Review Note

Continuum Flow vs. Free Molecular Flow

If λ is orders of magnitude smaller than the scale of the body, then the flow appears to the body as a continuous substance. The molecules impact the body surface so frequently that the body cannot distinguish the individual molecular collisions, and the surface feels the fluid as a continuous medium. Such flow is called continuum flow.
The other extreme is where λ is on the same order as the body scale; here the gas molecules are spaced so far apart that collisions with the body surface occur only infrequently, and the body surface can feel distinctly each molecular impact. Such flow is called free molecular flow.