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Nanofilms based on butterfly wings allow colorful cooling

08 Aug 2023

Shenzhen University materials could allow temperature control of buildings, vehicles and equipment.

A project at Shenzhen University has created nanofilms able to offer both passive cooling and vibrant colors to engineers and designers.

Reported in Optica, the findings join the lengthy list of optical design breakthroughs that take inspiration from the natural world, and in particular the wings of butterflies.

Nature's achievements in creating the wings of Lepidoptera have been studied and adapted by optics engineers for a long time, exploited in diverse applications including banknote security and photovoltaic panels.

The Shenzhen project was interested particularly in the relationship between light reflected from a thin film, its perceived color, and the potential for temperature control offered by those same optical parameters.

"Extensive efforts are expended to cool colorful objects to reduce their energy consumption," noted the project in its paper. "However, a strategy to cool colorful objects below ambient temperature while fully preserving their excellent color properties with high saturation and large viewing fields remains a significant challenge."

The project studied the way in which the wings of Morpho butterflies can produce highly saturated colors, attributed to a wing's multilayered nanostructure, along with a wide range of viewing angles, thanks to the presence of disordered structural elements and their scattering effects.

However, those same structures in butterfly wings can also give rise to energy absorption and inhibit any natural cooling effect. The Shenzhen project tackled this through a design which also incorporated a total reflection layer of silver, on which the layers of nanostructured and disordered components were integrated.

"The researchers placed a disordered material (rough frosted glass) under a multilayer material made of titanium dioxide and aluminium dioxide," commented Shenzhen University. "They then placed this structure on a silver layer that reflects all light, preventing the absorption of solar radiation and the heating associated with that absorption."

Towards carbon-neutral heating and energy sustainability

In trials this three-part design of multilayer structure, frosted glass and silver mirror, was optimized to achieve total reflection of unwanted yellow wavelengths but the highest transmissivity of blue light, so as to retain the vivid hue associated with Morpho butterflies.

The transmitted blue light is then diffusively reflected by the disordered frosted glass and silver mirror, and further transmitted through the multilayer structure to produce high saturation, according to the project.

The researchers created blue, yellow and colorless films, which they placed outdoors at Shenzhen University, on surfaces such as roofs, cars, cloth and cell phones in both winter and summer. Thermocouple sensors and infrared cameras showed that the cooling films were more than 15 degrees centigrade cooler than the surfaces they were placed on in the winter, and about 35 degrees cooler in the summer.

The same principle could now be modified by replacing the silver film with aluminium film, reducing the cost and potentially opening up scalable fabrication methods such as electron beam evaporation or magnetron sputtering.

"In buildings, large amounts of energy are used for cooling and ventilation, and running the air conditioner in electric cars can reduce the driving range by more than half," said Guo Ping Wang from Shenzhen University. "Our cooling films could help advance energy sustainability and carbon neutrality. They could even be used on textiles to create clothes of any color that are comfortable in hot temperatures."

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