  Nanofluids; dispersions of nanometer-sized particles in a liquid medium; have
been proposed for a wide variety of thermal management applications. It is
known that a solid-like nanolayer of liquid of typical thickness 0.5-1 nm
surrounding the colloidal nanoparticles can act as a thermal bridge between the
nanoparticle and the bulk liquid. Yet, its effect on the nanofluid viscosity
has not been elucidated so far. In this article, we compute the local viscosity
of the nanolayer using equilibrium molecular dynamics based on the Green-Kubo
formula. We first assess the validity of the method to predict the viscosity
locally. We apply this methodology to the calculation of the local viscosity in
the immediate vicinity of a metallic nanoparticle for a wide range of
solid-liquid interaction strength, where a nanolayer of thickness 1 nm is
observed as a result of the interaction with the nanoparticle. The viscosity of
the nanolayer, which is found to be higher than its corresponding bulk value,
is directly dependent on the solid-liquid interaction strength. We discuss the
origin of this viscosity enhancement and show that the liquid density increment
alone cannot explain the values of the viscosity observed. Rather, we suggest
that the solid-like structure of the distribution of the liquid atoms in the
vicinity of the nanoparticle contributes to the nanolayer viscosity
enhancement. Finally, we observe a failure of the Stokes-Einstein relation
between viscosity and diffusion close to the wall, depending on the
liquid-solid interaction strength, which we rationalize in terms of
hydrodynamic slip.
