Photophoresis arises when gas molecules recoil more strongly from a warmer surface than from a cooler one, producing a net push. The effect is negligible at sea level but grows in low pressure air, matching mesospheric conditions. The researchers redesigned devices so that this subtle force can overcome their weight.
They fabricated thin centimeter scale membranes of ceramic alumina with a chromium layer underneath to absorb light. Illumination creates a temperature gradient across the structure that drives a sustained photophoretic lift exceeding the device mass, allowing passive flight when exposed to sunlight in the upper atmosphere.
"We are studying this strange physics mechanism called photophoresis and its ability to levitate very lightweight objects when you shine light on them," said Ben Schafer, lead author and former Harvard graduate student, who worked with Joost Vlassak and David Keith on the project.
The idea traces back more than a decade, when Keith explored photophoretic particles for multiple uses, including possible climate applications. Progress accelerated with advances in nanofabrication that enable low mass devices with high precision, bringing the concept from theory toward practical atmospheric platforms.
"We developed a nanofabrication process that can be scaled to tens of centimeters," Vlassak said. "These devices are quite resilient and have unusual mechanical behavior for sandwich structures. We are currently working on methods to incorporate functional payloads into the devices."
Using those methods, the team built centimeter scale structures and directly measured photophoretic forces in a custom low pressure chamber assembled in Vlassak's lab by Schafer and former Harvard postdoctoral fellow Jong hyoung Kim. They validated calculations by matching measurements to real world atmospheric conditions; Kim led device design and fabrication.
"This paper is both theoretical and experimental in the sense that we reimagined how this force is calculated on real devices and then validated those forces by applying measurements to real world conditions," Schafer said.
A key test showed a 1 centimeter wide structure levitating at 26.7 Pascals under light at 55 percent of solar intensity, mirroring air at 60 kilometers altitude. "This is the first time anyone has shown that you can build larger photophoretic structures and actually make them fly in the atmosphere," said Keith. "It opens up an entirely new class of device: one that's passive, sunlight powered, and uniquely suited to explore our upper atmosphere. Later they might fly on Mars or other planets."
Potential uses include climate sensing to record wind, pressure, and temperature in a region long missing from models, improving weather and climate projections. Networks of flyers could also host communications payloads, forming low latency airborne links comparable to low orbit satellite constellations, and enabling operations in Mars like atmospheres.
The next step is integrating onboard communications for real time telemetry during flight. "I think what makes this research fun is that the technology could be used to explore an entirely unexplored region of the atmosphere. Previously, nothing could sustainably fly up there," Schafer said. "It's a bit like the Wild West in terms of applied physics."
Research Report:Photophoretic flight of perforated structures in near-space conditions
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Harvard John A. Paulson School of Engineering and Applied Sciences
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