The drag function is expressed as:

(12–53)

For sub-micron particles, a form of Stokes’ drag law more suitable for small particles is available [502]. In this case, is defined as

(12–54)

The factor is the Cunningham correction to Stokes’ drag law, which you can compute from

(12–55)

where is the molecular mean free path. The Cunningham correction improves drag predictions for the slip flow or transitional regime between the continuum and free-molecular flow regimes. It is not intended for the free-molecular flow regime where the mean free path of the gas molecules is much larger than the particle diameter.