Tiny discs can levitate in the upper atmosphere using sunlight alone

Fingernail-size discs that levitate in sunlight could someday carry sensors through some of the thinnest and coldest reaches of the atmosphere. By flying higher than commercial aircraft or weather balloons can, such swarms could reveal new insights about Earth’s changing weather and climate patterns.

The levitating devices harness a phenomenon called photophoresis. It was first discovered more than 150 years ago when chemist William Crookes invented the radiometer, a device with black and white vanes that spin when exposed to sunlight. This happens because the vanes absorb the light and give off heat, and this heat boosts the momentum of gas molecules around them. Because the black sides of the vanes are hotter than the white ones, they transfer more momentum to the gas, making the air flow in one direction with enough force to turn the vanes.

“We’ve taken this obscure piece of physics and applied it to something that could actually impact a lot of people – and help us better understand how things like weather and climate are evolving over time,” says Ben Schafer at Harvard University.

To develop the levitating discs, Schafer and his colleagues created a 1-centimetre-wide device made of two aluminium oxide sheets full of micro-scale holes. When exposed to light, the bottom sheet – which included alternating layers of chromium with the aluminium oxide – heated up more than the top sheet, like the black sides of a radiometer’s vanes. This also created a directional airflow, but moving upward instead of sideways.

Under white LEDs and laser light – set to intensities equivalent to about 50 per cent of natural sunlight – this lifting force levitated the device. That is an improvement on other solar-powered fliers, which require light intensities several times brighter than sunlight. But the demonstration also took place in laboratory conditions with air pressure several thousand times weaker than that at Earth’s surface.

Fortunately, those low air pressure conditions are common elsewhere – like the mesosphere, an upper layer of the atmosphere that extends 50 to 85 kilometres above the planet. The researchers say scaling up their discs to 3 centimetres would let them carry 10 milligrams of payload at an altitude of 75 kilometres, bringing sensors to a region so difficult to study it has been nicknamed the “ignorosphere”. Schafer co-founded the startup Rarefied Technologies to commercialise swarms of such high-flying devices for atmospheric monitoring and telecommunications.

After the sun sets, computer modelling suggests the discs could stay airborne by harnessing heat radiating from Earth’s surface. “If you can stay aloft at night, that’s a big change from just settling or falling,” says Igor Bargatin at the University of Pennsylvania, whose lab is doing similar research.