COMMUNICATION www.advmat.de Photophoretic Levitation of Macroscopic Nanocardboard Plates John Cortes, Christopher Stanczak, Mohsen Azadi, Maanav Narula, Samuel M. Nicaise, Howard Hu, and Igor Bargatin* in size, has demonstrated the ability to Scaling down miniature rotorcraft and flapping-wing flyers to sub-centimeter lift off, hover, and move in the horizontal dimensions is challenging due to complex electronics requirements, plane using power tethers or concentrated manufacturing limitations, and the increase in viscous damping at low sunlight.[5,6] Reynolds numbers. Photophoresis, or light-driven fluid flow, was previously Many emerging applications for aerial vehicles, such as search and rescue mis- used to levitate solid particles without any moving parts, but only with sizes sions, jet engine inspection, and even of 1–20 µm. Here, architected metamaterial plates with 50 nm thickness are satellites could benefit from even smaller, leveraged to realize photophoretic levitation at the millimeter to centimeter “smart dust-”scale flyers;[7] however, fur- scales. Instead of creating lift through conventional rotors or wings, the ther decreasing the size of conventional nanocardboard plates levitate due to light-induced thermal transpiration microflyers introduces many challenges. One key challenge is the increasing dif- through microchannels within the plates, enabled by their extremely low ficulty of miniaturizing both mechanical mass and thermal conductivity. At atmospheric pressure, the plates hover structures and electronic components. above a solid substrate at heights of ≈0.5 mm by creating an air cushion Another, more fundamental challenge is beneath the plate. Moreover, at reduced pressures (10–200 Pa), the increased the fact that the propulsion efficiency of speed of thermal transpiration through the plate’s channels creates an rotorcraft and winged flyers decreases as air jet that enables mid-air levitation and allows the plates to carry small the vehicle scale is reduced and Reynolds number decreases, making viscous forces payloads heavier than the plates themselves. The macroscopic metamaterial more dominant.[8] These limitations can structures demonstrate the potential of this new mechanism of flight to be overcome by using alternative methods realize nanotechnology-enabled flying vehicles without any moving parts in of propulsion, such as the photophoretic the Earth’s upper atmosphere and at the surface of other planets. forces described below. Solids illuminated by light while immersed in a gas experience a force variously known in different litera- Miniaturized unmanned aerial vehicles or drones typically rely ture domains as the photophoretic, radiometric, or Knudsen on rotorcraft or flapping wing propulsion, with recent advances force.[9] Different from the light pressure caused by a photon’s made possible by new actuators, processors, batteries, sen- momentum, this force appears when incident light is absorbed sors, and communication electronics.[1-3] The current record by a solid, heating the solid relative to the ambient gas. The holder for the smallest self-powered maneuverable drone is temperature difference can then drive gas flows that result in the Piccolissimo, which is about 28 mm in its largest dimen- a corresponding reaction force acting on the solid. The photo- sion, weighs ≈2.5 g, and uses rotorcraft propulsion.[4] Another phoretic/radiometric/Knudsen force has been studied for over approach is provided by winged robots inspired by insects. The a century, including by luminaries such as Reynolds, Maxwell, RoboBee, which weighs only ≈80 mg and is a few centimeters Einstein, and Knudsen, who primarily focused on explaining a common device called a Crookes radiometer or light mill, which [10,11] Dr. J. Cortes, M. Azadi, M. Narula, Dr. S. M. Nicaise, Prof. H. Hu, rotates when illuminated by sufficiently intense light. The Prof. I. Bargatin force pushing on the rotating centimeter-scale paper vanes of Department of Mechanical Engineering and Applied Mechanics a light mill is typically a few orders of magnitude smaller than University of Pennsylvania the weight of the vanes, which is enough to produce rotation on Philadelphia, PA 19104, USA E-mail: [email protected] a low-friction bearing, but not levitation. However, for microm- C. Stanczak eter-sized particles, the same force can exceed their weight. [12,13] [14] Vagelos Integrated Program in Energy Research Both experimental and theoretical studies have shown University of Pennsylvania the levitation of 1–20 µm diameter spherical particles in mid- Philadelphia, PA 19104, USA air by using light to create a temperature difference and thus The ORCID identification number(s) for the author(s) of this article gas flow across the particle surface. The corresponding relative can be found under https://doi.org/10.1002/adma.201906878. movement of particles and gas is called photophoresis, from DOI: 10.1002/adma.201906878 Greek words for light and motion. Adv. Mater. 2020, 32, 1906878 1906878 (1 of 7) © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advmat.de A i ii iii x = 0 mm x = 5 mm x = 13 mm 4 mm B C Gas Flow Tc Absorbing Th CNT Film D E 400 µm 10 µm Figure 1. A) Sequential screenshots of a low-height levitation test at atmospheric conditions inside a glass enclosure at an incident optical flux of 10 000 W m−2 (≈10 Sun). B) Schematic of the photophoretic gas flow through one nanocardboard channel. C) Cross-sectional schematic of the optimized nanocardboard structure, showing a cutaway view of the internal channels. D) Scanning electron microscopy (SEM) image of the five-channel-array optimized nanocardboard plate structure. E) SEM image of the 200 nm thick carbon nanotube film on the edge of one of the nanocardboard channels. Much of the recent research on photophoretic forces focused images.[21] Yet, despite multiple theoretical proposals,[22–24] sus- on the settling of aerosols in the atmosphere[15,16] and led to a tained levitation of larger, macroscale bodies has not yet been better understanding of how pollutants such as volcanic ash, demonstrated because it requires materials with both very low carbon soot, and other greenhouse gases affect atmospheric density and thermal conductivity. This creates an opportunity conditions.[17] Researchers have also modeled the photophoretic for metamaterials specifically engineered for photophoresis to lift forces generated on aerosol nanoparticles as a function realize new mechanisms of flight. of altitude, which could be useful for climate engineering While the previous research on photophoresis focused on efforts.[18,19] Finally, some of the most detailed theoretical the gas flows around solid bodies, such as spheres or plates, models of photophoretic forces have been developed to study photophoretic flows can also occur inside tubes and channels if the effect of photophoresis on planetary disk accretion.[20] On a temperature gradient is present along the inner channel wall, the more applied side, Smalley et al. recently created a 3D dis- as shown in Figure 1B. This phenomenon, known as thermal play by photophoretically trapping and moving 10 µm cellulose transpiration or thermal creep, has been widely studied both particles in free space while continuously illuminating them fundamentally[25–27] and with the goal of creating new types of with red, green, and blue light, providing highly detailed 3D gas pumps without any moving parts.[28,29] As we show below, Adv. Mater. 2020, 32, 1906878 1906878 (2 of 7) © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advmat.de thermal transpiration can also be used to pump air through chemically in xenon difluoride (XeF2) gas. The resulting hollow an architected plate to maximize the photophoretic forces and plates of alumina were manually cut with a razor blade to the realize photophoretic hovering and propulsion of macroscopic desired size (typically, ≈1 cm). For more details of the fabrica- structures at both atmospheric and reduced pressures. tion process, see the main text in the Supporting Information. The physical mechanism behind the photophoretic force Figure 1A and Movie S1 in the Supporting Information depends on the Knudsen number, Kn = λ/L, where λ is show our plates hovering above a transparent substrate at equi- the molecular mean free path and L the characteristic length librium heights ranging from 400 to 600 µm for arbitrarily long scale of the flow. For reference, the mean free path for air at periods of time. The levitation was most reliably observed above atmospheric pressure and room temperature is ≈70 nm and a micropatterned low-stiction glass substrate (see the main text scales proportionally to the absolute temperature and inversely in the Supporting Information). We observed the hovering in proportionally to the pressure.[30] In the free-molecular regime over 50 flights with a wide variety of samples. After around (Kn ≫ 1), the gas molecules that collide with the hotter sur- 50 tests, the corners and edges of the sample typically experi- face depart at a larger thermal velocity in comparison to the ence damage that prohibits further trials. The damage typically molecules colliding with the cooler surface. This momentum occurs when the samples land and their corners lock into the exchange results in a reaction force that is proportional to the mesh grid below. number of molecules colliding with the solid per unit time and, The levitation heights were estimated from microscope therefore, proportional to both the active area and the pressure video footage, such as the one shown in Figure 1A, by com- of the ambient gas (Figure S3A, Supporting Information). In paring the distance between the plate and its reflection to the contrast, in the continuum regime (Kn ≪ 1), the ambient gas 500 µm thickness of the substrate. The incident optical flux of will flow along the side walls of the object due to the thermal up to ≈1 W cm−2 was produced by a 100 W LED diode array, creep phenomenon,[18] producing a reaction force that usually as shown in Figure 2B.