Knudsen layer

The Knudsen layer, also known as evaporation layer, is the thin layer between liquid and vapour. It is named after Danish physicist Martin Knudsen (1871–1949).

Definition

On molecular level, the state of matter can be hard to define. From kinetic theory, it can be derived that if liquid is in contact with vapour, there is a small layer where the phase is between liquid and vapour. This region, several mean free path lengths thick, is called the Knudsen layer.

Knudsen layer thickness

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

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

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).