Shear and bulk acceleration viscosities in simple fluids

Inhomogeneities in the velocity field of a moving fluid are dampened by the inherent viscous behaviour of the system. Both bulk and shear effects, related to the divergence and the curl of the velocity field, are relevant. On molecular time scales, beyond the Navier-Stokes description, memory plays an important role. We demonstrate here on the basis of molecular and overdamped Brownian dynamics many-body simulations that analogous viscous effects act on the acceleration field. This acceleration viscous behaviour is associated with the divergence and the curl of the acceleration field and it can be quantitatively described using simple exponentially decaying memory kernels. The simultaneous use of velocity and acceleration fields enables the description of fast dynamics on molecular scales.