Demonstration of Atmospheric-Pressure Radiometer with Metamaterial Vanes

We report a Crookes radiometer that rotates at atmospheric pressure using architected microporous dielectric plates, known as nanocardboard, as vanes [1,2]. Compared to most light mills working at tens of Pascals [3,4], the functionality at pressures three orders-of-magnitude larger results from the metamaterial vanes’ unique features: (1) extremely low areal density (0.1 mg/cm2) that reduces the vane mass and hub friction force by almost 100 times; (2) high thermal resistivity that increases the cross-vane temperature difference; and (3) micro-channels that enable through-vane thermal transpiration gas flows. Each nanocardboard vane features a basketweave-style five-flow-channel pattern to amplify the thermal transpiration force. We manufactured these vanes using microfabrication techniques in four stages: (1) silicon mold creation using photolithography and reactive ion etching; (2) mold conformal coating using atomic layer deposition; (3) carbon nanotube drop-casting and oxygen plasma etching; and (4) mold cleaving and removing using XeF2 isotropic etching [1, 5]. We 3D-printed a 26-mm-diameter quad-arm hub and mounted the vanes to it using super glue. We measured the temperature and rotation speed of the radiometer using thermal and video cameras while illuminating it using an octagonal LED array. We found that our radiometer could operate at atmospheric pressure, and that its rotation rate increased with light intensity. To our knowledge, no other radiometers have achieved such functioning in ambient air. Lastly, we simulated the radiometer’s fluid dynamics, obtaining similar trends between its rotation speed and light intensity and achieving order-of-magnitude agreement with our experiments. Our photophoretically-propelled microstructures reveal new possibilities for light sensing and actuation, aerial microflyers, and photo-generators. References: [1] Lin, Chen, et al. “Nanocardboard as a nanoscale analog of hollow sandwich plates.” Nature Communications 9.1 (2018): 1-8. [2] Cortes, John, et al. “Photophoretic Levitation: Photophoretic Levitation of Macroscopic Nanocardboard Plates” (Adv. Mater. 16/2020). Advanced Materials 32.16 (2020): 2070127. [3] Han, Li-Hsin, et al. “Light-powered micromotor: design, fabrication, and mathematical modeling.” Journal of Microelectromechanical Systems 20.2 (2011): 487-496. [4] Wolfe, David, Andres Larraza, and Alejandro Garcia. “A horizontal vane radiometer: Experiment, theory, and simulation.” Physics of Fluids 28.3 (2016): 037103. [5] Azadi, Mohsen, et al. “Demonstration of Atmospheric-Pressure Radiometer With Nanocardboard Vanes.” Journal of Microelectromechanical Systems 29.5 (2020): 811-817.