MEAM Ph.D. Thesis Defense: “Photophoretic Light-flyers for Mesospheric Applications”

The Earth’s mesosphere plays a crucial role in weather forecasting, environmental conservation, and planetary exploration. However, traditional unmanned aerial vehicles (UAVs) such as balloons or spacecraft face significant challenges when attempting to access this layer. Balloons and airplanes struggle due to the low pressure, while satellites encounter high aerodynamic drag. To overcome these limitations, we have developed a photophoresis-based UAV capable of levitating in the Earth’s mesosphere for an extended duration of days or even months. Photophoresis refers to the movement of small particles suspended in fluids when illuminated by an intense beam of light. This phenomenon can be attributed to a difference in heat exchange of a particle illuminated in gases or liquids. Our research focuses on the levitation of mylar-based light-flyers utilizing the photophoretic force driven by the difference in the thermal accommodation coefficient (TAC). These levitations ultimately occur under light irradiances comparable to natural sunlight (1.36 kW/m2).

Our work encompasses several key contributions. First, we proposed a guideline to minimize the ground effect associated with testing in a vacuum chamber, ensuring realistic performance evaluations of light-flyers. Second, we utilized germanium coatings as selective absorbers to achieve levitation at irradiances as low as 1.5 kW/m2 with minimal ground effect. Third, we revised the previous semi-empirical model that underlies the photophoretic force based on our experimental observations. Finally, we predicted successful levitations of light-flyers in a mesosphere-like environment under natural sunlight, accommodating sub-milligram payloads. Overall, our study represents a significant advancement towards launching long-life UAVs into Earth’s mesosphere, which can offer substantial benefits to atmospheric science.