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Flying colours

How could Swiss cheese found in a butterfly wing affect the colour of your car?

Butterfly_Callophrys_rubi

Butterflies are pretty unassuming. They flit about, discreetly sipping nectar and quietly soaking up some sun. So who knew that they could be harbouring the blueprint for next-generation supercomputers or slow-release cancer drugs?

It turns out these kaleidoscopic critters are the owners of some pretty far-out geometrical structures that could prove useful in all sorts of areas. In fact, car companies are already using metallic iridescent paint based on this butterfly building block.

“It’s a really elaborate 3D sponge structure,” explains Professor Stephen Hyde from the ANU Department of Applied Mathematics.

“Think of it as Swiss cheese – it’s an incredibly complicated Swiss cheese.”

Known as photonic crystals, these labyrinthine configurations are responsible for some of nature’s most amazing colours.

“When you see a beetle or a butterfly or even a peacock feather that looks a bit metallic, it’s usually a giveaway that the colour is not due to pigmentation,” says Hyde. “Something funky is going on.”

Different insects have different types of photonic structures. The main aspect that affects the colour is the size of the structure, explains Hyde.

“The species we looked at, Callophrys rubi, has green underwings. They’re green because light comes into the sponge, bounces around, and only the green wavelength is the right size to escape; the rest of the colours get trapped inside.

“If you change the size of the opening, you change which colour can escape.”

Hyde and his team made the discovery by generating the first 3D reconstruction of butterfly photonic crystals using the computational and visualisation facilities at the National Computational Infrastructure, home to the Southern Hemisphere’s most powerful supercomputer.

“The Australian National Fabrication Facility has a really nice machine that allows you to image the surface of the wing using an electron microscope, then slice off a bit and image the surface again,” explains Hyde. “It’s like slicing bread, but extremely thin.

“Then you put all the slices back together to create a 3D image. That’s where we needed the National Computational Infrastructure’s computing power – there are so many slices you need a lot of crunching power.”

When Hyde saw the reconstruction, he immediately recognised a familiar pattern.

“We’ve known for more than 20 years that these sorts of structures appear spontaneously when you mix soap and water in a test tube,” he says.

“In the butterfly, it’s all made of chitin – the stuff insects use to make hard bits – and forms when the caterpillar becomes the butterfly, when it’s all wet inside the chrysalis. We think it’s just like in our test tube: it gives you the structure automatically.

“But the butterfly structure is 100 times larger than the ones we can create in the lab. The physics of how you can stabilise an ordered structure at that scale is completely unknown and that’s what’s really fascinating.”

butterfly wing_microscope_small

As per usual, it turns out nature perfected the process long ago and synthetic substitutes are well behind the game.

“It’s amazing if you think about it. It all works with just water and air, at room temperature. It’s much cleverer than any material science that we’ve got.

“The ultimate question is: what’s the minimum mix of chemicals we can throw in a test tube to create crystals of this size?

“That would be the Holy Grail for us.”

In the animal kingdom, these remarkable structures probably evolved for camouflage and mating purposes, but their potential uses in the synthetic world go well beyond colour.

“Some people are looking at using them for slow-release drugs because they’ve got this complicated sponge network,” says Hyde. “You could take a pill and the drug would slowly seep out into your body to control the dose.

“Other groups are starting to synthesise glass crystals to create optical resistors for computing.”

It certainly makes you wonder what other potential technologies the Earth’s million insect species are yet to let us humans in on.

This story was originally published in ANU Reporter.

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