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).

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

The Knudsen layer thickness can be approximated by ${\displaystyle l_{c}}$, given by³

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

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

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