Iridescent, plant-based coatings that cool buildings – and the Earth

Blue film with iridescent edges
This film is made from two layers of plant-based cellulose materials, and cools down when it’s in the sun. It can be tuned to different colours and textures. Credit: Qingchen Shen

Chemists have made an iridescent, plant-based film that gets cooler in sunlight.

The material, which comes in a range of shining colours, could one day coat buildings and cars, lowering the need for air conditioning.

The film exhibits a smart property: called passive daytime radiative cooling, or PDRC, it doesn’t absorb much light, and it radiates heat out at a wavelength that escapes the atmosphere and zooms straight into space.

“To make materials that remain cooler than the air around them during the day, you need something that reflects a lot of solar light and doesn’t absorb it, which would transform energy from the light into heat,” explains principal investigator Professor Silvia Vignolini, a chemist at the University of Cambridge, UK.

“There are only a few materials that have this property, and adding colour pigments would typically undo their cooling effects.”

Colour usually comes from a pigment that absorbs some sunlight: a green leaf absorbs red and blue light, for instance, leaving green to be reflected out.

This means that if you want something to reflect all sunlight back as light, it usually has to be white.

But colour can come from other properties: structural colour happens when light interacts with tiny structures inside a substance, which bounce the light around until it comes out in a different hue.

Structural colour often produces shining and iridescent results, like in beetles and hummingbirds.

Viglioni and colleagues figured out a way to create the effect with cellulose, which is one of the few naturally occurring materials known to be capable of forming PDRC materials.

Their film, which they’re discussing at the spring meeting of the American Chemical Society, is made from cellulose nanocrystals (CNC) – tiny particles about the same size as individual wavelengths of light.

These CNC films can be manipulated to look red, green, or blue, but they’re quite brittle. The researchers have figured out how to attach the films to ethyl cellulose: a more flexible white material.

When combined, the researchers had a flexible, colourful film that is 4°C cooler than the ambient temperature during the day.

They’ve also figured out how to make the films glittery, and show different textures, so they could be adapted to any architectural style.

The films should be easy to manufacture metres at a time, and cellulose is a cheap feedstock.

But before you can coat your car or house with them, the researchers want to see if they can introduce more functions: CNCs can be sensitive to pollutants, so they’re exploring whether they can build smoke or smog detectors into their films.

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