  We report light-driven levitation of macroscopic polymer films whose bottom
surface is engineered to maximize the thermal accommodation coefficient.
Specifically, we levitated centimeter-scale disks made of commercial
0.5-micron-thick mylar film coated with carbon nanotubes on one side. When
illuminated with light intensity comparable to natural sunlight, the polymer
disk heats up and interacts with incident gas molecules differently on the top
and bottom sides, producing a net recoil force. This lift force is maximized at
gas pressures corresponding to Knudsen number on the order of 0.3, and
correspondingly, we observed the levitation of 0.6-cm-diameter disks in a
vacuum chamber at pressures between 10 and 30 Pa. Moreover, we controlled the
flight of the disks using a shaped beam that optically trapped the levitating
disks. Our experimentally validated theoretical model predicts that the lift
forces can be many times the weight of the films, allowing payloads of up to 10
milligrams for sunlight-powered low-cost microflyers in the upper atmosphere at
altitudes of 50-100 km.
