#weekendusers How does nature make a colour whiter than white?

02-09-2016

This question puzzles a team from the University of Sheffield in the UK working this weekend on ID16B. The Cyphochilus (occurring in South East Asia) and Lepidiota stigma beetles produce an exceptionally white colour, using only a thin layer of material, in the range of 10-15 microns.

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These beetles' structural colours rely on the optical properties of their morphologies to produce colour, as opposed to pigments which rely on their absorption spectra. This means that pigments photodegrade, whilst the natural colours (also called structural colours) don´t fade as long as the structure remains uncompromised. Potential artificial structural colours would have extensive industrial applications in areas such as coatings, fabrics and sunscreens. .

So how do the beetles do it? The nanostructure responsible for their impressive optical scattering is a porous network. Because of the complexity of this network, 2D image slices taken with traditional microscopy methods provide a limited picture of the total structure. And this is the reason why the team is now working on a comprehensive 3-D analysis of the structures in the scales.

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The team in action in the experimental hutch of ID16B. 

The sample for this experiment is tiny: the scale of a beetle of 110microns by 300 microns. But the internal structure the team wants to study is actually 200 nanometres. “We need to use a non-destructive technique that can give us a high resolution 3-D image - a task which ID16B is uniquely suited for.”, explains Stephanie Burg, a PhD student who is the main proposer of the experiment. The goal is to understand and ultimately model the complex optical properties and design rules of these disordered systems.

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Julie Villanova (left), from the ESRF, and Stephanie Burg, main proposer, discuss the best way to align the minuscule samples.

With Burg, there are two other scientists from the University of Sheffield: Andrew Dennison and Adam Washington, as well as Paul Evans, an artist in residence, who is studying structural colour and developing a series of artworks in relation to this phenomenon as it occurs in nature. The team also collaborates with the researchers at the Natural History Museum in London, who helped provide zoological expertise. 

We might be still far away from being able to make these structures commercially, but there is a group of enthusiastic scientists who are determined to find out the key to the whitest white during several days and also – pun intended – very white nights…

Text by Montserrat Capellas Espuny

Top image: The Cyphochilus beetle. Credits: The wild center.