When it comes to colors, nature has invented all manner of pigments and microstructures to give it a breathtaking palette. By far the hardest to make, though, is white, because it can be created only when all visible wavelengths are scattered in a certain way.
That’s not to say nature abhors whiteness. It offers a multitude of examples: snow, milk, teeth, and cabbage butterflies, to name just a few. Our understanding of how these conventional surfaces scatter light has helped to improve the formulation of paint, paper, and many other things. But now the work of a physicist in England on an obscure Southeast Asian beetle is turning convention on its head, with tantalizing hints that we could make all things white—LEDs included—far more efficiently than we do now.
Pete Vukusic at the University of Exeter, in southwest England, specializes in teasing apart the ingenious mechanisms that nature has invented for generating color [see ”Butterfly Effect,”IEEE Spectrum , February 2006]. But the fingernail-size Cyphochilus beetle has been one of his greatest challenges: its ultrathin shell, which helps the beetle hide among the local fungi, turns out to be one of the whitest natural objects on the planet. The International Organization for Standardization (ISO) has measures of whiteness and brightness on which Cyphochilus measures 60 and 65, respectively. Human baby teeth have a whiteness of only 40 and a brightness of 53.
What really sets Cyphochilus apart is the thinness of the shell in which it achieves that whiteness, compared with competitors. ”Whiteness is the result of the random scattering of light within an object,” says Vukusic. That requires distances of the order of a kilometer for a cloud and a centimeter for milk. Cyphochilus does it in only 5 micrometers. That’s less than 1 percent of the thickness of common synthetic coatings with a similar whiteness.
The Cyphochilus shell is made up of elongated flat white scales consisting of a tough but flexible cuticle material. When Vukusic examined one of these scales under an electron microscope, he found an entirely random network of interconnecting cuticle filaments, each just 250 nanometers across. That’s curious, he says, because closely related beetles produce brilliant colors owing to the periodic order found in these kinds of cuticle structures.
One factor contributing to the beetle’s whiteness may have important industrial implications: the relatively large amount of empty space within the cuticle structure. It’s at the interface between the cuticle filaments and the air in the spaces that light scattering takes place. The large amount of space turns out to be perfect for producing whiteness, and Vukusic thinks he knows how to put this trick into industrial practice.
One possibility is that a synthetic version of the beetle shell could one day be used to make white LEDs more efficient for tasks such as household lighting. The trouble with LEDs is that they emit light in all directions, and as much as half of it can end up being absorbed by the substrate on which the LED sits. A way around the problem is to place an LED on a mirror to reflect this light. But Vukusic says that an even better option would be to place the LED on a synthetic version of the beetle shell, which would reflect white light as well as any mirror. ”It would be much thinner and lighter than a mirror,” he says.