical mapping of silicon wafer surfaces, while unable to provide a spatial resolution comparable to that of the AFM, clearly showed the initially formed contact line to be enriched in carbon. Therefore, hydrophobic and/or non-soluble (or slightly soluble) substances which are present on every surface exposed to air are expected to be responsible for the observed fractal structures. Reactions of the network structures toward changing environmental conditions were analyzed. When increasing the ambient humidity, the structures grew only slightly, which is indicative of their highly (but not totally) hydrophobic nature. Heating the sample above 100 °C for about 10 h led to an almost complete disappearance of these nanostructures. Due to the lateral extension of these stable network heterogeneities, they are expected to affect contact angle measurements in wetting studies, especially at the micro- and nanoscale. When acid-cleaned wafers are used as substrate, deposited water extends as a film over the silicon surface without droplet formation. No fractal structures are then observed.