Knudsen Gas
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A Knudsen gas is a
gas Gas is a state of matter that has neither a fixed volume nor a fixed shape and is a compressible fluid. A ''pure gas'' is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom ( elements such as ...
in a state of such low density that the average distance travelled by the gas molecules between collisions (
mean free path In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a ...
) is greater than the diameter of the receptacle that contains it. If the mean free path is much greater than the diameter, the flow regime is dominated by collisions between the gas molecules and the walls of the receptacle, rather than intermolecular collisions with each other. It is named after Martin Knudsen.


Knudsen number

For a Knudsen gas, the
Knudsen number The Knudsen number (Kn) is a dimensionless number defined as the ratio of the molecular mean free path length to a representative physical length scale. This length scale could be, for example, the radius of a body in a fluid. The number is nam ...
must be greater than 1. The Knudsen number can be defined as: \rm = \frac where \lambda is the mean free path L is the diameter of the receptacle When 10^<\rm<10, the flow regime of the gas is transitional flow. In this regime the intermolecular collisions between gas particles are not yet negligible compared to collisions with the wall. However when \rm>10, the flow regime is
free molecular flow Free molecular flow describes the fluid dynamics of gas where the mean free path of the molecules is larger than the size of the chamber or of the object under test. For tubes/objects of the size of several cm, this means pressures well below 10− ...
, so the intermolecular collisions between the particles are negligible compared to the collisions with the wall.


Example

For example, consider a receptacle of air at room temperature and pressure with a mean free path of 68nm. If the diameter of the receptacle is less than 68nm, the Knudsen number would greater than 1, and this sample of air would be considered a Knudsen gas. It would not be a Knudsen gas if the diameter of the receptacle is greater than 68nm.


See also

*
Free streaming In astrophysics, free streaming is the motion of particles through a medium without scattering. Free streaming is often considered in the context of photons, but it is also relevant for neutrinos, cosmic rays, and hypothetical dark matter parti ...
* Kinetic theory


References

Gases Phases of matter {{statisticalmechanics-stub