Controlled levitation of nanostructured thin films for sun-powered near-space flight

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Science Advances  12 Feb 2021:
Vol. 7, no. 7, eabe1127
DOI: 10.1126/sciadv.abe1127
  • Fig. 1 Force generation mechanism and samples.

    (A) Schematic diagram of the main mechanism behind the photophoretic force due to a difference in the thermal accommodation coefficient (in the free molecular regime). (B) Photograph of a 6-mm-diameter mylar disk covered by a layer of CNTs. (C) The porous surface of the CNT layer that traps incoming air molecules, allowing for the gas molecules to absorb more heat and approach unity thermal accommodation coefficient partially covering the smooth surface. The inset shows a closeup of the trap created by the nanotubes (D) Sequential screenshots of two levitating 6-mm-diameter disks under incident light intensity of 0.5Wcm2. Samples placed on a 74% transparent stainless-steel mesh ~6 cm above the light source (movie S1). Photo credit: Mohsen Azadi, University of Pennsylvania.

  • Fig. 2 Experimental data and theoretical prediction.

    (A) Areal density of a disk with a given radius and micron thickness that can be levitated under 0.5Wcm2 and ∆α = 0.15. The shaded area represents the domain that mylar can operate without undergoing thermal deformation due to temperatures exceeding 400 K (see the Supplementary Materials). (B) Comparison of the force and weight for a disk with 6-mm diameter with thermal deformation considerations (the size corresponding to the dashed line in Fig. 2A).

  • Fig. 3 Optical trap configuration.

    (A) (i) Side and (ii) top-view schematic diagram of the test setup consisting of eight LED arrays below an acrylic vacuum chamber, a 74% transparent metallic mesh placed several centimeters above the bottom surface of the acrylic chamber and a levitating disk sample. (B) Experimental measurements of the intensity of the trapping light beam from eight LED arrays at 7-cm (i and ii) and 10-cm (iii and vi) heights above the LEDs. Note that the high-intensity ring surrounding the microflyer confines its in-plane movement and that the intensity at the center drops as the height increases, which stabilizes the flight height.

  • Fig. 4 Near-space flight prediction.

    Contour plots of (A) areal density of the object able to be levitated (B) payload that can be lifted using mylar-CNT (white area represents no levitation for mylar areal density). (C) Temperature and (D) temperature difference between the disk and ambient for different sizes at different altitudes with ∆α = 0.5, ϵ = 0.5, and under natural sunlight (0.136Wcm2).

Supplementary Materials

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