ABSTRACT
We are all accustomed to liquid droplets being spherical, due to the tendency of the surface tension to minimize the surface area at fixed volume. In some contrast, Fig. 1 shows a number of macroscopic sessile droplets sharing the striking feature of being faceted along the vertical direction. This is because the substrate, a standard silicon wafer with a native oxide layer, has been structured chemically by microcontact printing. It bears vertical hydrophobic stripes about 200 nm in width, separated by hydrophilic domains of similar width. The whole sample is thus stripewise patterned as to its wettability and the spatial period of the pattern is 400 nm, well below the optical resolution of a standard optical microscope. Nevertheless, the impact of this
chemical structure on the substrate (which is almost perfectly flat) on the shape of the drops is quite obvious and easily visible. This simple experiment shows that in order to understand wetting even on a macroscopic scale, it is indispensable to study it on very small scales, down to those governed by, and comparable to, the typical range of wetting forces. It is therefore natural to consider modern techniques of submicrometer scale microscopy, such as scanning probe or electron microscopy, as important methods for investigating the fundamentals of wetting phenomena.
