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

Knudsen layer thickness

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

l c = k T s π d 2 p s {\displaystyle l_{c}={\frac {kT_{s}}{\pi d^{2}p_{s}}}},

where k {\displaystyle k} is the Boltzmann constant, T s {\displaystyle T_{s}} is the temperature, d {\displaystyle d} is the molecular diameter and p s {\displaystyle 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).