# 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

This region, several mean free path lengths thick, is called the Knudsen layer.[1]

## Knudsen layer thickness

The knudsen layer thickness can be approximated by $l_c$, given by[2]

$l_c =\frac{k T_s}{\pi d^2 p_s}$,

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).[3]

## References

1. ^ Aoki, K. (2003). Journal of Statistical Physics 112 (3/4): 629–655. doi:10.1023/A:1023876025363. edit
2. ^ 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. edit
3. ^ Huebner, W. F. (1987). "First Polymer in Space Identified in Comet Halley". Science 237 (4815): 628–630. doi:10.1126/science.237.4815.628. PMID 17758563. edit