Other Applications - Hierarchical Nanostructures for Energy Devices - page 283

lamellae and each row consists of thousands of primary setal stalks known
as setae. Autumn et al. 124 estimated that there are 14 400 setae mm 2 and
each seta is approximately 30-130 mm in length and 5-10 mm in diameter
consisting of three levels. 125 The secondary seta is about 20-30 mm in length
and 1-2 mm in diameter. At the end of each secondary seta, 100-1000 spa-
tulae with a diameter of 100-200 nm form the points of contact with a
surface. The tips of the spatulae are approximately 200-300 nm in width,
5500 nm in length, and 10 nm in thickness. 123 Cumulative van der Waals
interactions have been attributed to be the main adhesive mechanism
achieved through contact splitting of setae. However, it is the hierarchical
topography that allows for an effective compliance to a surface by decreasing
the stiffness of each level of seta with the spatula maintaining sucient
mechanical stability. 123 To have the same compliance to a surface without
the hierarchical structure, linear b-keratin setae would have to have a length
of 160 mm (aspect ratio of 100-160). At this aspect ratio, the mechanical
stability of the pillars would be insucient, resulting in clumping and
collapse of the pillars. Clumping of the setae is undesirable as this would
reduce the contact points. In addition, these clumped or bunched pillars
would have higher stiffness and thus lower compliance. 123
With the hierarchical nanostructure, Gao et al. 126 have shown that the
theoretical van der Waals adhesion strength between surfaces can be
reached as shown in Figure 12.10. Numerically, Kim et al. 127 showed that a
hierarchical setae structure provides geckos with the adaptability to have a
large effective area of contact with rough surfaces and he further showed
that the equivalent stiffness of a three-level hierarchical structure is ap-
proximately 40% lower than a one-level linear structure. That stiffness re-
duction resulted in more than 100% enhancement in adhesion energy. 128

d n 3 r 4 n g | 8

.

Figure 12.10 The hierarchical adhesive structures of the gecko. A gecko's toe con-
tains hundreds of thousands of setae and each seta contains hundreds
of spatulae. (a) and (b): scanning electron micrographs of rows of setae
at different magnifications and (c): spatulae, the finest terminal
branches of seta. ST: seta; SP: spatula; BR: branch.
Reproduced with permission from ref. 126. Copyright 2013, Elsevier.