Since the derivation of the Boltzmann Equation uses the molecular chaos assumption, it seems to me that it should not be valid for dense systems such as fluids. Now, according to Chapman-Enskog theory, the Boltzmann equation can be represented by a series expansion in powers of the Knudsen Number. If we take the first two terms in this series, we get the Navier-Stokes equation. But, the Navier-Stokes equation is valid for the motion of viscous fluid substances which disobey the molecular chaos assumption.
This sounds to me like a contradiction. How could the Boltzmann equation reduce to the Navier-Stokes equation even though it uses the molecular chaos assumption?
