Knudsen layer

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]

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

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

1. Dongari, Nishanth; Zhang, Yonghao; Reese, Jason M. (2008). "The 'Knudsen Layer' Effect on Thermal Transiporation of Rarefied Gases" (PDF). GASMEMS report.
2. Aoki, K. (2003). "Knudsen Layer for Gas Mixtures". Journal of Statistical Physics. 112 (3/4): 629–655. doi:10.1023/A:1023876025363.
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.