Consider a neutral gas consisting of identical hard-sphere molecules of mass and diameter . Admittedly, this is not a particularly physical model of a neutral gas, but we are only considering it for illustrative purposes. The fluid equations for such a gas are similar to Equations (4.47)-(4.49):
The mean-free-path, , for hard-sphere molecules is given by
In the Chapman-Enskog scheme, the distribution function is expanded, order by order, in the small parameter :
It is possible to linearize the kinetic equation, and then rearrange it so as to obtain an integral equation for in terms of . This rearrangement crucially depends on the bilinearity of the collision operator. Incidentally, the equation is integral because the collision operator is an integral operator. The integral equation is solved by expanding in velocity space using Laguerre polynomials (sometimes called Sonine polynomials). It is possible to reduce the integral equation to an infinite set of simultaneous algebraic equations for the coefficients in this expansion. If the expansion is truncated, after terms, say, then these algebraic equations can be solved for the coefficients. It turns out that the Laguerre polynomial expansion converges very rapidly. Thus, it is conventional to keep only the first two terms in this expansion, which is usually sufficient to ensure an accuracy of about percent in the final result. Finally, the appropriate moments of are taken, so as to obtain expression for the heat flux density and the viscosity tensor. Strictly speaking, after evaluating , we should then go on to evaluate , so as to ensure that really is negligible compared to . In reality, this is never done because the mathematical difficulties involved in such a calculation are prohibitive.
The Chapman-Enskog method outlined previously can be applied to any assumed force law between molecules, provided that the force is sufficiently short-range (i.e., provided that it falls off faster with increasing separation than the Coulomb force). For all sensible force laws, the viscosity tensor is given by
Equations (4.66) and (4.67) have a simple physical interpretation. The viscous and thermal diffusivities of a neutral gas can be accounted for in terms of the random-walk diffusion of molecules with excess momentum and energy, respectively. Recall the standard result in stochastic theory that if particles jump an average distance , in a random direction, times a second, then the diffusivity associated with such motion is (Reif 1965). Chapman-Enskog theory basically allows us to calculate the numerical constants and , multiplying in the expressions for and , for a given force law between molecules. Obviously, these coefficients are different for different force laws. The expression for the mean-free-path, , is also different for different force laws.