[1] The presence of free molecular flow can be calculated, at least in estimation, with the Knudsen number (Kn).
Practically, the "vapor" cannot move around bends or into other spaces behind obstacles, as they simply hit the tube wall.
For example, in the interplanetary medium, the plasma is in a free molecular flow regime in scales less than 1 AU; thus, planets and moons are effectively under particle bombardment.
However, on larger scales, fluid-like behavior is observed, because the probability of collisions between particles becomes significant.
The technique is used to separate isotopic mixtures, such as uranium, using gaseous diffusion through porous membranes.