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K. Uchida et al.
24.4 Photoinduced Fabrication of Double Roughness
Surface Mimicking Lotus Leaf
and the Superhydrophobicity
In nature, many plants and insects have superhydrophobic surfaces with double
roughness structure. For example, the lotus leaf and legs of a water strider have
the double roughness structure. Historically, the “lotus effect” found by Barthlott
and Neinhuis is the milestone [11]. They investigated the super water-repellent and
self-cleaning effect of the lotus leaf and attributed it to the double roughness structure with structures and nanostructures on its surface, together with the hydrophobic
properties of epicuticular wax [12]. As for the self-cleaning, the bouncing of a water
droplet plays an important role, and artificial double roughness structures have been
prepared [13, 14]. Here, we report a guiding principle to prepare a superhydrophobic
surface with the double roughness structure of a diarylethene microcrystal by using
crystal growth technique (CGT). The prepared surface bounced a water droplet on it
[15, 16].
The procedure to prepare the double roughness structured microcrystalline surface
of 1o is shown in Fig. 24.6. Initially, the microcrystalline surface of 1o (Fig. 24.6-1)
was irradiated with UV light. Then, the colorless 1o near the surface is photoisomerized to be 1c with blue color (Fig. 24.6-2). By storage at 50 °C, the needle-shaped
crystals of 1c grew on the surface (Fig. 24.6-3). Due to Ostwald ripening, the crystals
covering the surface changed from the needle-shaped to the rod-shaped crystals of
1c after storage for 24 h at 50 °C in the dark (Fig. 24.6-4) [5]. To make the double
roughness structure, we need to grow needle-shaped crystals on rod-shaped crystals
as in Fig. 24.6-9a, i.e., first make 1o on rod-shaped crystals by irradiation with visible
light, then grow needle-shaped crystals of 1c by irradiation with UV light. However,
the rod-shaped crystals of Fig. 24.6-4 were not thick enough; the crystals melt upon
visible-light irradiation.
To make thick rod-shaped crystals of 1c, we further applied the Ostwald ripening.
The surface of Fig. 24.6-4 was irradiated with UV light again and stored it at 70
°C for 3 h in the dark (Fig. 24.6-5). Then, the small rod-shaped crystals of 1c grew
on the surface (Fig. 24.6-6), and were incorporated in the larger rod-shaped crystals
(Fig. 24.6-7a). The average width and length of these rod-shaped crystals at the state
shown in Fig. 24.6-7a were 10 and 21 µm, respectively.
Upon visible-light irradiation to the surface of Fig. 24.6-7a, cycloreversion reaction from 1c to 1o proceeded, and the surface was covered with cubic-shaped crystals
of 1o (Fig. 24.6-8a) on the 10-µm rough structures. The size of each bumpy rodshaped crystal was almost the same as that of a projection on a lotus leaf covered
with plant wax tubes. The enlarged crystals were not melted [5, 6].
Upon UV irradiation to the surface, each rod-shaped crystal was covered with
needle-shaped crystals of 1c of 0.2–0.5 µm width and 3–5 µm length. Thus, double
roughness structures were obtained (Fig. 24.6-9a). The needle-shaped crystals were
much smaller than those grown on the flat microcrystalline surface of 1o due to the
small supply of 1c crystals to form needle-shaped crystals on each projection. The
K. Uchida et al.
24.4 Photoinduced Fabrication of Double Roughness
Surface Mimicking Lotus Leaf
and the Superhydrophobicity
In nature, many plants and insects have superhydrophobic surfaces with double
roughness structure. For example, the lotus leaf and legs of a water strider have
the double roughness structure. Historically, the “lotus effect” found by Barthlott
and Neinhuis is the milestone [11]. They investigated the super water-repellent and
self-cleaning effect of the lotus leaf and attributed it to the double roughness structure with structures and nanostructures on its surface, together with the hydrophobic
properties of epicuticular wax [12]. As for the self-cleaning, the bouncing of a water
droplet plays an important role, and artificial double roughness structures have been
prepared [13, 14]. Here, we report a guiding principle to prepare a superhydrophobic
surface with the double roughness structure of a diarylethene microcrystal by using
crystal growth technique (CGT). The prepared surface bounced a water droplet on it
[15, 16].
The procedure to prepare the double roughness structured microcrystalline surface
of 1o is shown in Fig. 24.6. Initially, the microcrystalline surface of 1o (Fig. 24.6-1)
was irradiated with UV light. Then, the colorless 1o near the surface is photoisomerized to be 1c with blue color (Fig. 24.6-2). By storage at 50 °C, the needle-shaped
crystals of 1c grew on the surface (Fig. 24.6-3). Due to Ostwald ripening, the crystals
covering the surface changed from the needle-shaped to the rod-shaped crystals of
1c after storage for 24 h at 50 °C in the dark (Fig. 24.6-4) [5]. To make the double
roughness structure, we need to grow needle-shaped crystals on rod-shaped crystals
as in Fig. 24.6-9a, i.e., first make 1o on rod-shaped crystals by irradiation with visible
light, then grow needle-shaped crystals of 1c by irradiation with UV light. However,
the rod-shaped crystals of Fig. 24.6-4 were not thick enough; the crystals melt upon
visible-light irradiation.
To make thick rod-shaped crystals of 1c, we further applied the Ostwald ripening.
The surface of Fig. 24.6-4 was irradiated with UV light again and stored it at 70
°C for 3 h in the dark (Fig. 24.6-5). Then, the small rod-shaped crystals of 1c grew
on the surface (Fig. 24.6-6), and were incorporated in the larger rod-shaped crystals
(Fig. 24.6-7a). The average width and length of these rod-shaped crystals at the state
shown in Fig. 24.6-7a were 10 and 21 µm, respectively.
Upon visible-light irradiation to the surface of Fig. 24.6-7a, cycloreversion reaction from 1c to 1o proceeded, and the surface was covered with cubic-shaped crystals
of 1o (Fig. 24.6-8a) on the 10-µm rough structures. The size of each bumpy rodshaped crystal was almost the same as that of a projection on a lotus leaf covered
with plant wax tubes. The enlarged crystals were not melted [5, 6].
Upon UV irradiation to the surface, each rod-shaped crystal was covered with
needle-shaped crystals of 1c of 0.2–0.5 µm width and 3–5 µm length. Thus, double
roughness structures were obtained (Fig. 24.6-9a). The needle-shaped crystals were
much smaller than those grown on the flat microcrystalline surface of 1o due to the
small supply of 1c crystals to form needle-shaped crystals on each projection. The