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average diameter and length were approximately 20 nm and 200
nm, respectively, which is smaller than the typical dimensions of
the nanograss in 'black silicon' [67, 68]. The SEM image (Fig. 4.18)
showed the two-tier, grassy air-like structure with a pillar spacing
of 5 µm. The air fraction, calculated from the top-view image, was
approximately 99.98%.
Figure 4.18
SEM images of nanograss-on-nanopillar structures after 9 s
of CHF
plasma treatment. (a) Cross-sectional image of
an air-like surface having an air fraction of 99.98%. (b,c)
High-magnification images on the bottom and top surfaces,
respectively, of the nanopillar structure. (d) Top-view image
of this air-like sample. Reprinted with permission from Ref.
[65], Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA,
Weinheim.
3
Contact line structure (shape, length, continuity of contact,
amount of contact) is of practical importance on a three-phase (solid-
liquid-air) real surface. During a droplet movement on the surface,
the contact line advances (on the downhill side) and recedes (on the
uphill side) over the barriers (roughness, chemical heterogeneity,
defects) from one metastable state to another metastable state.
However, there will be energy barriers for this kind of movements.
These kinetic barriers are responsible for contact line pinning and
hysteresis. Actually the air-liquid-solid three-phase contact line
length, which is proportional to the energy barrier for the drop
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