ABSTRACT

Photophoresis, a light-induced thermal transport and levitation mechanism, has been studied and used with micrometer-scale particles for over a century. The principal investigator's group recently discovered photophoretic levitation and propulsion of macroscopic (millimeter-scale) plates thousands of times larger than ever before realized. This project will study the mechanisms of heat transfer that lead to the levitation of ultrathin macroscopic structures. The research can bring about a paradigm shift in unmanned aerial vehicle (drones) technology, eliminating the need for onboard propellers and batteries, and enabling persistent microflyers that can be powered by sunlight alone and operate at altitudes ranging from the sea level to mesosphere (an underexplored region of the atmosphere at altitudes of 50-80 km). Separately from photophoresis, thermal absorption and emission in ultrathin structures are important for thermal engineering of miniaturized spacecraft, including interstellar light sails, which may one day transmit images from other stars' planetary systems. The integrated research, education, and outreach plan will train a new generation of engineers to work at the intersection of thermodynamics, heat transfer, and nanotechnology through interdisciplinary courses and outreach activities.

The research objective of this project is to study radiation absorption, radiation emission, and thermal accommodation in ultrathin metamaterial structures that produce photophoretic forces exceeding their weight. The photophoretic force arises when a solid is illuminated by light, which heats the solid relative to the ambient gas, producing momentum exchange between them. The force is maximized in structures that absorb light on the bottom while keeping the top side cool, or in heated structures with different thermal accommodation coefficients on opposite sides. The PI will use photophoretic levitation as an experimental platform to study light absorption, thermal infrared emission, and thermal accommodation in ultrathin structures, including micro- and nano-patterned structures as well as novel 2D materials. Simultaneously, theoretical modeling will be performed to understand the heat transfer across multiple length scales, including molecular dynamics of thermal accommodation of gas molecules, finite-element simulation of ultrathin selective absorbers and emitters, and analytical modeling of the solid-gas momentum exchange. The intellectual merit of this proposal lies in the performance of novel experiments and the development of new theoretical models to understand photophoresis and heat transfer in structures with nanoscale thickness but macroscopically large lateral scales.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

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