Thermal transpiration

Thermal transpiration (or thermal diffusion) refers to the thermal force on a gas due to a temperature difference. Thermal transpiration causes a flow of gas in the absence of any pressure difference, and is able to maintain a certain pressure difference called thermomolecular pressure difference in a steady state. The effect is strongest when the mean free path of the gas molecules is comparable to the dimensions of the gas container.
Thermal transpiration appears as an important correction in the readings of vapor pressure thermometers, and the effect is historically famous as being an explanation for the rotation of the Crookes radiometer.