YOU dip a plastic wand into a mixture of water and washing up liquid and raise it to your lips. With a gentle blow, you unleash a stream of bubbles that bob about in the air, winking a swirling rainbow of colours back at you as they reflect the light. This is the beauty of iridescence.
You can also see it on the wings of birds and the shells of beetles. It is a completely different way of creating colour than the pigments and dyes we typically load onto brushes and into printers, one that is more subtle and adaptable. Now, we are starting to get a deeper understanding of iridescence. We are learning that animals make use of it in surprisingly varied ways, and it seems we could soon join them in harnessing this phenomenon to pull a few tricks of our own.
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Most of us think we know the basics of how colour works. A ray of white light, composed of many different wavelengths or colours, strikes an object. Some of the wavelengths are absorbed by pigment molecules and the remaining ones are bounced back and seen as a particular colour. All this is true – but it isn’t quite the full story.
Colour can also be produced by surfaces that reflect, or scatter, different wavelengths of light back in slightly different directions. This is what happens when we look at the surface of a bubble or a bird’s wing and see that characteristic iridescent shimmer. If you move your eyes, the colour of the surface seems to shift and dance. The reflections are caused by tiny structures – bumps, hairs, ridges – on the surface that are somewhere around a billionth of a metre across.
We have known of iridescent structures in nature for a long time. Robert Hooke identified them in peacock feathers in the 1600s. Since then we have discovered that iridescence is responsible for the hues of many other living things. That includes insects, like jewel beetles, and parts of plants, like the marble berry, which looks just like a deep blue marble. There is even a mole with an iridescent coat.
More recently, we have learned that surface structures don’t always produce that characteristic shimmering iridescence. Tiny structures on surfaces can also produce more ordinary looking colours – and they can be some of the darkest and brightest in the natural world.
In 2014, biochemist white Cyphochilus beetle from South-East Asia uses this structural effect to achieve its colour. Their scales . Since these beetles like to hide among a certain kind of white fungi, it isn’t difficult to see why an ultra-white shell is handy. The secret of the insect’s dazzling look seems to be that the tiny rods on the scales are of a very particular size. “We know that if structures are similar in size to the wavelength of light, then they can scatter the light,” says Qingchen Shen, who is based at Vignolini’s lab.
The beetles’ nanorods look white because they reflect nearly all light. But other structures can scatter light in a way that makes them appear intensely black. In 2018, the .
The team found that a typical black feather reflects between 3 and 5 per cent of light, but when viewed from certain angles, . Part of the work involved coating the feathers in gold dust, to make them amenable to analysis by electron microscope – and they still looked black even then. McCoy says this super-blackness helps set off the vibrant colours elsewhere in the birds’ plumage, which they use to attract mates.
Beat the heat
Strange as it sounds, structural colour isn’t just about colour. Because the size of the nanostructures involved determines the wavelength of radiation they affect, slightly larger structures scatter infrared radiation – otherwise known as heat – not visible light.
Nature hasn’t missed this. Some butterflies are thought to use such effects to absorb heat and warm their bodies ready for flight. Other insects use it to beat the heat. Saharan silver ants scurry across the desert, where air temperatures can hit 50°C. A few years ago, we . This reduces their body temperature by a couple of degrees, helping them survive conditions in which other creatures perish.
In 2020, materials scientist Han Zhou at the Shanghai Jiao Tong University in China looked at a species of longhorn beetle that lives on active volcanoes in Indonesia, where ground temperatures can top 70°C. She found .
Inspired, Zhou wondered if she could design a structural colour coating that could be applied to buildings to lower temperatures. We already paint houses white for this reason, of course, as part of our quest to hit net-zero carbon emissions. But Zhou suspected that structural colour could perform better than regular white paint.
“Iridescent materials can be made from almost anything”
She developed a polymer film coated in microscopic pyramids and a random arrangement of ceramic particles. She says this mimics the effect of the beetles’ nano-prisms and the coating reflects around 95 per cent of light. Zhou tested it by applying the material and a sheet of white paper to a car bonnet. She found that the snazzy new polymer kept the car about 5°C cooler.
Others, including Shen and the team that discovered the secrets of the Cyphochilus beetles’ scales, say they are also working on . But would these really be better than white paint?
David Sailor at Arizona State University and his colleagues recently conducted a modelling study in which they compared the cooling effects of white paint, which is about 70 per cent reflective, with a supercooling coating that is about 95 per cent reflective. They modelled this across eight US cities and found that the reduction in demand for energy for cooling could be .
Structural coatings could also be more environmentally benign to produce. White paint often uses titanium dioxide as a pigment. “You usually extract it by melting rocks,” says Benjamin Droguet, who also works in Vignolini’s lab. “It is definitely not something which is good for the environment.” Because their colour comes from structure, not specific chemicals, iridescent materials can be made from almost anything, including renewable materials like recycled plastic or cellulose derived from wood pulp.
An unexpected bonus of supercooling structural colour materials is that they don’t have to be white. Not everyone wants a white building, and they can be so bright that they dazzle passers-by. The trouble is that conventional coloured paints absorb a lot of infrared radiation, so white is usually the best option by far for keeping cool.
To get around this, Yuan Yang at Columbia University in New York and his colleagues have developed a coating with two layers: one has an iridescent structure that reflects 90 per cent of infrared light, then regular coloured paint is added on top. Tests have shown that .
Yang says the coatings work, though, in any shade or hue. The cities of our net-zero future will need to be cooler, but that doesn’t mean they can’t be as colourful as a rainbow.
