Butterflies are often described as colourful, when in fact they are structureful: the bright colours of their wings are a product of light refraction in tiny repetitive patterns within the wings rather than reflection from coloured tissue. But how to turn them into a temperature sensor readable to the human eye?
By Guo Ping Wang and Wang Zhang
What we see as natural white light is a mix of all colours – different wavelengths – from purple through blue, green and yellow to red.
The most common mechanism for a natural surface to have a colour is to absorb all the other wavelengths and reflect that colour, which the observer then sees.
The zebra’s stripes – a natural form or air conditioning
A zebra’s stripes not only act as an effective camouflage from preditors, the black and white pattern also keeps them cool under the hot African sun. The monochrome stripes result from fur that absorbs all light, while the white areas reflect all of it. The way dark and light fur absorbs and gives off heat cools the zebras down. Accordingly, a green leaf ‘keeps’ most the blue and red hues and reflects most of the green wavelengths.
Photonic crystals – the shapes in wings
Butterfly wings, however, have no colour. Instead, what the observer sees is the result of sunlight entering the tiny shapes within the wings and being affected by them. These shapes are the size of a billionth of a metre and therefore invisible to the human eye. One such nanostructure alone has no effect that humans can see. Effects only become visible for us when many nanostructures appear in an equidistant pattern, called photonic crystals.
Research inspired by butterfly wing structures is seeking ways to use such photonic crystals to act upon a change in their surroundings to tell us about it by changing the colour of light they let escape. While it was found in the past that a change in temperature can affect the colour of a photonic crystal through thermal expansion of the crystal structure, the change in wavelength is too modest for a viable temperature sensor suitable for naked eye use.
Scientists from Shenzhen University have found a way to magnify the visible effect by introducing a stronger link between the photonic crystal and the ambient temperature. Their work was published in the open access journal Nanophotonics.
The trick is to make temperature changes affect the refractive index of the open spaces within the photonic crystal by adding a temperature-dependent substance to the air in them. In their paper, the authors demonstrate how encapsulating a butterfly wing structure between glass plates together with an amount of ethanol leads to a device that is highly sensitive to temperature. Changing the latter affects the balance in the enclosed space between liquid and gaseous ethanol, increasing or decreasing the amount of ethanol vapour lacing the air. This in turn shifts the refractive index of the open spaces in the photonic crystal.
The paths of each wavelength through the photonic crystal change dramatically depending on that refractive index. Accordingly, the colour actually capable of escaping the structure again at the given temperature does too – and the human eye can see the marked difference.
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