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. 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 the Boltzmann 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).