A smooth future?

When a water droplet rolls easily, retaining a nearly perfect spherical shape on a surface that remains essentially dry, such a surface is super-repellent or superhydrophobic. Common superhydrophobic materials include many plant leaves, duck feathers, and glass surfaces coated by black soot dispersed from a lighted candle (see Fig. 1a,b). By convention, for a surface to be superhydrophobic, the effective contact angle of the liquid droplet with the surface — a simple and robust characteristic — should exceed 150°. The issue of the large contact angle of water droplets on plant leaves and animal surfaces was addressed over 80 years ago by Wenzel, Cassie and Baxter, who pointed out the two physical ingredients necessary for a surface to be superhydrophobic: a bare hydrophobic coating, such as wax crystalloids on plant leaves, and roughness at the micro-scale. Strong water repellency occurs because the roughness effectively increases the liquid–solid free energy. This is the consequence of either a larger real contact area between the two components, the case of the liquid impregnating the surface (the so-called Wenzel state1), or a replacement of the true liquid–solid contact by a highly energetic liquid–vapour interface — the case of the liquid interface suspended on an air cushion on top of the roughness peaks (the so-called Cassie1 or fakir state). Although these early works spurred industrial interest in textile and glass coatings, the field of superhydrophobic materials remained essentially asleep for more than 50 years. In the late 1990s, as research in wetting reached maturity2, publications by Onda and collaborators3 and Neinhuis & Barthlott4 demonstrated the possibility to reach nearly perfect nonwettability, far exceeding the performance of bare chemical coatings, such as Teflon (the contact angle of water on Teflon is about 120°). Subsequently, Bico & Quéré5 laid out the basic physical picture for the transition between the Wenzel and Cassie states. This helped revive the enthusiasm for the field, and to attract scientists from physics, engineering, mathematics and chemistry. What was behind this ‘gold rush’? Taking a closer look at the literature on superhydrophobic materials over the past decade we see that, in response to the challenge of building cheap and scalable surfaces, research efforts have mainly focused on two questions: the design of new materials and the characterization of their wetting properties. Design-wise, imparting superhydrophobicity to a surface is not a difficult task. However, attaining robustness of the material over time and/or under external constraints has proved challenging, and much work has been dedicated to developing chemical recipes for superhydrophobic coatings6. Importantly, the popularization of nanofabrication tools has allowed wellcontrolled textured surfaces to be easily built (see Fig. 1c,d), thus providing ideal systems on which to test fundamental ideas. In parallel to the progress made in designing superhydrophobic materials, characterization of their wetting properties has been addressed with experiments, theory and computer simulations. For example, much insight has been gained into the relative stability of the Cassie and Wenzel states, the control of their stability by electric fields7–9, and the transitions and hysteresis between these states.