Wetting Properties of Natural and Plasma-Processed Biomimetic Surfaces - Biomimetic Architectures by Plasma Processing - page 142

compared to the other two, indicating that pinning of the receding
contact line led to larger hysteresis [70] and, thereby, the larger
sliding angle [71]. In comparison, for two-tier topography the
pinning effect nearly disappears due to the shortest contact line.
Wetting properties can be modified not only by the
nanostructures but also by coating materials that can alter the
SFE. Lo

,
which formed particle-like features, to render them hydrophobic. A
similar coating of TiO

et al. [58] coated their moth eye-type SiNTs with TiO

2

≈ 72°
(Fig. 4.20a). However, when deposited on the SiNTs, they exhibited
hydrophobicity. On coating with TiO

on a polished Si wafer produced

q

2 act

, the super-hydrophilic SiNT

2

with

q

≈ 2° (Fig. 4.14a) exhibits

q

≈ 140° (Fig. 4.20b), with

q

act act rec

= 74°,

q

adv = 110°. A comparison of an uncoated as-grown SiNT

and a TiO coated nanotip revealed that switching from wetting

to de-wetting behaviour could be attributed to the TiO

2-

layer.

2

Figure 4.20 -coated polished

silicon wafer. (b) Tilted view SEM image of a TiO

(a) Cross-sectional SEM image of a TiO 2

-coated SiNT.
The inset shows an optical photograph of a water droplet
indicating the CAs on the respective substrates. Reprinted
from Ref. [58] with permission from IOP Publishing, copyright
IOP Publishing (doi:10.1088/0957-4484/17/10/017).

2

With available data from literature, we give a rough estimate of
the CA variation of different material surface as a function of their
pitch value as shown in Fig. 4.21. Most of the nanostructures show
hydrophobic behaviours with CAs more than 150° for low pitch value.
However, with increasing pitch value hydrophilicity is expressed, as
expected, since the roughness decreased on approaching a flat surface.