  We present a molecular dynamics study of the flow of rigid spherical
nanoparticles in a simple fluid. We evaluate the viscosity of the dispersion as
a function of shear rate and nanoparticle volume fraction. We observe shear
thinning behavior at low volume fractions, as the shear rate increases, the
shear forces overcome the brownian forces, resulting in more frequent and more
violent collisions between the nanoparticles. This in turn results in more
dissipation. We show that in order to be in the shear thinning regime the
nanoparticle have to order themselves into layers longitudinal to the flow to
minimize the collisions. As the nanoparticle volume fraction increases there is
less room to form the ordered layers, consequently as the shear rate increases
the nanoparticles collide more which results in turn in shear thickening. Most
interestingly, we show that at intermediate volume fractions the system
exhibits metastability, with successions of ordered and disordered states along
the same trajectory. Our results suggest that for nanoparticles in a simple
fluid the hydro-clustering phenomenon is not present, instead the
order-disorder transition is the leading mechanism for the transition from
shear thinning to shear thickening.
