Coherent translation and libration as seen by a shear viscosity of a classical hard sphere fluid

Thermodynamic transport properties, such as a shear viscosity, can provide evidence of coherent phenomena. In a dilute gas, the coherence takes the form of free translation interrupted by collisions and this effect is reflected in the wavevector dependence of the shear viscosity. In a dense fluid, near the solidification transition or in a glass, the viscosity begins to diverge as a consequence of highly correlated recollisions. In this work, a generalized Langevin formulation of the kinetic theory of hard spheres is used to calculate the shear viscosity in these two limiting regimes. In the collision dominant (i.e., caging) regime, the divergence of the shear viscosity can be compensated by the vanishing self diffusion coefficient. The Dη product is well behaved, illustrates aspects of Stokes–Einstein behavior, in rough accord with the hydrodynamic predictions (obtained using slip boundary conditions) and suggests a Stokes–Einstein breakdown in accord with experiments on glassy ortho-terphenyl liquids.