Demonstration of Atmospheric-Pressure Radiometer With Nanocardboard Vanes

Crookes radiometers have been the subject of numerous theoretical, numerical, and experimental studies because of the complicated forces they exhibit as well as their potential applications to light sensing and actuation. The majority of these studies have focused on classical radiometers, which function under low vacuum pressures. In contrast, here we report a radiometer with microengineered vanes that rotates at atmospheric pressure. Its functionality at pressures thousands of times higher than previous light mills is due to unique attributes of the nanocardboard that forms its vanes: 1) the extremely low areal density (0.1 mg/cm²) of nanocardboard reduces the vane masses by two orders of magnitude; 2) its lower thermal conductivity allows a greater cross-vane temperature difference; and 3) its microchannels dramatically increase the thermal transpiration flow that drives the rotation. Intriguingly, the experimentally observed rotation speeds are substantially higher than those theoretically expected. Our device demonstrates new possibilities for micromanipulation, propulsion of aerial vehicles, and light-powered generators. [2020-0165].

I. Introduction

Traditional Crookes radiometers, shown in Fig. 1, are well-known examples of opto-thermo-mechanical coupling [1]. They usually consist of a low-friction hub and four lightweight paper vanes that are colored black and white on opposing faces and are placed inside a glass chamber under low vacuum ( Pa). When exposed to sunlight, the black sides of the vanes are radiatively heated to a higher temperature than the white sides. Two forces arise as a result. First, molecules impacting the hotter black sides on average depart with higher velocities than those contacting the white sides; this results in a small differential recoil force. Second, as a result of the temperature gradient, gas molecules tend to creep around the edges of the vanes from the white sides to the black sides. This motion, called thermal transpiration, also causes the vanes to move in a direction toward the white sides. However, as the pressure rises above Pa, both the differential recoil and thermal transpiration forces decrease and eventually become smaller than the static friction and aerodynamic drag forces, preventing the hub from turning. Fig. 1.

(a) Photorealistic rendering of a traditional radiometer in a vacuum jar. Light heats the dark sides of the vanes, inducing a photophoretic force that causes the vanes to rotate. (b) Schematic diagram showing forces on traditional radiometer vanes. The differential momentum force arises from molecules departing the light and dark-colored faces with different velocities, whereas the thermal transpiration force arises from gas molecules slipping around the edges of the vanes.