The glossy, water-repellent leaves of lotus plants (Nelumbo nucifera and N. lutea) have inspired many synthetic superhydrophobic surfaces. But the best of these are difficult to make, and can be very fragile. Steven Bell and colleagues now describe a simple method to prepare robust superhydrophobic surfaces of high quality (I. A. Larmour et al. Angew. Chem. Int. Edn doi:10.1002/anie.200604596; 2007).

The secret of lotus leaves' water-repellency is the double roughness of their surfaces — caused by the presence of nanohairs on microbumps — coupled with a waxy coating. The authors recreate this double roughness by coating metal substrates with a textured layer of another metal, simply by immersing them in a metal-salt solution. Scanning electron microscopy shows that the deposited metal forms flower-like structures (0.20 to 1 µm across) that are made up of smaller crystallites (about 60 to 200 nm in size), simulating the complexity of lotus leaf surfaces.

Dipping the substrates in a solution of a chemical surface-modifier, HDFT, supplies a monolayer of hydrophobic molecules. These molecules are highly fluorinated, just like the Teflon lining of non-stick frying-pans. The resulting surfaces show almost perfect super-hydrophobicity: a drop of water on a perfectly water-repellent surface forms a contact angle θ of 180°, and the surfaces produced by this method have θ values consistently greater than 170°.

The approach is so simple that it can be applied to metal objects of any reasonable size or shape. The authors again turned to nature for inspiration. Pond skaters (Gerridae) use superhydrophobic legs to walk on water. Bell and colleagues made a model pond skater from copper (pictured), with legs that had been treated with silver and HDFT. Despite having ten times the mass of a real pond skater, the metallic insect was able to rest comfortably on the surface of water.

The authors suggest that their method will aid research into superhydrophobic surfaces. This should hasten the arrival of practical applications, such as reducing turbulent flow in water-bearing pipes.