  Photophoretic levitation is a propulsion mechanism in which lightweight
objects can be lifted and controlled through their interactions with light.
Since photophoretic forces on macroscopic objects are usually maximized at low
pressures, they may be tested in vacuum chambers in close proximity to the
chamber floor and walls. We report here experimental evidence that the terrain
under levitating microflyers, including the chamber floor or the launchpad from
which microflyers lift off, can greatly increase the photophoretic lift forces
relative to their free-space (mid-air) values. To characterize this so-called
"ground effect" during vacuum chamber tests, we introduced a new miniature
launchpad composed of three J-shaped (candy-cane-like) wires that minimized a
microflyer's extraneous interactions with underlying surfaces. We compared our
new launchpads to previously used wire-mesh launchpads for simple levitating
mylar-based disks with diameters of 2, 4, and 8 cm. Importantly, wire-mesh
launchpads increased the photophoretic lift force by up to sixfold. A
significant ground effect was also associated with the bottom of the vacuum
chamber, particularly when the distance to the bottom surface was less than the
diameter of the levitating disk. We provide guidelines to minimize the ground
effect in vacuum chamber experiments, which are necessary to test photophoretic
microflyers intended for high-altitude exploration and surveillance on Earth or
on Mars.
