Devising better cooling materials has become a pressing issue as the climate warms, and some scientists are turning to nature for ideas. Small creatures with low body mass, such as insects, have to deal with the fact that they warm up much faster than large mammals. When butterflies land on tree branches to bask in the sun, for example, their relatively large wings can overheat within seconds. So they have evolved sophisticated ways to cool themselves. Researchers at Columbia University and Harvard University have now uncovered these colorful insects’ built-in cooling mechanisms. Their wings behave a bit like nanoscale radiators and could inspire new lightweight materials to beat the heat.
Heat is electromagnetic radiation generated by the vibration of molecules, explains study co-author Nanfang Yu, an associate professor of applied physics at Columbia’s Fu Foundation School of Engineering and Applied Science. The more molecules a material can expose on its surface, the more heat it can dissipate in a process called radiative cooling. So depending on their structure, some materials can release heat more quickly than others. Those made of corrugated layers shaped like waves, pleats or cylindrical tubes, for example, cool off much faster than solid objects because they have more exposed surface area. This observation is why home radiators are usually constructed to run heat through numerous metal folds, which efficiently release warmth into a room.
It turns out that parts of butterflies’ wings exploit a similar principle. The wings themselves are complex systems that contain both living and nonliving structures, covered by various types of scales made of chitin—a rigid substance that is also found in some shellfish exoskeletons and fungi. The living parts of butterfly wings include scent pads and patches that release pheromones, as well as veins for hemolymph, a circulatory fluid in arthropods that is comparable to blood. The nonliving structures include chitin membranes that stretch between wing veins.
The researchers found that the scales covering these structures have different shapes that radiate heat in different ways. The scent-pad scales are shaped like a myriad of tiny tubes with their openings facing outward, each about one micron (one thousandth of a millimeter) in diameter. Such nanostructures dissipate warmth very efficiently, protecting the pheromone organs from overheating. Because these finely corrugated structures expose more surface molecules than flat ones would, Yu explains, “microscopically, you have more radiators.” Scales covering the wing veins are thicker but contain a lot of holes, through which they also emit heat efficiently. The scales over the nonliving wing parts, which are not as easily damaged by heat, do not have such high “thermal emissivity.”
To elucidate how these complex systems work, the team anesthetized several types of butterflies and used a small paintbrush to remove wing scales so it could see the underlying structures. The scientists also injected a small amount of blue dye into the insects’ thorax. Carried by the hemolymph, the dye helped visualize living cells and tissues. Thermal cameras also helped Yu and his colleagues analyze the wings. The devices showed the living structures glowing brighter than their surroundings, proving they were dissipating heat. The team also discovered that butterflies have a “wing heart,” which pumps hemolymph through the scent pads, says co-author Naomi E. Pierce, a biologist and curator at Harvard’s Museum of Comparative Zoology. The study was published last month in Nature Communications.
The scientists’ work is “remarkable,” says Aaswath Raman, who studies light and heat interactions with nanoscale structures at the University of California, Los Angeles, and was not involved in the study. “The specific microstructures they uncover here, we can conceivably make into our own artificial processes,” he says. “One thing that’s potentially attractive is that butterfly wings are extremely light, and the microstructures involved are extremely small and thin. So one thought that comes from this [paper] is that it can inspire ways of efficiently getting rid of heat in very lightweight systems.”
The researchers are already working to develop a cooling material based on their findings in butterflies, along with their similar research on sub-Saharan ants. They hope to devise a cooling polymer and spin it into forms resembling the butterfly wings’ nanostructures. Although far from a commercial application, the resulting material could eventually be used for purposes such as painting buildings.
Along with its light weight, a butterfly-inspired cooling material might have another advantage: working in a variety of colors. Many conventional heat-repellant coatings, such as those on cool roofs, tend to be white because that hue does not absorb heat. But painting too many things white would not be aesthetically pleasing, says Qiaoqiang Gan, a photonics scientist at the University at Buffalo, State University of New York, who was not involved in the study. “We still need a colorful world,” Gan says, noting how vivid butterflies are in nature. “Butterfly wings give us an example that colorful structures can also achieve radiative cooling in specific cases.”