Secrets behind the Super Water Repellency of Lotus Leaves
If you wonder why lotus leaves can always be kept clean, there is something "nano" on their surfaces that you should know about!
The lotus effect
Cutting-edge development of synthetic liquid-repellent surfaces is inspired by the lotus effect (Barthlott, W. & Neinhuis, C. Planta 1997, 202:1-8): water droplets are supported by surface textures on a composite solid/air interface that enables them to roll off easily, carrying the dirt away with them. Over the years, the phenomenon has been observed in many different natural surfaces on animals, insects and plants.
Recently, micro/nanostructures on many of these surfaces have been attributed to the observed high surface hydrophobicity. These observations have led to enormous interests in manufacturing biomimetic water-repellent surfaces owing to their broad spectrum of potential applications, ranging from liquid-repellent fabrics to friction-reduction surfaces.
Intriguing scientific observations
Despite these tremendous bio-inspired innovations, an important scientific observation is that the characteristic lengths of surface textures on these natural surfaces are consistently on the order of 10 nm to 1000 nm. Is it a merely coincident? Or is there a scientific basis why specific length scale of textures is utilized by nature for the water repellent function? This question led to my earlier work to study the relationship between the length scale of surface textures and the macroscopic wetting phenomena.
Dependence of macroscopic wetting on nanoscopic surface textures
It is commonly known that the hydrophobicity of a surface can be enhanced by physical textures. However, no existing theories of surface wetting can provide a guidance to pinpoint the texture size requirement to achieve super/ultrahydrophobicity (i.e., a contact angle approaching 180 degrees). In the earlier study, we show that the three-phase contact line tension is an important link to understand the dependence of macroscopic wetting on physical texture size in an ideal Cassie regime. Specifically, we show that texture size is the dominant parameter in determining surface hydrophobicity when the size approaches to a limiting physical length scale, as defined by line and surface tension of liquid. Simply put, the surfaces become more hydrophobic when the surface textures are "small" enough. The new theory suggests that near spherical water droplets can be supported by textured surfaces with high solid fraction, which is in contrast to the classical Cassie-Baxter theory (1944), and may offer new physical insight to the biological design of superhydrophobic surfaces.
1. Tak-Sing Wong and Chih-Ming Ho, “Dependence of Macroscopic Wetting on Nanoscopic Surface Textures”, Langmuir, vol. 25, 12851 – 12854 (2009).
2. Tak-Sing Wong, Adam Huang, and Chih-Ming Ho, “Wetting Behaviors of Individual Nanostructures”, Langmuir, vol. 25, 6599 – 6603 (2009).
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Copyright by Tak-Sing Wong 2013. All rights reserved.
Department of Mechanical and Nuclear Engineering
Materials Research Institute
Huck Institutes of the Life Sciences
The Pennsylvania State University, University Park, PA 16802