Session: 17-04-01: Posters Related to Advances in Aerospace Technology

Paper Number: 99576

99576 - Minimizing Ground Effect for Photophoretic Force-Driven Mylar-Based Microflyers 

We have developed launchpads that can minimize the ground effect experienced by microflyers in order to simulate their flight more accurately at mesosphere-like low pressures in laboratory test chambers. These launchpads, comprised of triads of cane-shaped steel sticks, minimize the enhancement of lift forces due to the ground effect, thereby leading to more realistic experimental conditions. Our measurements using these novel launchpads indicate that traditional launchpads, composed of grids of steel wires, lead to overestimates of lift forces by as much as 500%.

Microflyers are airborne entities that have at least one microscopic dimension with potential and powerful applications in sensing and scientific measurement. They are usually driven by wind or solar energy and do not need a battery, engine, or motor to stabilize their mid-air flight. These miniaturized and unpowered airborne vehicles have been implemented in plant seed-inspired wind dispersion [1] and sunlight-empowered photophoretic levitation [2]. Microflyers are superior in being lightweight, energy conservative, environmentally friendly, and low-cost, in contrast with traditional macroscopic flying systems like weather balloons, airplanes, and satellites. They can potentially improve the next-generation microelectromechanical system in atmospheric monitoring, climate warning, and biosphere surveillance.

Although microflyers are designed for mesospheric functioning far from the ground, they are commonly tested in laboratory vacuum chambers in close proximity to walls and supporting frames. These underlying entities often enhance the aerodynamic forces experienced by the microflyers in a phenomenon referred to as the ground effect [3]. This work demonstrates that inappropriate launchpads can overvalue the total lift force by up to five times, which gives an inaccurate approximation of environmental conditions for microflyers to levitate. A proper characterization of the ground effect is thus imperative for rigorously designing and testing microflyers.

Therefore, we designed new launchpad configurations that dramatically reduce the ground effect’s interference and tested them against traditional launchpads. Whereas prior launchpads consisted of grids of steel wires, our novel designs employed three vertical curved wires that minimized the contact area with the microflyer. Since the light intensity reaches a global minimum around a certain pressure and becomes larger otherwise [4], we define this pressure as optimal pressure, implying the most responsive reaction of the launchpad-microflyer system to photophoretic stimulus. The ground effect can be indirectly validated from the non-distinguishment of optimal pressures of disk-like microflyers with very different diameters. We argue that the wire spacing impacts the ground effect more than the wire diameter. Compared to steel meshes interwoven by the same-diameter wires, cane-shaped sticks show a sharper variation in optimal pressures for all three sizes of microflyers. This observation reveals that the sparsity of the launchpad and thus contact area ensures that the lift force is unlikely to be amplified by the ground effect.

We highlight the necessity of diminishing the ground effect when testing structures that need an underlying surface to take off in a virtual laboratory environment but are fashioned for real-world applications. Our new ultra-sparse cane-shaped steel stick-based launchpads give a more realistic flying ability evaluation of microflyers, making it possible to simulate low-pressure off-the-ground levitation in a vacuum chamber. The advancement in understanding aerodynamical and levitational forces provides a cheaper and more convenient alternative for conducting performance tests.

 

[1] Kim, Bong Hoon, et al. "Three-dimensional electronic microfliers inspired by wind-dispersed seeds." Nature 597.7877 (2021): 503-510.

[2] Azadi, Mohsen, et al. “Controlled levitation of nanostructured thin films for sun-powered near-space flight.” Science Advances 7.7 (2021): eabe1127.

[3] Rozhdestvensky, Kirill V. “Wing-in-ground effect vehicles.” Progress in aerospace sciences 42.3 (2006): 211-283.

[4] Rohatschek, Hans. “Semi-empirical model of photophoretic forces for the entire range of pressures.” Journal of Aerosol Science 26.5 (1995): 717-734.

Presenting Author: Zhipeng Lu University of Pennsylvania

Presenting Author Biography: Zhipeng Lu received his Bachelor of Science in Chemistry at Nanjing University (Nanjing, China) in 2018. He joined Prof. Igor Bargatin's lab at the University of Pennsylvania in 2019, working on micron-scale lightweight materials with extraordinary heat resistivity, mechanical robustness, and aerodynamical mobility.

Authors:

Zhipeng Lu University of Pennsylvania
Miranda Stern University of Pennsylvania
Jinqiao Li University of Pennsylvania
David Candia University of Pennsylvania
Lorenzo Yao-Bate University of Pennsylvania
Thomas Celenza University of Pennsylvania
Mohsen Azadi University of Pennsylvania
Matthew Campbell University of Pennsylvania
Igor Bargatin University of Pennsylvania