Climate change is causing cities around the world to become uncomfortably hot. Researchers are increasingly looking to nature for answers, with several cool new materials in the pipeline, reports Jasmin Fox-Skelly
Last year, 2019, was the Earth’s second hottest year on record, with cities in Belgium, The Netherlands, Germany and France experiencing their hottest day ever. As the Earth continues to warm, heatwaves will become more common and extreme, with the Intergovernmental Panel on Climate Change (IPCC) warning that if the world fails to dramatically reduce carbon emissions, global temperatures will increase by 2.6-4.8°C by 2050-2100, from the 1850–1900 average.
Many of the world’s cities are simply not designed with heat in mind. Concrete and asphalt absorb heat during the day, and then release it slowly at night, a phenomenon known as the urban heat island effect. This is made even worse by modern architectural trends favouring glass buildings, which become unbearably hot without fossil fuel-guzzling air conditioning systems.
Those most at risk are the elderly and people with existing health conditions. During the height of the summer UK heatwave from 25 June to 9 July 2018, there were 663 more deaths than the average recorded for those weeks over the previous five years.
Devising better ways of cooling buildings is therefore a pressing issue, and some scientists are turning to nature for ideas. A famous example of this is the Eastgate Centre in Harare, Zimbabwe, designed by architect Mick Pearce and engineering firm Arup. The building takes inspiration from termite mounds in Africa, which stay remarkably cool inside, even in blistering heat. A constant draft of air flows from pockets in the bottom of the termite mound to holes in the top of the structure, through convection.
The Eastgate Centre copies this by allowing cool air to enter the bottom of the building at night, causing hot daytime air to escape through vents in the roof. Cool air is also stored in hollow floors and baseboard vents and then released into offices the next day. The system uses only 10% as much energy as conventional air-conditioning to drive fans that keep the air circulating.
Other scientists are devising unique materials inspired by specific animals. Nanfang Yu, professor of applied physics at Columbia University, US, for instance, is taking inspiration from butterflies and sub-Saharan ants to develop a cooling polymer that could eventually be used for painting buildings in hot cities.
In 2015, Yu began studying the Silver Ant (Cataglyphis bombycina), which lives in one of the hottest terrestrial environments on Earth; the scorching Sahara desert. Saharan silver ants go out foraging in the full midday sun when surface temperatures reach up to 70°C but must keep their body temperatures at or below their critical thermal maximum of 53.6°C.
Researchers have long wondered how these tiny insects – about 10 mm long – can survive under such extreme conditions, and Yu wondered whether the ants’ conspicuous silvery coats could play a role in keeping them cool.
Using electron microscopy, Yu’s group discovered that the top and sides of the silver ants’ bodies are covered with a coating of uniquely shaped hairs with triangular cross-sections, and with air voids between and beneath them.1 The hairs work in two different ways. First, they are very good at reflecting the visible and near-infrared light of the solar spectrum, which means that they do not absorb much heat from the sun. Secondly, they are also highly emissive in the mid-infrared portion of the spectrum, which means that they are good at offloading extra body heat by radiating it back to the sky. An example of cooling via thermal radiation is the cold feeling you get when you get out of bed in the morning, when your skin temperature is temporarily much higher than that of the surrounding environment.
‘The solar reflectivity of the hairs, coupled with their enhanced thermal radiative efficiency means that they are able to reduce the silver ants’ body temperatures by 5 to 10°C, compared with if the ants were without the hair cover,’ says Yu.
Yu wondered whether the same properties found in the ants could be used to design cooling materials. Ideally, any material would need to be excellent at passive daytime radiative cooling (PDRC), a phenomenon where a surface spontaneously cools by reflecting sunlight and radiating heat to the colder atmosphere.
PDRC is most effective if a surface has a high solar reflectance that minimises solar heat gain, and a high thermal emittance that maximises radiative heat loss. If both are sufficiently high, a net heat loss can occur, even under hot sunlight. However, the cooling effect works best in dry climates and with clear skies, as when it’s cloudy or humid, water vapour traps the infrared radiation.
In 2018, Yu and Yuan Yang, a professor of materials science and engineering at Columbia University, US, successfully made a polymer paint capable of cooling surfaces to around 6°C below ambient temperatures through PDRC.2 The polymer – polyvinylidene fluoride-co-hexafluoropropene – has a porous thin film structure containing a high density of nanosized air voids, similar to those found in the hair cover of the Saharan silver ants. The air voids in the polymer strongly scatter and reflect sunlight, due to the difference in the refractive index between the voids and the surrounding polymer.
To make the air voids, the scientists mix the polymer with acetone and water. When the mixture is painted onto a surface, the acetone evaporates, leaving just the polymer crystallised around the water droplets. Finally, the water evaporates leaving the air voids behind.
The size and density of the air voids can be finely adjusted by varying the percentage of water in the mix, allowing scientists to maximise the ability of the paint to reflect sunlight. As well as being highly reflective in the solar spectral region, which stops the paint from absorbing solar heat in the first place, the air voids give the coating an extremely high thermal emissivity over a broad angular range, which means that the coating can be used to cool objects that are already hot.
Compared with other highly reflective paints, such as those containing TiO2 nanoparticles, the paints developed by Yu and Yang are easier to disperse evenly and absorb little UV light.
‘Our polymer has essentially zero absorption across the solar spectrum, which includes UV, visible, and infrared components,’ says Yu. ‘It can be applied to rooftops, building facades, water tanks, vehicles, and shipping containers. Anything that can be painted.’
What’s more, the paint’s high solar reflectance (R > 99%) and high thermal emittance (Ɛ ~ 97%) allow it to stay cool under widely different environments. In tests, it was able to maintain temperatures 6˚C cooler than ambient temperatures in the warm, arid desert of Arizona, and 3˚C cooler in the foggy, tropical environment of Bangladesh.
Squid have organs called chromatophores that can rapidly expand and contract, going from tiny pinpricks of colour to spots 14 times wider in less than a second. The changing dimensions of their spots allow them to precisely control the wavelengths of light reflected from their skin at any one time.
‘We have also conducted ageing tests on our polymer paint applied to asphalt, which is the most commonly used roofing material in the US.’ says Yu. ‘During the course of two years of application, we found that while commercial cool-roof coatings are gradually contaminated by dirty chemicals migrating up from the asphalt, our coating is still quite white.’
The team is currently looking for partners in the paint industry to further develop and commercialise the polymer cool-roof paint.
Other research by the team has focused on the unique structure of butterfly wings. When butterflies land on tree branches to bask in the sun, their relatively large wings can overheat within seconds. To cope, they have evolved sophisticated ways to keep their wings cool.
In a 2020 study, Yu and Naomi Pierce, a professor of biology at Harvard University, US, examined the wings of butterflies using an optical microscope.3 They identified a number of living tissues within the wings, such as nerve fibres, blood vessels, and tracheae along wing veins, pheromone-producing cells in the scent pads, and mechanical and temperature sensors.
These were either protected by thick layers of cuticles made of the long-chain polymer chitin (C8H13O5N)n, or covered by special nanostructured wing scales. The nanostructured wing scales are shaped like the folds of a radiator and have an extremely high surface to area ratio, enhancing heat loss through thermal radiation and allowing the living regions of the wings to rapidly cool down in the sun.
Yu’s lab varied experimental conditions that mimic the butterflies’ natural environment, such as the intensity of sunlight, the temperature of the terrestrial environment and the ‘coldness’ of the sky. The team found that the areas of butterfly wings containing living cells were always cooler than the ‘lifeless’ regions, irrespective of the outside conditions. For example, the white scent pad of the butterfly Rekoa meton was 15°C cooler than the hottest part of the wing, and the light-coloured scent patch and pad of Bistonina biston were 10°C cooler than the hottest part of the wing.
The researchers are in the process of developing a colourful cooling paint based on their findings.
‘Our colourful coating consists of two layers, a very thin top layer with dyes to impart colour, and a thick underlying porous layer similar to the scent pad scales of the butterfly wings to strongly enhance solar reflection,’ says Yu.
An ant-inspired polymer paint was able to maintain temperatures 6˚C cooler than ambient temperatures in the warm, arid desert of Arizona.
While most conventional heat-repellent coatings are white, the vivid colour of butterfly wings could inspire the invention of colourful cooling paints. ‘Colourful cooling coatings are very desirable as white coatings are not always aesthetically pleasing’, adds Yu.
Yu’s team aren’t the only scientists developing cooling materials inspired by nature. Researchers at Özyegin University in Turkey have designed a material based on the iridescent blue butterfly Morpho didius, which inhabits the tropical forests of Latin America.4
Azadeh Didari and Pinar Mengüç used a transmission electron microscope to analyse microscopic scales on the butterfly’s wings.
They found that ‘Christmas-tree’- or ‘pine-tree’-like nanostructures in the scales are able to precisely reflect and scatter blue light and are therefore responsible for the striking blue colour of the butterfly.
Inspired by the geometric design of the scales, the researchers produced a material that is extremely good at reflecting light in the mid-infrared part of the spectrum, the region responsible for heat.
The material consists of a palm-tree like structure made from silicon carbide (SiC), where each branch is separated from other branches by a nanosized gap. This palm-tree is then situated close to a separate thin film of SiC. The researchers suggest that the right combination of their biomimicry designs and materials has potential use in advanced radiative cooling applications.
Alon Gorodetsky, professor of chemical & biomolecular engineering at the University of California, Irvine, US, meanwhile, has taken inspiration from squid – and their remarkable ability to rapidly change the skin colour – to create a new material that can keep in or let out an adjustable amount of heat.5 Squid have organs called chromatophores that can rapidly expand and contract, going from tiny pinpricks of colour to spots 14 times wider in less than a second. The changing dimensions of their spots allow the sea creatures to precisely control the wavelengths of light reflected from their skin at any one time.
Instead of looking at visible light waves, Gorodetsky and his team devised a material that is able to precisely control the amount of reflected infrared light, which we feel as heat.
The material consists of a thin film of copper on top of a stretchy layer of rubber. In its inactive and unstretched state, the copper pieces reflect infrared light, however when the material is stretched the copper pieces pull apart, exposing the inside rubber layer which allows heat to pass through the cloth. The material was designed with copper as an inside layer, helping to keep body heat in. But if you flipped the material, it would keep heat out, like a sunshade placed on a car windshield.
In total, stretching the material can vary its temperature by up to 8.2°C. At 50% stretch, the material keeps heat in like wool; however, when stretched to double its length it lets heat escape in a similar way to cotton. At its warmest, the material traps heat almost as effectively as a space blanket – a material used to reflect the sun’s glare in space and keep marathon runners warm after a race. For example, a sleeve made from the material increased the temperature of a wearer’s forearm by nearly 1°C.
‘Similar to the space blanket, this material reflects heat, keeping your body warm when wearing it,’ says Erica Leung, a UCI researcher who was first author on the paper.
‘However, the space blanket is static, and so can’t change its properties whereas this material is dynamic and can change the amount of heat it reflects upon stretching. This means that you can tune the material to reflect the amount of heat you want, allowing you to maintain specific temperatures in varying environments, eg a warm versus. a cool day.’
As well as clothing, the copper material could be used as a reflective insert in buildings – reducing the energy needed to keep buildings cool and allowing fine control of internal environmental conditions. It could also be used to dissipate heat from electronics, or to fabricate tents that would keep occupants comfortable outdoors.
The findings suggest that when it comes to keeping cool, nature has already solved many of the problems with which we are grappling.
‘Certain animals and plants have survived for millennia under harsh conditions,’ says Leung. ‘By learning how these organisms have adapted to increasing temperatures, we can base new materials and systems on these adaptations to improve our own survivability in increasingly hot cities.’
Gorodetsky agrees: ‘Nature has come up with remarkably creative solutions for thermal management, and I think scientists are only beginning to scratch the surface of what is possible.’
Berry’s bobtail squid (Euprymna berryi) burying itself in the sand. This small squid uses two arms to sweep sand over its body after burying itself. Both squids and cuttlefish use small sacs of pigment in their skin to to change their colouring and markings
References
1 Science, 2015, doi: 10.1126/science.aab3564
2 Science, 2018, doi: 10.1126/science.aat9513
3 Nature Communications, 2020, doi: 10.1038/s41467-020-14408-8
4 Scientific Reports, 2018, doi: 10.1038/s41598-018-35082-3
5 Nature Communications, 2019, doi: 10.1038/s41467-019-09589-w
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