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

由于气流和重力的影响，小颗粒对温度梯度的响应在地球空气中的运动是具有挑战性的。大气中存在着丰富的温度梯度，因此，热潜水作用影响大气气溶胶的迁移，而大气气溶胶通过反射或吸收阳光并影响云的形成来影响地球气候。然而，热泳对这些过程的贡献很难从其他因素中分离出来，包括气流、重力、蒸发和电荷，这加剧了围绕气溶胶在推动和补救气候变化方面所起作用的不确定性。这项工作的目的是描述微小颗粒在微重力下的热电泳性，在微重力条件下，这些混杂因素是不存在的。微粒子将被包装在地面上专门设计的试管中，并发射到国际空间站(ISS)，在那里它们的运动将被可视化地描述。温度梯度可能是通过加热试管的一个面从外部施加的，也可能是由吸收光线不均匀的颗粒自身产生的，这一现象被称为“自身热导入”。在水中观察到了自热潜水作用，但在空气中从未观察到过；这项工作将揭示不对称气溶胶在多大程度上可以经历同样的现象。通过提供一系列相关材料的数据，这项工作将为气候模型提供信息，并对其他应用有用，例如利用热泳从空气中收集气溶胶，包括传播传染病的生物气溶胶。这项工作的研究目标是观察和量化微重力下空气中的热泳和自我热泳。热泳速度将使用地面上设计、制造和包装的密闭试管进行目测(通过国际空间站上的科米特显微镜)。表征的颗粒将是二氧化硅、氧化铝和高岭石，所有这些都存在于大气中，但其热电特性尚未完全表征。发射后，国际空间站的实验将分两个阶段进行。首先，在热泳实验中，各种微粒的速度将被表征为大小、形状和试管内气压的函数(以模拟高海拔)。其次，自体热泳实验将使半包覆金的二氧化硅微球暴露在红外和可见光下，从而首次通过粒子产生的温度梯度观察到自体热泳运动。这项工作将提供有史以来第一次胶体通过气体自推进的实验演示。此外，这项研究还可以为向大气中释放地球冷却气溶胶的地球工程建议提供信息，目前正在对其好处和副作用进行调查。最后，这项工作还为国防、环境或空间应用的三维“微型飞行器”工程的未来创新提供了一个平台。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.