Thermal transpiration (or thermal diffusion) refers to the thermal force on a

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

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 In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a ...

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 thermometer A vapour pressure thermometer is a thermometer that uses a pressure gauge to measure the vapour pressure Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pre ...

s, and the effect is historically famous as being an explanation for the rotation of the

Crookes radiometer The Crookes radiometer (also known as a light mill) consists of an airtight glass bulb containing a partial vacuum, with a set of vanes which are mounted on a spindle inside. The vanes rotate when exposed to light, with faster rotation for more i ...

References

Non-equilibrium thermodynamics {{thermodynamics-stub

See also

References