414
K. Uchida et al.
SEM images of the surfaces in Fig. 24.6-7a,8a,9a are shown in Fig. 24.6-7b,8b,9b,
respectively. Corresponding SEM images of cross sections of samples in Fig. 24.67b,8b,9b are shown in Fig. 24.6-7c,8c,9c, respectively. The size distribution of the
microstructures on natural lotus leaf was reported by box-counting fractal analysis
[17]. The size distribution of the double roughness structure was very similar to that
of natural lotus leaf.
The CAs of water droplets on the surfaces of rod-shaped crystals of 1c (Fig. 24.67a,7b,7c) and cubic crystals of 1o (Fig. 24.6-8a,8b,8c) were around 130°. The water
droplets were pinned on the surface, even when the surfaces were upside down
(Fig. 24.6-7d,8d). The CA and SA on the surface with double roughness structures
were 161.9 ± 2.0° and 2°, respectively. The advanced (CA ad ) and receding (CA rec )
CAs of a water droplet were 162.0 ± 1.6° and 158.3 ± 1.4°, respectively. Thus, a
remarkable lotus effect was observed (Fig. 24.6-9d). The reversibility of the melting
and reformation of double roughness structure by alternate irradiation with UV and
visible light were successfully achieved.
The static CAs and SAs of water droplets on the microcrystalline surface with
double roughness structure resembled those obtained for single roughness surfaces of
the same diarylethene 1. To further examine the hydrophobic property of the surface,
a water-droplet bouncing experiment on the diarylethene microcrystalline surface
with double roughness structures was carried out by dropping the water droplet
from h = 1.8 mm high. The result was compared with those of diarylethene with
single roughness structure and natural lotus leaf (Fig. 24.7). On the lotus leaf, water
droplets bounced (Fig. 24.7a), while bouncing was not observed on a diarylethene
microcrystalline surface with single roughness structure (Fig. 24.7b). The bouncing
was reproduced only on the microcrystalline surface with double roughness structure
(Fig. 24.7c). The importance of double roughness structure over single roughness
structure is thus confirmed.
24.5 Dual Wettability on Diarylethene Microcrystalline
Surface Composited with Two Types of Needle-Shaped
Crystals of Two Diarylethene Derivatives
In nature, many plants and insects have specific interesting properties with specific
rough structures. The termite wing has a specific property of wetting in contact with
a water droplet: it adsorbs water mist, whereas larger water droplets are bounced
on the surface [18]. It is called “dual wettability.” The surface of termite wing is
covered with two types of projections of different sizes. One is a hair-like projection
with about 50 µm long and 1 µm wide, whereas the other is a star-shaped projection
of 5–6 µm in height and width. These hairs protect the wing from the contact with
large water droplets, whereas the small projections work to keep the small droplets
on the surface of the wing, followed by gathering to build up large ones to remove
them via the hair arrays. Here, we reproduce the dual wettability of termite wing by
K. Uchida et al.
SEM images of the surfaces in Fig. 24.6-7a,8a,9a are shown in Fig. 24.6-7b,8b,9b,
respectively. Corresponding SEM images of cross sections of samples in Fig. 24.67b,8b,9b are shown in Fig. 24.6-7c,8c,9c, respectively. The size distribution of the
microstructures on natural lotus leaf was reported by box-counting fractal analysis
[17]. The size distribution of the double roughness structure was very similar to that
of natural lotus leaf.
The CAs of water droplets on the surfaces of rod-shaped crystals of 1c (Fig. 24.67a,7b,7c) and cubic crystals of 1o (Fig. 24.6-8a,8b,8c) were around 130°. The water
droplets were pinned on the surface, even when the surfaces were upside down
(Fig. 24.6-7d,8d). The CA and SA on the surface with double roughness structures
were 161.9 ± 2.0° and 2°, respectively. The advanced (CA ad ) and receding (CA rec )
CAs of a water droplet were 162.0 ± 1.6° and 158.3 ± 1.4°, respectively. Thus, a
remarkable lotus effect was observed (Fig. 24.6-9d). The reversibility of the melting
and reformation of double roughness structure by alternate irradiation with UV and
visible light were successfully achieved.
The static CAs and SAs of water droplets on the microcrystalline surface with
double roughness structure resembled those obtained for single roughness surfaces of
the same diarylethene 1. To further examine the hydrophobic property of the surface,
a water-droplet bouncing experiment on the diarylethene microcrystalline surface
with double roughness structures was carried out by dropping the water droplet
from h = 1.8 mm high. The result was compared with those of diarylethene with
single roughness structure and natural lotus leaf (Fig. 24.7). On the lotus leaf, water
droplets bounced (Fig. 24.7a), while bouncing was not observed on a diarylethene
microcrystalline surface with single roughness structure (Fig. 24.7b). The bouncing
was reproduced only on the microcrystalline surface with double roughness structure
(Fig. 24.7c). The importance of double roughness structure over single roughness
structure is thus confirmed.
24.5 Dual Wettability on Diarylethene Microcrystalline
Surface Composited with Two Types of Needle-Shaped
Crystals of Two Diarylethene Derivatives
In nature, many plants and insects have specific interesting properties with specific
rough structures. The termite wing has a specific property of wetting in contact with
a water droplet: it adsorbs water mist, whereas larger water droplets are bounced
on the surface [18]. It is called “dual wettability.” The surface of termite wing is
covered with two types of projections of different sizes. One is a hair-like projection
with about 50 µm long and 1 µm wide, whereas the other is a star-shaped projection
of 5–6 µm in height and width. These hairs protect the wing from the contact with
large water droplets, whereas the small projections work to keep the small droplets
on the surface of the wing, followed by gathering to build up large ones to remove
them via the hair arrays. Here, we reproduce the dual wettability of termite wing by