A colourful material made from wood cools down even when under direct sunlight, meaning it could be used to decorate the outside of buildings while lowering their internal temperature without the need for air conditioning.
Most materials warm up when exposed to sunlight because they absorb a combination of light. But some have a cooling effect because they reflect these wavelengths instead and on top of that also give off some of the warmth within the material through the atmosphere into space.
Adding colors to such a material normally increases the amount of light it absorbs, but Silvia Vignolini at the University of Cambridge and her colleagues have now found a way to make versions in red, green or blue that have a cooling effect and the key ingredient comes from wood, in the form of two types of cellulose (纤维素).
The chemicals in cellulose are particularly good for giving off the right kind of light to have a cooling effect, while cellulose nanocrystals (纳米晶体)—which can be got from sources such as wood or cotton—produce different colors. This effect is similar to how soap bubbles show colors on their surface by spreading different wavelengths of light in different directions.
Vignolini and her colleagues combined these features by layering the nanocrystals on top of a highly reflective sheet (层) made from a kind of cellulose.
By adjusting the arrangement of the nanocrystals, they produced versions of this material in red, green and blue, and found they were on average 3℃ cooler than the surrounding temperature in daylight.
This early version is promising, but the coloured layer is sensitive to environmental effects like weather, says Vignolini, so more work is needed to make it a practical decoration. "Going from here to an application on a wall or on a device, you would need to have it stable enough," she says.
