Radiometric phenomena: From the 19th to the 21st century

Many of the major scientists of the late 19th and early 20th century have weighed in on the mechanisms that influence the forces produced in the Crookes radiometer. However, even today, there is still some uncertainty, and in fact confusion, about the order of magnitudes of forces produced by these mechanisms and the dominant mechanisms in various operational regimes and configurations. The development of a better understanding of the radiometric force, aimed at resolving that confusion, has been helped by modern computational techniques. In the transitional flow regime, where the radiometric force is observed, kinetic approaches are necessary for valid results. These modern kinetic approaches, coupled with significant increases in computational speed, have qualitatively improved the level of understanding of radiometric flows. For the first time, flowfields near the vanes of a radiometer can be fully investigated in an attempt to quantify the importance of various mechanisms involved in the production of the radiometric force. In many cases, the computational techniques have been validated by highly accurate experiments. Various operational regimes and geometric configurations have been studied in an attempt to maximize the force. This review looks at the historic context of three types of radiometers, most notably the Crookes radiometer, develops a modern understanding of the mechanisms behind the radiometric force, and documents recent applications.