Origin of normal stress differences in rapid granular flows

A method for performing a Chapman-Enskog-like expansion of the Boltzmann equation corresponding to granular gases is presented. A calculation of the stress tensor corresponding to a two-dimensional gas of inelastically colliding smooth disks serves to demonstrate the method. This calculation provides an answer to the long sought source of the normal stress differences in granular fluids. It turns out that, like in molecular fluids, this effect is second (Burnett) order in the shear rate but, unlike in simple molecular fluids, it is a sizeable effect; as such it can be considered as a measurable manifestation of the Burnett correction for simple fluids.