The Beetle’s ‘Whitest White’ Album
Many things in this world promise “the brightest white.” Toothpaste, laundry detergent, paper, paint and even LED lights all boast the ability to produce a white that nothing else can. But these products have nothing on a seemingly lowly insect that uses light-scattering scales to reign supreme.
A new investigation of the Cyphochilus and Lepidiota stigma beetles at the University of Cambridge and the University of Florence has shown that the scales covering their bodies scatter light more efficiently than any other biological tissue or man-made material – truly achieving the brightest white.
Animals produce bright colors for several purposes, from camouflage to communication, mating and thermoregulation, and they’re usually produced using pigment. Colors are produced by materials absorbing certain wavelengths of light and reflecting others. To appear white, however, a tissue needs to reflect all wavelengths of light equally, with the same efficiency – and these beetles are producing a type of white that has never before been seen, reproduced or understood.
Photo courtesy of Lorenzo Cortese and Silvia Vignolini.
To find out how the beetles could achieve this unprecedented level of white, the researchers gathered single scales from the beetles’ bodies and placed them on microscope glass slides, where they were immobilized by electrostatic forces. By the cross-sectioning of focused ion beam milling, the researchers found inside the scales a network of chitin – a molecule similar in structure to cellulose, found throughout nature in the shells of mollusks, the exoskeletons of insects and the cell walls of fungi. The chitin filaments, just a few billionths of a meter thick, act as a dense light-scattering material for the beetle.
Typically, bright white requires a thick system comprising randomly positioned high-refractive-index scattering centers. The beetles were bypassing this prerequisite with their very thin scales.
“Current technology is not able to produce a coating as white as these beetles can in such a thin layer,” said Dr. Silvia Vignolini of Cambridge, who led the research. “To survive, these beetles need to optimize their optical response, but this comes with the strong constraint of using as little material as possible in order to save energy and to keep the scales light enough in order to fly.”
The internal structure is key. Over millions of years of evolution, the beetles developed an anisotropic, or directionally dependent, network of chitin filaments that allow high intensities of reflected light for all colors simultaneously. This network makes the scales appear white at every angle of observation. The researchers discovered that the scales force light to undergo pronounced multiple scattering, with the lowest transport mean free path ever reported for low-refractive-index systems.
“These scales have a structure that is truly complex, since it gives rise to something that is more than the sum of its parts,” said co-author Dr. Matteo Burresi of the University of Florence. “Our simulations show that a randomly packed collection of its constituent elements by itself is not sufficient to achieve the degree of brightness that we observe.”
Time-resolved and total-transmission experiments were performed using a probe pulse laser focused with an aspheric lens. Total light transmission was measured using a large-area silicon photodiode and a lock-in amplifier. And thanks to these technologies, the newly found natural structures may become inspirations for commercial paints, papers, lights and displays.
“The lessons we are learning from these beetles is twofold,” Vignolini said. “On one hand, we now know how to look to improve scattering strength of a given structure by varying its geometry. On the other hand, the use of stron
gly scattering materials, such as the particles commonly used for white paint, is not mandatory to achieve an ultrawhite coating.”
The study was published in Scientific Reports (doi:10.1038/srep06075).
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