Minimizing the Ground Effect for Photophoretically Levitating Disks

TL;DR

This study investigates how the proximity of surfaces in vacuum chambers enhances photophoretic lift forces on levitating disks, introducing new launchpad designs and guidelines to minimize ground effects for accurate microflyer testing.

Contribution

The paper introduces a new miniature launchpad design and provides guidelines to reduce ground effects in vacuum chamber tests of photophoretic microflyers.

Findings

Wire-mesh launchpads increase lift by up to six times.

Ground effects are significant when the chamber bottom is close to the disk.

Guidelines are provided to minimize ground effects in experiments.

Abstract

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.