The Cyphochilus beetle, native to Southeast Asia, may hold the key to more energy-efficient cooling of buildings, thanks to its brilliant white shell that reflects sunlight and radiates heat with incredible efficiency. By studying the unique structure of this beetle‘s scales, researchers have unlocked new insights into the science of radiative cooling and how it can be harnessed for sustainable temperature regulation.
The Whitest Insect on Earth
What makes the Cyphochilus beetle so remarkable is the ultra-white coloration of its scales, which cover its body in a dense network. These scales are some of the brightest white surfaces found in nature, reflecting up to 97% of visible light. This is a remarkable feat considering how challenging it is for insects to produce white coloration.The secret lies in the microscopic structure of the scales. Using electron microscopy, researchers found the scales contain a highly randomized network of chitin filaments, each about 250 nanometers wide.
This disordered filament network is optimized to scatter light of all wavelengths, resulting in the brilliant white appearance. Interestingly, the chitin material only makes up about 60% of the scales’ volume – the remaining 40% is air.
Principles of Radiative Cooling
The Cyphochilus beetle’s scales not only reflect sunlight, but also excel at radiative cooling – emitting thermal radiation to shed heat. All objects emit infrared radiation, with the peak wavelength determined by the object’s temperature. For terrestrial temperatures around 300 K, this peak occurs at wavelengths between 8-13 μm, which fortunately coincides with a “transparency window” in Earth’s atmosphere where air absorbs minimal radiation.
Microscopy images of the ultra-white beetle scales (https://www.nature.com/articles/s42004-019-0202-8)
The internal nanostructure of the scales from the Cyphochilus and L. stigma beetles consists of a highly interconnected network of chitin and air voids, with a characteristic length scale around 300-500 nm. This structure is reminiscent of morphologies formed by spinodal decomposition phase separation. Optical simulations showed that the measured beetle scale nanostructures have a chitin volume fraction around 30-35%, close to the optimum 25% predicted to maximize reflectance for spinodal structures.
By having surfaces that strongly emit in this 8-13 μm band, objects can radiate heat directly to the cold sink of outer space, even during the day. This is the basis of passive radiative cooling. The most effective radiative cooling surfaces have two key properties:
- High reflectance (>95%) of sunlight (wavelengths 0.3-2.5 μm) to minimize solar heating
- High emissivity (>95%) in the atmospheric transparency window (8-13 μm) to maximize thermal radiation
The Cyphochilus beetle’s scales display both of these properties. The random chitin network backscatters nearly all incident sunlight, while also providing a high infrared emissivity around 97% in the 8-13 μm band. As a result, the beetle can stay cool in the hot sun by reflecting solar energy and shedding its own heat through thermal radiation.
Biomimetic Radiative Cooling Materials
Inspired by the Cyphochilus beetle, material scientists have developed new types of radiative cooling surfaces that mimic the beetle’s scale structure. One example is a porous polymer coating embedded with randomly dispersed silicon dioxide spheres. This material reflects 96% of sunlight while emitting strongly in the atmospheric transparency window, achieving a cooling power of over 110 W/m2 – enough to cool surfaces below ambient air temperature under direct sunlight.
Other radiative cooling materials use photonic designs, such as multi-layer optical films or metamaterials, to tailor the optical properties for optimal solar reflection and infrared emission. Some are able to stay 5-10°C cooler than ambient air temperature, even in the peak of the day.
The applications for radiative cooling materials are vast, including:
- Cooling roofs, walls, and pavements in buildings and cities
- Improving efficiency of solar cells by keeping them cool
- Nighttime cooling systems that require zero energy input
- Thermoregulation of satellites, vehicles, and electronics
As the world faces increasing cooling demands in a warming climate, the Cyphochilus beetle provides a blueprint for how to achieve efficient, sustainable cooling by tapping into the power of thermal radiation. Through deeper study and biomimicry of this remarkable insect, breakthroughs in radiative cooling materials could play a key role in reducing global energy consumption and greenhouse gas emissions. Nature still has much to teach us in our quest for sustainable technology.