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.

References
  1. Watkins, R. A. (1967). "Thermomolecular Pressure Difference Measurements for Precision Helium−3 and Helium−4 Vapor-Pressure Thermometry". The Journal of Chemical Physics. 46 (3): 1007–1018. Bibcode:1967JChPh..46.1007W. doi:10.1063/1.1840762.
  2. Pobell, F. (2007). Matter and Methods at Low Temperatures (3rd ed.). Springer. ISBN 978-3-540-46356-6.
  3. Reynolds, Osbourne (1879). "On certain dimensional properties of matter in the gaseous state. - Part I. Experimental researches on thermal transpiration of gases through porous plates and on the laws of transpiration and impulsion, including an experimental proof that gas is not a continuous plenum. - Part II. On an extension of the dynamical theory of gas, which includes the stresses, tangential and normal, caused by a varying condition of gas, and affords an explanation of the phenomena of transpiration and impulsion". Philosophical Transactions of the Royal Society of London. 170 (170): 727–845. doi:10.1098/rstl.1879.0078.
  4. Maxwell, James Clerk (1879). "On stresses in rarified gases arising from inequalities of temperature". Philosophical Transactions of the Royal Society of London. 170: 231–256. doi:10.1098/rstl.1879.0067. S2CID 110911902.


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