Differences in fluid dynamics associated with an atomic versus a molecular description of the same system

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Abstract
A fluid of diatomics is usually described entirely in terms of the motions of the molecules themselves. On the other hand, since the molecules are composed of atoms, a purely atomic description of the fluid is also possible. This paper explores the relationships and differences between these descriptions. Thus atomic mass, momentum, and energy densities, as well as the atomic momentum flux and heat flux, differ from their molecular counterparts. While some of these differences vanish in the Navier–Stokes regime (small spatial inhomogeneities), others do not. In particular, the atomic pressure tensor is necessarily symmetric while the natural molecular pressure tensor has an antisymmetric component.