where T0 is the (constant) temperature of the gas,
Mn is the molar mass of the gas, and
R is the universal gas constant. Here the total gas pressure,
pA = pref+pf, where
pA is the absolute pressure and
pf is the pressure developed as a result of the flow. Substituting this relation into
Equation 4-3 and dividing through by the constant
Mn/RT0 results in a modified form of the Reynolds equation:
For a gas, the slip length is often expressed using the mean free path, λ, and a tangential momentum accommodation coefficient,
α. For compatibility with the existing literature on thin-film gas flow the following definition of the mean free path is used by COMSOL Multiphysics in the Thin-Film Flow interfaces: