Xiulin Ruan, a mechanical engineer at Purdue University, showed off samples of ultra-white paint developed in his laboratory. 

(Inside Science)-Typical commercial white paint can only reflect 80% to 90% of the sunlight, and cannot help the building stay cool during the day. For the past six years, Xiulin Ruan, a mechanical engineer at Purdue University in West Lafayette, Indiana, and his colleagues have tried to create whiter paint to cool buildings, "similar to air conditioning, but without electricity," he said . They explored more than 100 materials, reduced them to 10, and tested about 50 different formulations for each material.

The best performer is barium sulfate, a compound used to brighten photographic paper and cosmetics. Its molecular structure can make it highly reflective to light of the sun's wavelength. When it does absorb energy from heat or other sources, it usually emits at infrared wavelengths that pass through the air unhindered into outer space, rather than heating the surrounding environment.

The scientists ensured that the paint contained a high concentration of barium sulfate particles of many different sizes. The wavelength of light scattered by each particle depends on its size, so a wide range of particle sizes helps the paint scatter more light from the sun.

Compared with the sunlight reflected by the researchers' previous ultra-white paint based on calcium carbonate, this new paint can reflect up to 98.1% of the sunlight. They think this white may be equivalent to the blackest black "Vantablack", which is made of carbon nanotubes and can absorb up to 99.9% of visible light. 

Outdoor testing of the new paint showed that it was the coolest on record, keeping the surface temperature 19 degrees Fahrenheit lower than the surrounding surface at night, and 8 degrees lower than the ambient temperature under strong sunlight at noon. It works even in the middle of winter-when the ambient temperature is 43 degrees Fahrenheit, it still manages to reduce the temperature by 18 degrees Fahrenheit. (The latter quality suggests that this coating may be best suited for hot climates that benefit the most from cooling, although Ruan points out that they "may come up with dynamic coatings that can change properties for other climates in the future.")

Scientists estimate that if a 1,000-square-foot roof is coated with this coating, it will provide 10 kilowatts of cooling power, which is more powerful than the central air-conditioning used in most houses. The scientists pointed out that they might increase the particle concentration to make the paint whiter, but too high a concentration will make the paint easier to peel off. Ruan pointed out that using their paint to smear 0.5% to 1% of the earth's surface, for example, by smearing roofs, roads, unused land, etc., may even help prevent global warming.

Ruan said that barium sulfate is very common and safe for the environment. The cost of its powder is about half of the titanium dioxide powder commonly used in commercial white coatings. The scientists also pointed out that the method they used to make the paint was compatible with commercial paint manufacturing technology and might be able to cope with traditional outdoor conditions.

In the future, researchers hope to optimize and commercialize their coatings for a wide range of applications including buildings, automobiles, and outdoor equipment. They also want to develop a color version. Although it will not achieve the same cooling effect, "compared with commercial paints of the same color, it can still save energy significantly," Ruan said.

The scientists detailed their findings online in the ACS Applied Materials & Interfaces magazine on April 15. 

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Charles Q. Choi is a science journalist who has written for Scientific American, New York Times, Wired, Science, Nature, and National Geographic News. 

Inside Science is an independent editorial news service of the American Physical Society

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