Knudsen gas

As a Knudsen gas (by the Danish physicist Martin Knudsen ) is a very thin gas ( vacuum ) means in which the gas molecules due to the low density seldom abut against each other.

“Rarely” specifically means that the mean free path is greater than the dimensions of the vessel. This means that the collisions between the gas molecules are negligible, so there can be no laminar flow and no boundary layer in Knudsen gas, for example .