Schafer Benjamin C, Kim Jong-Hyoung, Sharipov Felix, Hwang Gyeong-Seok, Vlassak Joost J, Keith David W
John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA.
Department of Materials Science and Engineering, Pukyong National University, Busan, South Korea.
Nature. 2025 Aug;644(8076):362-369. doi: 10.1038/s41586-025-09281-8. Epub 2025 Aug 13.
Lightweight nanofabricated structures could photophoretically loft payloads in near-space. Proposed structures range from microscale engineered aerosols, to centimetre-scale thin disks with variations in surface accommodation coefficients, to sandwich structures with nanoscale thickness that might be extended to metre-scale width. Quantitative understanding of how structural and surface properties determine photophoretic lofting forces is necessary to develop a practical flying device. Here we focus on thermal transpiration as the most promising photophoretic mechanism for lofting large devices and present a hybrid analytical-numerical model of the lofting force on a structure that consists of two perforated membranes spaced a small distance apart. We identify optimal structural parameters, including device size, membrane perforation density and distribution of the vertical ligaments that connect the two membranes, each as a function of atmospheric altitude. Targeting these optimal parameters, we fabricate structures with a heterogeneous ligament distribution, which efficiently compromises between structural rigidity and photophoretic performance. We measure how lofting forces generated by these structures depend on pressure using gases with three different molecular weights. We observed photophoretic levitation of a 1-cm-wide structure at an air pressure of 26.7 Pa when illuminated by 750 W m, about 55% the intensity of sunlight. Lastly, we describe the preliminary design of a 3-cm-radius device with 10-mg payload capacity at 75-km altitudes and discuss horizontal motion control, overnight settling, and applications in climate sensing, communications and Martian exploration.
轻质纳米制造结构可通过光泳作用将有效载荷提升至近太空。提议的结构范围从微观尺度的工程气溶胶，到具有不同表面适应系数的厘米级薄盘，再到纳米级厚度且可能扩展至米级宽度的夹层结构。要开发一种实用的飞行装置，定量了解结构和表面特性如何决定光泳提升力是必要的。在此，我们聚焦于热 transpiration 作为提升大型装置最有前景的光泳机制，并提出了一种关于提升力的混合解析 - 数值模型，该模型针对由两个间距很小的穿孔膜组成的结构。我们确定了最优结构参数，包括装置尺寸、膜穿孔密度以及连接两个膜的垂直韧带的分布，它们均是大气高度的函数。针对这些最优参数，我们制造了具有非均匀韧带分布的结构，该结构在结构刚度和光泳性能之间实现了有效折衷。我们使用三种不同分子量的气体测量了这些结构产生的提升力如何随压力变化。当受到 750 W m 的光照时，我们观察到一个 1 厘米宽的结构在 26.7 Pa 的气压下实现了光泳悬浮，光照强度约为太阳光强度的 55%。最后，我们描述了一种半径为 3 厘米、在 75 千米高度具有 10 毫克有效载荷能力的装置的初步设计，并讨论了水平运动控制、夜间沉降以及在气候传感、通信和火星探测中的应用。
