A new way to fly, built up from the nanoscale

Take a sheet of paper and blow on it. What happens? It might soar very briefly, but quickly flops back down, the air you’re expelling blowing around and through it.

Now, think about an ultra-light material that’s stiff enough to sustain that lift, but light enough that an amazingly small amount of energy—just a beam of light—can make it rise.

Inside mechanical engineer Igor Bargatin’s lab, this possibility is reality. After years of work and refinement, he and his team have developed what they call “nanocardboard,” a material that’s as thin as a few strands of DNA and weighs less than a thousandth of a gram, but stiff enough to resist flopping.

“What we have here is actually a composite of solid and empty space. We’re specifically putting the solid material only in places where it matters the most and take the advantage of the weightless empty space,” says Sam Nicaise, a postdoctoral researcher in Bargatin’s lab.

Over the past few years, Bargatin and researchers in his lab have been working on these plate mechanical metamaterials: Structures that are super thin, perhaps just a few tens of nanometers, but big enough that you can hold them in your hands. The nanocardboard breakthrough was published in Nature Communications last year.

As they honed their materials, they realized that the very lightness of the material opened the door to light-driven movement—initially, just brief levitation but eventually, perhaps, sustained controlled flight. Bargatin just received a prestigious CAREER Award from the National Science Foundation to pursue this tantalizing possibility.

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Bargatin was inspired by a simple instrument that’s been around for more than a century, a device known as a Crookes radiometer. (You may have had one in your house as a kid or seen one in a glassblower's shop.) Also known as a light mill, the instrument is essentially a glass bulb under partial vacuum with paper vanes that are black on one side and white on the other.

When the radiometer is exposed to sunlight, the black sides of the vanes absorb some of the energy and heat up a little bit.  

“What’s happening then is the air molecules hit the black side, absorb some of the heat, and then leave with a higher speed than it came in with. In physics, we know that whenever you have a change of momentum, or speed, there must be a recoil, there must be a reaction force. The recoil pushes harder on the hotter black sides than the cooler white sides of the vanes and can make the Crookes radiometer rotate pretty quickly if you put it out in direct sunlight,” Bargatin says.

“Sometimes people think that the Crookes radiometer is driven by light pressure,” he adds. “The light pressure is obviously there, but it is much smaller and points the opposite way. The force that we observe and use is a thermal effect, based on the interaction between the gas molecules and a heated solid.”

Although the Crookes radiometer has been around for more than a century, no one has yet been able to use these forces to overcome gravity and make the vanes levitate. That’s where the new nanocardboard plates come in. Because they’re so light, thermal forces that are too small to lift paper can lift the nanocardboard pieces into the air.

“It’s exciting because it’s essentially a new mechanism of flight,” Bargatin says. “We’re talking about a structure half an inch in size that can fly around without any moving parts. These structures create a jet of air and a corresponding lift force, just based on the temperature differences that exist in the structure.

“The most exciting thing is thinking about how far we can push it in terms of payloads and dreaming about what else we could do.”

The possibilities are dazzling. Using light, whether it’s sunlight or a laser beam, to power a flying device opens the door to a huge range of potential uses.

For about 20 years, academics have explored the concept of “smart dust”: ultra-small particles that can work as sensors, measuring temperature, pressure, and more.

Imagine a search and rescue operation using “microflyers” that could dart in and out of tiny holes in the rubble, to help find trapped people without putting first responders at risk. Or a tiny robot with a camera that could inspect a jet engine for wear and other problems without taking the engine apart.

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Getting smaller can impact performance, however, and conventional microflyers with rotors and wings require more and more energy as they shrink down because of the increased role of air's viscosity at small scales. That’s another advantage for a material that can instead use photophoresis, from the Greek words for light-induced motion.

“The thing about photophoretic propulsion is it actually works better if you make the structures smaller,” Bargatin says. “In fact, photophoretic levitation of microscopically tiny particles is well established and has even been used to create a 3D display.

“So that’s another thing we’re very excited about, exploring what is the optimal size for our plates and what are the optimal payloads. The plates could be a perfect carrier for smart dust sensors.” 

The current version of nanocardboard can't fly using photophoresis near the sea level yet. But in the mesosphere—the area above the stratosphere and below the thermosphere, about 30 to 50 miles above the Earth—it could be a perfect fit because the photophoretic jet becomes much faster at reduced air pressures.

“The mesosphere is too high for airplanes and balloons, which fly up to about 50 kilometers in altitude, and too low for satellites, which orbit at about 100 kilometers or higher,” Bargatin says. “It’s a region of the atmosphere that is almost empty—only rockets briefly pass through it on their way up or down. It’s generally unused, and under-explored. What we’re doing would allow these new structures to stay aloft in that region of the atmosphere using only the sun’s energy.”

Why would the mesosphere aircraft be valuable? It might be a good place for weather sensors, Bargatin says, or surveillance, just to name a few applications. Humans have generally found new uses for aircrafts at all altitudes.

The material that shows the most promise for photophoretic levitation is a sandwich of aluminum oxide film, layered together to maximize the amount of air while still maintaining the stiffness. Bargatin likens its structure to corrugated cardboard, which also uses a combination of multiple layers and empty space.

“If you try to make a plate the thickness of a few nanometers, it’s going to be floppy. Think about Cling Wrap, and that’s a thousand times thicker than what we’re working with,” he says. “This nanocardboard, or nanoscale equivalent of cardboard or sandwich composite, allows you to create structures that are extremely thin and lightweight but also very stiff.”

Nicaise has been working on the nanocardboard for more than two years, spending countless hours in the fabrication rooms at the Singh Center for Nanotechnology working on different permutations of the material.

“It took a lot of engineering and prototyping and failure,” he says.

Now, he says, “it’s really amazing to be able to see these structures that are tens of nanometers in thickness—literally things that you can barely see with your naked eyes otherwise—being built into a macroscopic structure that you can see with your eyes, pick up with tweezers or your hand, and you can make move around with light.”

The plates are more durable than you might think, Bargatin says, because they’re so thin. Poke them, and they spring back to their original shape, kind of like an inflatable mattress.

But they can still be breathtakingly tricky to work with. Literally: John Cortes and Mohsen Azadi, two graduate students in Bargatin’s lab, hold their breath when handling the material, since even one exhale can send a tiny piece flying across the room.

“These plates are heartbreakers. The fact that they’re so lightweight, and they’re fairly transparent—you can’t see them unless you shine some light on them,” says Cortes, a fifth-year Ph.D. student. “When you fabricate them and you release them from the larger wafer where they’re made, they tend to want to fly away.

“We have seen weeks of work literally just fly into the air and stick to the ceiling, and once they’re gone, they’re gone.”

Azadi, a second-year Ph.D. student, has also experienced the sinking feeling of losing several weeks of work in one breath as he finished cutting a piece of nanocardboard out.

“And when I say ‘cut,’ it’s literally a razor blade in your hands cutting something that’s a thousand times thinner than hair,” he says.

Azadi, who is studying mechanical engineering, came to the project after an element of another project he was working on, involving blood filters, seemed applicable to the levitation project.

He’s been trying to fit what the team sees when the nanocardboard levitates into the broader realm of fluid mechanics.  

“You do the experiment, and you want to validate these, so you have to go through the theories that exist, but you also have to see where the theory lags so you have your own you can add to it,” he says. “For many hours I literally had to sit, with a piece of paper and a pen and my imagination, to come up with the theory to describe what we are seeing.”

Cortes’ focus was on fabricating the material, then building the perfect platform for allowing the material to fly. Cortes says he entered a Ph.D. program for one reason: to be challenged. He’s found that working on this project.

“The coolest part of it is to be doing something nobody else is doing,” Cortes says. “To create something this new that could actually move us forward, technologically, is really interesting.”

He’s interested in space exploration, and it’s not lost on him that the atmosphere of Mars is within the same pressure range as Earth’s mesosphere. The possibility of these tiny materials playing a role in a Mars mission one day is thrilling, he says.

“How cool would it be to have our plates or arrays of our plates be released by, say, a rover, and have them controlled by low-powered lasers and fly around and explore from a different perspective than what we have now?” he asked.

“One day—hopefully in my lifetime—if we could see this fundamental research be applied to something in space exploration, I think I will die a very happy guy.”

Igor Bargatin is the Class of 1965 Term Assistant Professor in the Department of Mechanical Engineering at the School of Engineering and Applied Science at the University of Pennsylvania.

Homepage photo: Graduate student Mohsen Azadi holds a small sample of the nanocardboard material inside the framework used to test its ability to levitate.