Wettability and adhesional differences on a natural template: the cicada wing

Natural nanostructures represent an interesting group of materials which have been finely tuned through the imperatives of species survival and which represent suitable templates for man-made applications. We have investigated the surface properties on two species of cicadas, Cryptotympana mandarina and C. atrata. These were chosen as they present hydrophilic (C. mandarina) and hydrophobic (C. atrata) membrane surfaces with a similar lateral nanostructure arrangement. The study illustrates the relationship between wettability, nanostructure and adhesion on the wing surfaces. The elemental components (as determined by X-ray photoelectron spectroscopy (XPS)) showed a high percentage of carbon in accord with an epicuticle layer of hydrocarbon. The wing nanostructured protrusions were found to posses similar values of basal diameter (80-130 nm) and spacing (67-107 nm) but significant differences in height (from 157 nm to 464 nm). The contact angles (CAs) ranged from 58.1° to 137.9°. The results show that the surfaces with higher protrusions typically have larger CAs. Adhesion of the wing surfaces correlated strongly to the surface wetting properties. The data demonstrates that, in principle, tuning of nanostructure height with structuring at these lateral dimensions can be used to exhibit particular wetting and adhesional properties.