© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2010, XX, 1–6 1 www.advmat.de www.MaterialsViews.com RESEARCH NEWS wileyonlinelibrary.com By Tuukka Verho, Chris Bower, Piers Andrew, Sami Franssila, Olli Ikkala, and Robin H. A. Ras* Mechanically Durable Superhydrophobic Surfaces 1. Introduction A myriad of reports have been published on ways to fabricate superhydrophobic non-wetting surfaces. [1–7] These surfaces, which possess the virtue of having a very large water con- tact angle and exhibiting little sticking to water drops, have numerous applications in self-cleaning paints and windows, [8] non-wetting fabrics, [9–12] anti-fogging, [13] anti-icing, [14] buoyancy [15] and flow enhancement [16] to name a few. How- ever, the practicality of non-wetting surfaces is hampered by the poor mechanical stability of the micro- scopic surface topography that is essen- tial for very large contact angles. Despite the importance of mechanical durability in applications, this aspect has received relatively little attention until very recently. Furthermore, mechanical contact may also leave impurities on non-wetting surfaces, causing a decline in their properties. Here, we present a short overview of the recent advances in developing mechanically resil- ient superhydrophobic surfaces and review the approaches that can be taken to avoid degradation due to contamination. Usually, two criteria are applied in defining superhydrophobicity. First, the equilibrium water contact angle θ of a superhydrophobic surface must be larger than 150 °. Second, water must not stick to the surface, i.e. droplets must roll off easily. The second condition is related to the contact angle hysteresis  θ of the sur- face — the difference between the largest (advancing) and smallest (receding) stable contact angle - θ θ adv rec . The maximum lateral force F lat that a distorted droplet can build up depends on θadv and θrec as [17] F lat rec adv ∝ cos - cos θ θ (1) which can be approximated for small hysteresis as F ∝Δ si lat nθ θ . In many cases, damage and contamination do not dramatically decrease the equilibrium contact angle θ of a non-wetting surface (or the measured ‘static’ contact angle) but do decrease the receding angle rec θ and so cause a large hyster- esis, affecting the rolling behavior of water droplets due to a larger F lat ( Figure 1 ). Smooth surfaces can have an intrinsic contact angle only up to about 120 °. [18,19] Superhydrophobicity — angles over 150 ° — can be achieved by roughening a hydrophobic surface to establish a stable Cassie state, i.e. a state where the grooves of the surface pattern are not wetted by water. [20] A well-known example of such a surface is the leaf of the Lotus plant, [21] on which a water droplet sits on the tops of the micropapillae that grow on the leaf, leaving most of the surface dry. In general, if the tips of the asperities that are wetted compose only a small area fraction of a patterned surface, the wetting properties are mostly determined by the trapped air layer between water and the surface. The apparent contact angle can approach 180 ° as shown by the Cassie-Baxter equation Development of durable non-wetting surfaces is hindered by the fragility of the microscopic roughness features that are necessary for superhydrophobicity. Mechanical wear on superhydrophobic surfaces usually shows as increased sticking of water, leading to loss of non-wettability. Increased wear resistance has been demonstrated by exploiting hierarchical roughness where nanoscale roughness is protected to some degree by large scale features, and avoiding the use of hydrophilic bulk materials is shown to help prevent the formation of hydrophilic defects as a result of wear. Additionally, self-healing hydrophobic layers and roughness patterns have been suggested and demonstrated. Never- theless, mechanical contact not only causes damage to roughness patterns but also surface contamination, which shortens the lifetime of superhydrophobic surfaces in spite of the self-cleaning effect. The use of photocatalytic effect and reduced electric resistance have been suggested to prevent the accumu- lation of surface contaminants. Resistance to organic contaminants is more challenging, however, oleophobic surface patterns which are non-wetting to organic liquids have been demonstrated. While the fragility of superhydro- phobic surfaces currently limits their applicability, development of mechanically durable surfaces will enable a wide range of new applications in the future. DOI: 10.1002/adma.201003129 [∗] T. Verho, Prof. O. Ikkala, Dr. R. H. A. Ras Molecular Materials Department of Applied Physics Helsinki University of Technology/Aalto University Puumiehenkuja 2, FI-00076 Aalto, Espoo (Finland) E-mail: robin.ras@tkk.fi Dr. C. Bower, Dr. P. Andrew Nokia Research Center Broers Building (East Forum) 21 JJ Thomson Avenue, Madingley Road Cambridge CB3 0FA (UK) Prof. S. Franssila Department of Materials Science and Engineering Helsinki University of Technology/Aalto University Vuorimiehentie 2, FI-00076 Aalto, Espoo (Finland)