  The presence and the microscopic origin of normal stress differences in dense
suspensions under simple shear flows are investigated by means of inertialess
particle dynamics simulations, taking into account hydrodynamic lubrication and
frictional contact forces. The synergic action of hydrodynamic and contact
forces between the suspended particles is found to be the origin of negative
contributions to the first normal stress difference $N_1$, whereas positive
values of $N_1$ observed at higher volume fractions near jamming are due to
effects that cannot be accounted for in the hard-sphere limit. Furthermore, we
found that the stress anisotropy induced by the planarity of the simple shear
flow vanishes as the volume fraction approaches the jamming point for
frictionless particles, while it remains finite for the case of frictional
particles.
