December 2, 2021

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Researchers Levitated a Small Tray Using Nothing but Light

After an ambient gas molecule from the air collides with a warm object, it picks up a small amount of energy and bounces off faster than it arrived. (Thermodynamics dictates that a hotter particle is a faster particle.) But not every surface transfers that energy to gases equally. Some, like a smooth sheet of Mylar, spring gas molecules away with only a little boost. Other surfaces, like a tangled mess of carbon nanotubes, can trap and heat gas molecules so much that they fire away a lot faster.

When this jet-black carbon carpet absorbs light, its tangled mess of nanotubes warms. Gas molecules that slip into the shag then collide with so many nooks that they heat up more than the molecules ricocheting off the smooth upper surface. This rush of molecules shooting down from the bottom surface faster than up from the top creates a lift force, says Bargatin. “You throw enough molecules down, you’re gonna create a jet,” says Bargatin. “That’s what helicopters do.”

On that day in late 2019 when Azadi and the rest of the team gathered around the vacuum chamber to try out the nanotube design for the first time, Azadi let the mini magic carpets float a few millimeters above the surface at mesosphere-like pressure. In one instance, two mylar plates circled each other as though they were dancing. “We decided to name the move because it worked so beautifully,” Azadi says. “It looked like two of them danced with the same very harmonic dance. It was like, let’s call it ‘Tango.’”

By surrounding one central LED with a ring of more intense LEDs set beneath the vacuum chamber, they were also able to demonstrate stable levitation. This setup keeps the levitating plate confined to an optical trap—if the plate begins tilting and zooming away, the light boundary forces it back to the center. Levitating without this balancing force is like balancing a pea on the underside of a spoon.

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“When they said that they have a centimeter-sized object that they can levitate using photophoretic forces, I was very skeptical,” says Yael Roichman, a physicist at Tel Aviv University who was not involved in the study. Roichman studies optical trapping and has used lasers to levitate dust particles. Conventional photophoresis experiments rely on a temperature gradient—a hot face and cold face—to propel objects. This restricts an object to only moving away from an energy source, nixing hopes of sun-powered levitation. But she says Bargatin’s idea is different. Regardless of where the light originates in relation to the levitator, it will reach the down-facing nanotubes and provide lift. “What they did doesn’t depend on the temperature gradient, which gives you very small forces, but depends on something completely different,” she says. “I think this is actually potentially very useful and innovative. It seems simple, but it’s not simple.”

Courtesy of Mohsen Azadi
Courtesy of Mohsen-Azadi

Immediately after Azadi first captured the levitation, he rushed to his computer and punched the experiment’s exact physical parameters into his theoretical model. The hovering behavior they observed matched the theory they had developed. “The range of pressure that it works at, the range of light intensity where the forces maximize—they all matched what I had seen,” says Azadi. “So that was a very exciting moment, to see that the theory works and it matches the experiments really well.” That validation meant they could now use their model to predict how microflyers of different sizes would behave in any atmospheric condition. They could calculate, for instance, the diameter of a plate that could carry the heaviest payload at a particular altitude without being too wide to float.

Their simulations estimated that a 6-centimeter plate could carry 10 milligrams of cargo in the mesosphere under natural sunlight. Ten milligrams may not sound like much; a drop of water weighs five times as much. But engineering advances have shrunk silicon chips into dust-sized sensors far smaller than that. These “smart dust” systems can fit a power source, radio communication, and a data-collecting sensor in cubes only a millimeter across. “Researchers can do a lot when you give them a cubic millimeter of silicon,” says Bargatin. “And a cubic millimeter of silicon weighs a couple of milligrams.”