Ellen Phiddian
Researchers have made a ceramic, inspired by the world’s whitest insect, that can reflect 99.6% of solar light aimed at it: a “near-perfect” reflectivity.
The strong, recyclable ceramic could be a major energy saver if used to coat buildings as a passive cooling device.
A description of the ceramic was published in Science.
To develop their material, the researchers looked at a Cyphochilus beetle – a genus native to Southeast Asia.
Cyphochilus beetles are some of the whitest insects in the world, thanks to their tiny scales. In the wild, these beetles use their whiteness to camouflage against fungi. But it has another advantage for engineers – very high reflection of sunlight.
“Nature offers us an abundance of intricate designs, efficient systems and sustainable solutions that have evolved over millions of years,” says study co-author Professor Zuankai Wang, from Hong Kong Polytechnic University’s Department of Mechanical Engineering.
“Through careful study of these natural phenomena, we can uncover innovative ideas and principles that can be translated into practical applications.”
Using a scanning electron microscope, Wang and colleagues found a beetle specimen had 15,000 chitin scales per square centimetre of exoskeleton.
These teardrop-shaped scales were clustered in random ways, allowing for a very porous exoskeleton filled with air bubbles.
The combination of tiny chitin scales and air bubbles scattered light extremely well.
Based on this discovery, the researchers made a ceramic out of alumina and some carbon- and silicon-based polymers.
The ceramic had a “hierarchical porous structure”: it had pores of different sizes throughout.
As well as allowing the ceramic to reflect nearly all the sunlight aimed at it – 99.6% of the sunlight, to be precise – this structure gives the material a few useful properties.
One is its passive cooling: because it reflects light so well, the material can cool surfaces beneath it.
Another is its water-repelling properties, which inhibits the Leidenfrost effect: the formation of a tiny layer of vapour when liquid hits a hot surface, letting the liquid levitate.
The Leidenfrost effect makes cooling surfaces less efficient at high temperatures, but Wang leaned on previous research done by his team to avoid it.
“It is this intricate structure that enables the ceramic to effectively draw in and evaporate liquid, thereby efficiently inhibiting the Leidenfrost effect,” says Wang.
The water-repelling also makes the surface self-cleaning, and the structure makes it mechanically strong.
The material can be easily recycled, according to the researchers. They were able to grind it into a powder and then re-make it 10 times without any loss in performance.
“All these characteristics make it ready for real-world applications,” says Wang.