Engineers in the US claim to have created 'the whitest paint yet' – which they say will help tackle global warming.
The ultra-white coating, developed at Purdue University in Indiana, reflects up to 98.1 per cent of sunlight while sending away infrared heat.
Currently, paints on the market designed to reject heat only reflect somewhere between 80 and 90 per cent of sunlight.
Buildings in hot climates coated with the new paint would stay cool more efficiently, reducing the need for air conditioning technology.
Paints could offer an environmentally-friendly way of reducing the power required to keep them constantly running in balmy weather.
"Xiulin Ruan, a Purdue University professor of mechanical engineering, holds up his lab's sample of the 'whitest paint on record'
The Purdue University team last October revealed a white paint that reflects 95.5 per cent of sunlight – but at 98.1 per cent, this new version is even more effective.
Last year's 'whitest paint' was a formulation made of calcium carbonate, an earth-abundant compound commonly found in rocks and seashells.
But this new version uses a chemical compound called barium sulfate.
'If you were to use this paint to cover a roof area of about 1,000 square feet, we estimate that you could get a cooling power of 10 kilowatts,' said Xiulin Ruan, a professor of mechanical engineering at Purdue University.
'That's more powerful than the central air conditioners used by most houses.'
The new record set by Purdue's white paint, which is detailed in the journal ACS Applied Materials & Interfaces, is the result of six years of research.
The project has built on attempts going back to the 1970s to develop radiative cooling paint as a feasible alternative to traditional air conditioners.
Ruan's lab had considered over 100 different materials, narrowed them down to 10 and tested about 50 different formulations for each material.
The researchers showed in their study that like commercial paint, their barium sulfate-based paint can potentially handle outdoor conditions.
Two features give the paint its extreme whiteness – one is the paint's very high concentration of barium sulfate, which is also used to make photo paper and cosmetics white.
'We looked at various commercial products, basically anything that's white,' said Xiangyu Li, a postdoctoral researcher at the Massachusetts Institute of Technology (MIT) who also worked on this project.
'We found that using barium sulfate, you can theoretically make things really, really reflective, which means that they're really, really white.'
The second feature is that the barium sulfate particles are all different sizes in the paint.
How much each particle scatters light depends on its size, so a wider range of particle sizes allows the paint to scatter more of the light spectrum from the sun.
'A high concentration of particles that are also different sizes gives the paint the broadest spectral scattering, which contributes to the highest reflectance,' said Joseph Peoples, also at Purdue University.
The technique that the researchers used to create the paint also is compatible with the commercial paint fabrication process.
At 98.1 per cent reflectivity, researchers say there's only a little bit of room to make the paint whiter, but not much without compromising the final product.
'Although a higher particle concentration is better for making something white, you can't increase the concentration too much,' Li said.
'The higher the concentration, the easier it is for the paint to break or peel off.'
The researchers believe that this white may be the closest equivalent to some of the blackest blacks ever developed.
A product called Vantablack, developed by Surrey NanoSystems, absorbs up to 99.965 per cent of visible light.
However, in 2019, Vantablack was bettered by a team at MIT, who pushed this to absorption rate to 99.995 per cent.
MIT covered a £1.6 million diamond with the Spinal Tap-worthy substance in a bid to prove its blackness – and it promptly appeared to vanish.
The MIT material was made from vertically aligned carbon nanotubes – microscopic filaments of carbon – grown on a surface of chlorine-etched aluminium foil.
A black object absorbs all wavelengths of light and converts them into heat, so the object gets warm.
A white object reflects all wavelengths of light, so the light is not converted into heat and the temperature of the object does not increase noticeably.
Different wavelengths (colours) of light have different amounts of energy. Violet light has more energy than red light.
If we compare an object that absorbs violet light with an object that absorbs the same number of photons (particles of light) of red light, then the object that absorbs violet light will absorb more heat than the object that absorbs red light.
The amount of heat absorbed is also affected by how light or dark an object is.
A dark object of a given colour will absorb more photons than a light object of the same colour, so it will absorb more heat and get warmer.
Source: UC Santa Barbara