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The Knudsen layer, also known as evaporation layer, is the thin layer of vapor near a liquid or solid. It is named after Danish physicist Martin Knudsen (1871–1949).

Definition

At the interface of a vapor and a liquid/solid, the gas interaction with the liquid/solid dominates the gas behavior, and the gas is, very locally, not in equilibrium.^[1] This region, several mean free path lengths thick, is called the Knudsen layer.^[2]

Knudsen layer thickness

The Knudsen layer thickness can be approximated by $l_{c}$ , given by^[3]

$l_{c}={\frac {kT_{s}}{\pi d^{2}p_{s}}}$ ,

where $k$ is Boltzmann's constant, $T_{s}$ is the temperature, $d$ is the molecular diameter and $p_{s}$ is the pressure.

Applications

One of the applications of Knudsen layer is in the coma of comets. It has been used specially in the coma chemistry model (ComChem model).^[4]

References

^ Dongari, Nishanth; Zhang, Yonghao; Reese, Jason M. (2008). "The 'Knudsen Layer' Effect on Thermal Transiporation of Rarefied Gases" (PDF). GASMEMS Report. Archived from the original (PDF) on 2016-06-03.
^ Aoki, K. (2003). "Knudsen Layer for Gas Mixtures". Journal of Statistical Physics. 112 (3/4): 629–655. doi:10.1023/A:1023876025363. S2CID 116416271.
^ Gusarov, A. V.; Smurov, I. (2002). "Gas-dynamic boundary conditions of evaporation and condensation: Numerical analysis of the Knudsen layer". Physics of Fluids. 14 (12): 4242. Bibcode:2002PhFl...14.4242G. doi:10.1063/1.1516211.
^ Huebner, W. F. (1987). "First Polymer in Space Identified in Comet Halley". Science. 237 (4815): 628–630. Bibcode:1987Sci...237..628H. doi:10.1126/science.237.4815.628. PMID 17758563. S2CID 22850190.

[1] Dongari, Nishanth; Zhang, Yonghao; Reese, Jason M. (2008). "The 'Knudsen Layer' Effect on Thermal Transiporation of Rarefied Gases" (PDF). GASMEMS Report. Archived from the original (PDF) on 2016-06-03.

[2] Aoki, K. (2003). "Knudsen Layer for Gas Mixtures". Journal of Statistical Physics. 112 (3/4): 629–655. doi:10.1023/A:1023876025363. S2CID 116416271.

[3] Gusarov, A. V.; Smurov, I. (2002). "Gas-dynamic boundary conditions of evaporation and condensation: Numerical analysis of the Knudsen layer". Physics of Fluids. 14 (12): 4242. Bibcode:2002PhFl...14.4242G. doi:10.1063/1.1516211.

[4] Huebner, W. F. (1987). "First Polymer in Space Identified in Comet Halley". Science. 237 (4815): 628–630. Bibcode:1987Sci...237..628H. doi:10.1126/science.237.4815.628. PMID 17758563. S2CID 22850190.

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