Shin'ya Yoshioka

Production of Colored Pigments with Amorphous Arrays of Black and White Colloidal Particles

There are many technical and industrial applications for colored pigments with nonfading properties. The development of a low-cost, high-volume production method for nonfading pigments with low toxicity and minimal environmental impact may promote their widespread use. To accomplish this goal, pigments need to be prepared using abundant and environmentally friendly compounds. Here, we report on the variously colored aggregates formed by spraying fine, submicrometer-sized spherical silica particles. The microstructure of the aggregate is isotropic with a shortrange order on a length scale comparable to optical wavelengths, and exhibits an angle-independent structural color as a result of wavelength-specific constructive interference. Interestingly, the color saturation of these aggregates can be controlled by the incorporation of a small amount of conventional black particles, such as carbon black (CB). We demonstrate that a Japanese-style painting can be successfully drawn with this method. Silicon dioxide, which is a major component of silica particles, is chemically stable and used in scientific glassware suitable for chemical experiments. It is also a primary component of soil and found in abundant supply in nature. Furthermore, in vivo toxicity of silica particles that are greater than 300 nm in diameter has not been detected. Therefore, submicrometer-sized silica particles are one of the best candidates for fabricating environmentally friendly materials. Fine submicrometer-sized spherical silica particles usually appear white to the human eye when they are in powdered form. However, assemblies of these particles can appear colored because of wavelength-specific optical interference, 5, 7] despite the absence of light-absorbing pigments and dyes. Such color is generally referred to as structural color, because it is essentially caused by the microstructure through optical phenomena, such as interference, diffraction, and scattering. 9] Crystalline arrays of fine submicrometersized spherical silica particles (colloidal crystals) are well known examples of assembled particles that have structural colors as a result of a very high reflectance at a certain wavelength of light. However, the structural colors produced by colloidal crystals show distinct variations, which depend on viewing and light illumination angles. Such iridescence makes the use of colloidal crystals as pigments difficult, because typical pigments generally require a constant color at different viewing angles. The iridescences of the colloidal crystals originate from Bragg reflection, which is the reflection mechanism that occurs as a result of the long-range order in the particle arrangement. Thus, if the arrangement is changed from the crystalline structure to the amorphous state, which has only a short-range order, iridescence is expected to be suppressed. In fact, amorphous aggregates of colloidal particles have been reported to exhibit angle-independent structural colors. 4, 5,12] However, amorphous colloidal arrays are difficult to fabricate because submicrometer-sized particles have a strong tendency to crystallize. Previously, amorphous colloidal arrays have been prepared by mixing two different kinds of submicrometer-sized silica particles. 4, 5, 13] These mixtures exhibit structural colors, but the colors are very pale. 4,5] Therefore, such amorphous colloidal arrays are unsuitable for use as brightly colored pigments. A simple synthetic method for the preparation of assemblies of submicrometer-sized particles with angle-independent brilliant structural colors for use as pigments has not yet been reported. Herein, we report a simple and reproducible synthetic procedure for the preparation of pigments that exhibit angleindependent, bright structural colors from amorphous colloidal arrays by spraying fine submicrometer-sized spherical silica particles of uniform size. We added a small amount of black particles to the colloidal amorphous array to enhance the saturation of the structural color by reducing incoherentlight scattering across the entire visible spectrum. Variously [*] Prof. Y. Takeoka, Prof. A. Takano, M. Teshima, Y. Ohtsuka, Prof. T. Seki Graduate School of Engineering, Nagoya University Furo-cho, Chikusa-ku, Nagoya, 464-8603 (Japan) E-mail: ytakeoka@apchem.nagoya-u.ac.jp

Mechanism of variable structural colour in the neon tetra: quantitative evaluation of the Venetian blind model

The structural colour of the neon tetra is distinguishable from those of, e.g., butterfly wings and bird feathers, because it can change in response to the light intensity of the surrounding environment. This fact clearly indicates the variability of the colour-producing microstructures. It has been known that an iridophore of the neon tetra contains a few stacks of periodically arranged light-reflecting platelets, which can cause multilayer optical interference phenomena. As a mechanism of the colour variability, the Venetian blind model has been proposed, in which the light-reflecting platelets are assumed to be tilted during colour change, resulting in a variation in the spacing between the platelets. In order to quantitatively evaluate the validity of this model, we have performed a detailed optical study of a single stack of platelets inside an iridophore. In particular, we have prepared a new optical system that can simultaneously measure both the spectrum and direction of the reflected light, which are expected to be closely related to each other in the Venetian blind model. The experimental results and detailed analysis are found to quantitatively verify the model.

The Weak Iridescent Feather Color in the Jungle Crow Corvus macrorhynchos

Abstract We investigated the origin of the iridescent violet-bluish feathers of the adult Jungle Crow, Corvus macrorhynchos, using microscopic and optical techniques. A single layer of melanin granules was found below the surface of the barbules in the feathers of male crows. Although the barbule microstructure was clearly sexually dimorphic, neither the appearance nor the optical measurements were notably different between the sexes, which indicated that the single layer of melanin granules did not contribute to the iridescent color of the feathers. We also found a thin layer, which we refer to as the epicuticle, at the surfaces of the barbules of both male and female feathers, indicating thin-film interference as the most likely cause of the iridescent color. We investigated this possibility by measuring reflection patterns and spectra. Our results suggest that the weak violet-bluish feather color of the feathers of the Jungle Crow is caused by thin-film interface due to the presence of an epicuticle on the feather barbules.

Structural color of a lycaenid butterfly: analysis of an aperiodic multilayer structure

We investigated the structural color of the green wing of the lycaenid butterfly Chrysozephyrus brillantinus. Electron microscopy revealed that the bottom plate of the cover scale on the wing consists of an alternating air-cuticle multilayer structure. However, the thicknesses of the layers were not constant but greatly differed depending on the layer, unlike the periodic multilayer designs often adopted for artificial laser-reflecting mirrors. The agreement between the experimentally determined and theoretically calculated reflectance spectra led us to conclude that the multilayer interference in the aperiodic system is the primary origin of the structural color. We analyzed optical interference in this aperiodic system using a simple analytical model and found that two spectral peaks arise from constructive interference among different parts of the multilayer structure. We discuss the advantages and disadvantages of the aperiodic system over a periodic one.

Elucidation and reproduction of the iridescence of a jewel beetle

It is known that the structural colors of some beetles originate from multilayer thin-film interference. We investigated such an example, a jewel beetle Chrysochroa fulgidissima, to quantitatively characterize the coloration mechanisms. The essential physical factors of the iridescence were elucidated by careful determinations of the structural parameters, measurements of angle and polarization-dependent reflection spectra, and theoretical modeling of the multilayer system. On the basis of the elucidated coloration mechanisms, we successfully prepared a dielectric thin film structure that reproduces the iridescence of the jewel beetle.

Time and Temperature Dependence of Recovery Behavior of Residual Strains in Largely Deformed Glassy Poly (Methyl Methacrylate)

Time and temperature dependence of recovery behavior of residual strains in largely compressed glassy poly (methyl methacrylate) (PMMA) has been studied. At several temperatures lower than the glass transition temperature Tg, uniaxially compressed specimens were isothermally kept in the stress-free state for a variation of time tf, and then subjected to thermally stimulated strain recovery measurement. Thermally stimulated recovery of residual strains in specimens right after the compression began at heating temperatures lower than Tg. With increasing time tf, residual strains recoverable at heating temperatures below Tg decreased simply from those recoverable at the lowest heating temperature. The decrease of residual strains recovering at a heating temperature less than Tg was examined as a function of time tf and was found to be expressed by a single exponential retardation function. Thus, the sub-Tg strain recovery was revealed to be a simple viscoelastic process with distribution of retardation times. Temperature dependence of the retardation time allowed us to calculate activation energy of sub-Tg strain recovery as a function of heating temperature. The activation energy of sub-Tg strain recovery was found to be approximately the same function of temperature as that of the linear viscoelastic relaxation of the polymer. This result led us to a conclusion that, as a first approximation, the molecular mechanism governing the sub-Tg recovery of strains given by large deformation is almost the same as that of linear viscoelastic relaxation.