Structural color and the lotus effect.

The study of biological microstructure is one of the most important research areas in biomimicry. [1–3] Microstructure plays many important roles in living things. [2,3] For example, the charming blue color of the Morpho sulkowskyi butterfly originates from light diffraction and scattering, which results from the ordered microstructure of its scales. This form of color is usually known as structural color, which is utilized by animals both for protection and as a warning. Today, the study of structural color has been extended from biology to optics. [4–6] As well as affecting coloration, microstructure also plays an important role in self-cleaning. [2, 7] For the butterfly, the specific nanostructure enhances the hydrophobicity of its wings, which allows droplets of water to be dispersed more easily. During this process, dust particles on the surface of the wings are removed. This phenomenon is known as the “lotus effect”, which is not only very useful for natural species, but also for materials applications, such as for decoration where a natural force might be used to clean a surface. It would be interesting to discover whether it is possible to design a material that incorporates both structural color and the lotus effect, thus mimicking the wings of a butterfly. Such a material should be of great biological and technological importance. In this paper, we will show one approach to fabricating such a biomimetic decorative material by taking advantage of a nanostructured inverse opal surface. Inverse opal is a solid material that consists of a threedimensional network. [6,8–10] Orderedmonodisperse air spheres throughout the network contribute to an optical stop band, the position of which can be tuned by careful control of the periodicity of the air spheres. Colors can be observed by the naked eye when the stop band falls in the visible region. As a consequence of its unique optical properties, inverse opal has been regarded as a new-generation decorative material, in addition to its application as a photonic crystalline material. [6,11] Recently, we realized that inverse opal might also be incorporated into the design of a hydrophobic material. The solid material network of inverse opal contributes a rough surface composed of well-ordered meshes. According to the Cassie–Baxter law, the intrinsic wettability of the solid material can be greatly reduced. [12] Such a decorative material, which exhibits both structural color and the lotus effect, would be environmentally friendly and energy-efficient. For practical applications, a convenient method of fabricating a uniform inverse opal film over a large area is required. In addition, the rough inverse opal surface needs to be further optimized to imbue the surface with superhydrophobic character. We describe here the development of a dipping method that can be used to meet these criteria, and which can derive uniform inverse opal films with a nanostructured surface. The procedure for the fabrication is as follows: First, submicron-sized monodisperse polystyrene spheres and nanosized particles were ultrasonically dispersed into deionised water. A glass substrate was then immersed into the solution and withdrawn at a constant speed. It is known that a mixture of spheres with different sizes cannot be used to fabricate colloidal crystals with long-range structural order by such a deposition method,[13–15] as phase separation occurs, or an amorphous structure is formed. In our experiment, we found that this conclusion is only partially correct. A structure with long-range order can be derived when the ratio of the diameters of the spheres falls into a particular regime. Figure 1a shows an image of a structure composed of monodisperse spheres, while Figure 1b–d displays three images of structures composed of spheres of two sizes, with diameter ratios of 0.94, 0.34, and 0.07, respectively. The structure formed by the spheres of varying size depends on the diameter ratio. A structure with long-range order can be observed in films composed of monodisperse spheres, however, such order is absent in films composed of spheres of two sizes, where the diameter ratio is larger than 0.15. Usually, the particles form a structure with discernible separation when the ratio between the two types of sphere is larger than 0.5 (Figure 1b), while the domains formed by different types of particles are separated when the ratio is smaller than this value (Figure 1c). When the diameter ratio between the [7] W. P. Rothwell, W. Shen, J. H. Lunsford, J. Am. Chem. Soc. 1984, 106, 2452 – 2453. [8] Each unit cell of CHA contains 36 T sites and three cages. The HSAPO-34 used has one Si, five P, and six Al atoms per cage. [9] J. F. Haw, P. W. Goguen, T. Xu, T. W. Skloss, W. Song, Z. Wang, Angew. Chem. 1998, 110, 993 – 995; Angew. Chem. Int. Ed. 1998, 37, 948 – 949.