Collaborative Research: ISS: Colloidal Microflyers: Observation and Characterization of (Self-)Thermophoresis through Air in Microgravity

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Thermophoresis, the motion of small particles in response to temperature gradients, is challenging to study in air on Earth because of the influence of air currents and gravity. Temperature gradients abound in the atmosphere, and accordingly thermophoresis affects the migration of atmospheric aerosols, which influence Earth’s climate by reflecting or absorbing sunlight and effecting cloud formation. However, the contributions of thermophoresis to these processes are difficult to disentangle from other factors, including air currents, gravity, evaporation, and electrical charge, exacerbating uncertainty surrounding the role aerosols play in both driving and remediating climate change. The objective of this work is to characterize thermophoresis of small particles in microgravity, where these confounding factors are absent. Microparticles will be packaged into specially-designed cuvettes on the ground and launched to the International Space Station (ISS), where their motion will be characterized visually. The temperature gradient may be externally imposed via heating one face of the cuvette, or it could be self-generated by particles that absorb light unevenly, a phenomenon known as “self-thermophoresis.” Self-thermophoresis has been observed in water but never in air; this work will reveal the extent to which asymmetric aerosols can undergo this same phenomenon. By providing data for a range of relevant materials, this work will inform climate models and be useful for other applications such as the use of thermophoresis to collect aerosols from air, including bioaerosols that transmit infectious diseases.The research objective of this work is to observe and quantify thermophoresis and self-thermophoresis through air in microgravity. The thermophoretic speeds will be measured visually (via the KERMIT microscope on the ISS) using airtight cuvettes designed, fabricated, and packaged on the ground. The particles characterized will be silica, alumina, and kaolinite, all of which are found in the atmosphere but whose thermophoretic properties have been incompletely characterized. After launch, the ISS-based experiments will proceed in two phases. First, in the thermophoresis experiments, the velocities of various microparticles will be characterized as a function of size, shape, and air pressure inside the cuvette (to simulate high altitudes). Second, the self-thermophoresis experiments will expose silica microspheres half-coated in gold to infrared and visible light, allowing the first-ever observation of self-thermophoretic motion by particle-generated temperature gradients. This work will provide the first-ever experimental demonstration of colloidal self-propulsion through a gas. In addition, this research could inform geoengineering proposals to release planet-cooling aerosols into the atmosphere, for which there is ongoing investigation into both their benefits and side effects. Finally, this work also provides a platform for future innovations in the engineering of three-dimensional “microflyers” for defense, environmental, or space applications.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.