Towards photophoretic levitation of macroscopic structures in near-space

Abstract

Ultralightweight, nanofabricated structures of macroscopic size could photophoretically fly in near-space. Three photophoretic mechanisms, Δα photophoresis, ΔT photophoresis, and thermal transpiration, each enable a variety of structure designs and levitation characteristics. Proposed devices range from microscale engineered aerosols for solar geoengineering, to centimeter-scale thin disks with variations in surface accommodation coefficients, to bilayer “nanocardboard” structures that could be extended to meter-scale sizes. Despite progress towards these designs, the feasibility of photophoretic levitation for near-space applications requires deeper analysis. Better understanding of (1) the interplay among photophoretic mechanisms across pressure regimes, (2) the optimal structure design for maximized photophoretic lofting forces in the atmosphere, and (3) practical device fabrication, performance, and deployment are all necessary to establish feasibility. This thesis presents analytical models, numerical simulations, experimental measurements, and design analysis to evaluate the feasibility of photophoretically levitating macroscopic structures in near-space. Focusing on thermal transpiration as the most promising photophoretic mechanism to do so, we develop an analytical model of the lofting force on a generalized bilayer structure using first principles. Several optimal structural parameters were obtained for maximizing the lofting force at upper atmospheric pressures, and the remainder were determined by augmenting the analytical model with numerical simulations of thermal transpiration flows through and around a thin, permeable membrane. The models informed the design of 1 cm-width nanocardboard-like structures for experimental testing. After fabricating several test samples, their lofting forces were measured when illuminated at mesospheric pressures. The results validate the analytical and numerical approaches. We propose a practical 10-cm diameter device that could loft a 7 mg payload at altitudes of 60-80 km. While not optimal, this structure can be pursued as a basis for photophoretic aircraft design. This work presents several novel results in the emerging field of applied photophoresis. First, we identified the optimal structural parameters for the nanocardboard architecture at specific atmospheric altitudes. Second, we constructed a database of dimensionless numerical data that can be applied to any thin, permeable disk structure within the architecture to calculate the expected photophoretic forces as a function of rarefaction parameter and temperature distribution. Third, we identify that permeable 2D structures like nanocardboard only improve the lofting forces over nonpermeable structures if their temperature gradients are maintained at high values of the rarefaction parameter (> 10). At lower rarefaction parameters, perforations are detrimental to the lofting force. Fourth, we designed and fabricated nanocardboard-like structures with heterogeneous ligament (post) distributions, which efficiently compromise between structural rigidity and photophoretic performance. Lastly, we report the first photophoretic levitation of a macroscopic structure when illuminated by less than the equivalent intensity of sunlight.