The color and structure looked interesting, Bartl says. The question was: What was the exact three-dimensional structure that produces these unique optical properties"
The Utah teams study is the first to show that just as atoms are arranged in diamond crystals, so is the chitin structure of beetle scales, he says.
Galusha determined the 3-D structure of the scales using a scanning electron microscope. He cut a cross section of a scale, and then took an electron microscope image of it. Then he used a focused ion beam sort of a tiny sandblaster that shoots a beam of gallium ions to shave off the exposed end of the scale, and then took another image, doing so repeatedly until he had images of 150 cross-sections from the same scale.
Then the researchers stacked the images together in a computer, and determined the crystal structure of the scale material: a diamond-like or champion architecture, but with building blocks of chitin and air instead of the carbon atoms in diamond.
Next, Galusha and Bartl used optical studies and theory to predict optical properties of the scales structure. The prediction matched reality: green iridescence.
Many iridescent objects appear that way only when viewed at certain angles, but the beetle remains iridescent from any angle. Bartl says the way the beetle does that is an ingenious engineering strategy that approximates a technology for controlling the propagation of visible light.
A single beetle scale is not a continuous crystal, but includes some 200 pieces of chitin, each with the diamond-based crystal structure but each oriented a different direction. So each piece reflects a slightly different wavelength or shade of green.
Each piece is too small to be seen individually by your eye, so what you see is a composite effect, with the beetle appearing green from any angle, Bartl e
|Contact: Lee Siegel|
University of Utah