Rarefied gas flow occurs when the mean free path, λ, of the molecules becomes comparable with the length scale of the flow, l. There are four flow regimes depending on the value of the Knudsen number, Kn = λ/l (Ref. 5):

These flow regimes are shown in Figure 2-4. In the continuum flow regime the Navier–Stokes equations are applicable, as discussed in Modeling Microfluidic Fluid Flows. Gases flowing in the slip flow regime can be modeled using the Navier–Stokes equations with modified boundary conditions. The Slip Flow interface is included with the Microfluidics Module and can be used to model gas flows at these moderate Knudsen numbers. The Molecular Flow Module includes the Transitional Flow and the Free Molecular Flow interfaces, which can be used to model fully kinetic gas flows, at Knudsen numbers greater than 0.1.