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

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

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

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

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