Biologically Inspired Design: Color on Wings
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ON WINGS
Mohan Srinivasarao*and Luis Padilla Department of Textile Engineering, Chemistry and Science Fiber & Polymer Science Program North Carolina State University Raleigh, NC 27695-8301 INTRODUCTION Brilliant, iridescent colors found on the bodies and wings of many birds, butterflies and moths are produced by structural variations and have been the subject of study for centuries. Such brilliant colors have been described as metallic colors due to the saturation or purity of the color produced and have attracted the attention of great scientists like Newton, Michelson and Lord Rayleigh. It was recognized early on that such colors arise from physical effects such as interference or diffraction as opposed to colors that are normally produced due to the presence of chromophores which absorb or emit light. Common examples of physical colors are some butterfly wings [1], color of Indigo snake skin [2], hummingbird feathers [3,4], arthropod cuticles [which are due to selective reflection of color from the solidified cholesteric phase of chitin crystallites] [5], gemstones like opal [6,7], and some crystals like potassium chlorate [8]. While the origins of such colors are well understood the properties of color and color specification have not received much attention. It is worthwhile at this point to ask the question "what is color?". One can turn to The Oxford English Dictionary for help, which states: "The particular colour of a body depends upon the molecular constitution of its surfaces, as determining the character and number of light vibrations it reflects. Subjectively, colour may be viewed as the particular sensation produced by the stimulation of the optic nerve by particular light vibrations----. This sensation can be produced by other means, such as pressure on the eye-back or an electric current". In order for us to see or perceive object color, purely from a physical standpoint, three things are essential: a light source, an object that it illuminates, and the eye (and brain) to perceive the color. In order to specify color according to a universal standard the eye is replaced by a photodetector for making quantitative measurements of the light that would have reached the eye. However, this color specification depends on spectral color matching functions (spectral tristimulus values) provided by panels of human observers. We will not deal with the color perception as it is beyond the scope of this article but we will address the issue of color measurement. This study focuses on butterfly wings, and cholesteric liquid crystals. We are interested in specifying color from these materials in the language of color science. A common unifying theme in considering this set of materials is the nature of color generation. In both of these materials color is generated in the absence of chromophores, and primarily by structural variations resulting in interference or diffraction. There are three main attributes that need to be considered for specifying color: hue, saturation, and in the case of nonself-
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