By Ines Urdaneta, PhD in Physics and Researcher at Resonance Science Foundation.
Image: Evan Leeson/Bob Peterson/lowjumpingfrog. None of these animals contain a single trace of blue pigment.
Colors in nature come mainly from three sources: pigments, structural colors, and bioluminescence.
Have you noticed that some colors are more intense than others in nature?
Such is the case of blue and green colors, compared to reds and the rest. The main reason is that blue and green can be structural colors, while the remaining colors seem to not be part of the team.
Structural coloring is the result of microscopically fine structured surfaces that interfere with visible light, sometimes in combination with pigments. For example, peacock tail feathers are brown pigmented, but because of their microscopic structure, they also reflect blue, turquoise and green light. And they are often iridescent. Thus, structural coloring is a classic optical effect of interference and diffraction, rather than a quantum property of photon absorption and emission, which is responsible for color in pigments (as plants, which efficiently absorb red light and the green is reflected) and bioluminescence.
British scientists Robert Hooke and Isaac Newton were the first to observe structural coloration, while its principle - wave interference - was explained by Thomas Young a century later, in 1803. Young described iridescence as the result of interference between reflections from two or more thin film surfaces, combined with refraction when light enters and leaves the film. The geometry determines the angles at which the light reflected from both surfaces interferes constructively and is amplified, and the angles at which the light interferes destructively and is cancelled out. Therefore, different colors appear at different angles.
Well known examples include the colors produced by the bright iridescent scales of butterfly wings and the chins of bird feathers, such as the tail of a peacock.
Recent computer models carried out by researchers from the Chemistry Department at Cambridge University explain why blues and greens are nature's brightest colors.
The appearance of the matte or iridescent color will depend on how the internal structures are arranged on the nano scale. Ordered or crystalline structures result in iridescent colors, which change when viewed from different angles. Most examples of structural color in nature are iridescent.
Unordered structures result in matte colors, independent of viewing angles. Since the color of structures does not fade, these matte colors would be very useful for applications such as paints or coatings where metallic effects are not desired. So far, examples of natural matte structural color only exist in shades of blue or green.
"When we try to artificially recreate structural matte color for reds or oranges, we end up with a poor-quality result, both in terms of saturation and color purity”. Lukas Schertel, co-author of the research
“In addition to its intensity and resistance to fading, a matte paint that uses structural color would also be much more environmentally friendly, since no toxic dyes and pigments would be needed," said the first author, Gianni Jacucci.
By modeling the optical response and color appearance of nanostructures as found in the natural world, the researchers found that saturated and matte structural colors cannot be recreated in the red region of the visible spectrum, which could explain the absence of these tones in natural systems. The same difficulty was found for yellow and orange tones.
The researchers suggest that these apparent limitations of structural colors can be overcome by using other types of nanostructures, such as network structures or multi-layer hierarchical structures. Further investigations and understanding of these systems are required to achieve it.