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 other 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.

Diagram showing pressure difference induced by a temperature difference.

Thermal transpiration appears as an important correction in the readings of vapor pressure thermometers,^[1][2] and the effect is historically famous as being an explanation for the rotation of the Crookes radiometer. ^{[3] [4]}

See also

• Knudsen pump — a gas pump with no moving parts which functions via thermal transpiration.
• Thermophoresis (Soret effect) — diffusion of colloidal particles in a liquid, induced by a temperature gradient.