Other Applications - Hierarchical Nanostructures for Energy Devices - page 277

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Figure 12.5 (A) Large area-SEM image of the surface of a lotus leaf. Every epidermal

cell forms a papilla and has a dense layer of epicuticular waxes super-
imposed on it. (B) Enlarged view of a single papilla from (A). SEM
images of the lower surface of the lotus leaf. (D) The fitted curve based
on calculated data (contact angle, in degrees, against the mean outer
diameter of protruding structures, in micrometres).
Reproduced with permission from ref. 76. Copyright 2013, Wiley-VCH.

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Conventionally, superhydrophobic surfaces have been produced in mainly
two ways: (1) by creating a rough structure on a hydrophobic surface, and (2)
by modifying a surface using materials with a low surface free energy. Until
now, many methods have been developed to produce rough surfaces, in-
cluding solidification of melted alkylketene dimer (AKD, a kind of wax), 84
plasma polymerization/etching of polypropylene (PP) in the presence of
polytetrafluoroethylene (PTFE), 85 microwave plasma-enhanced chemical
vapor deposition (MWPE-CVD) of trimethylmethoxysilane (TMMOS), 86 an-
odic oxidization of aluminium, 87 immersion of porous alumina gel films in
boiling water, 88 mixing a sublimation material with silica or boehmite, phase
separation, 88 and molding, 89 To obtain superhydrophobic surfaces, coating
with low-surface-energy materials such as fluoroakylsilane (FAS) is often
necessary. 87-90
Hierarchical nanostructures can be used to enhance the super-
hydrophobicity of a surface. This idea came from the surface of a plant.
The self-cleaning effect of some plant leaves (such as the lotus) is of great
interest for practical applications in various fields. 76 The observation of
hydrophobicity related to the topology of the surface of a plant leaf was