If the Knudsen number is near or greater than one, the mean free path of a molecule is comparable to a length scale of the problem, and the continuum assumption of fluid mechanics is no longer a good approximation.
One of the most widely used applications for the Knudsen number is in microfluidics and MEMS device design where flows range from continuum to free-molecular.
[3] In recent years, it has been applied in other disciplines such as transport in porous media, e.g., petroleum reservoirs.
[citation needed] Airflow around an aircraft such as an airliner has a low Knudsen number, making it firmly in the realm of continuum mechanics.
[5] Mixtures of gases with different molecular masses can be partly separated by sending the mixture through small holes of a thin wall because the numbers of molecules that pass through a hole is proportional to the pressure of the gas and inversely proportional to its molecular mass.
The technique has been used to separate isotopic mixtures, such as uranium, using porous membranes,[7] It has also been successfully demonstrated for use in hydrogen production from water.
[8] The Knudsen number also plays an important role in thermal conduction in gases.