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Nanofluids; dispersions of nanometer-sized particles in a liquid medium; havebeen proposed for a wide variety of thermal management applications. It isknown that a solid-like nanolayer of liquid of typical thickness 0.5-1 nmsurrounding the colloidal nanoparticles can act as a thermal bridge between thenanoparticle and the bulk liquid. Yet, its effect on the nanofluid viscosityhas not been elucidated so far. In this article, we compute the local viscosityof the nanolayer using equilibrium molecular dynamics based on the Green-Kuboformula. We first assess the validity of the method to predict the viscositylocally. We apply this methodology to the calculation of the local viscosity inthe immediate vicinity of a metallic nanoparticle for a wide range ofsolid-liquid interaction strength, where a nanolayer of thickness 1 nm isobserved as a result of the interaction with the nanoparticle. The viscosity ofthe nanolayer, which is found to be higher than its corresponding bulk value,is directly dependent on the solid-liquid interaction strength. We discuss theorigin of this viscosity enhancement and show that the liquid density incrementalone cannot explain the values of the viscosity observed. Rather, we suggestthat the solid-like structure of the distribution of the liquid atoms in thevicinity of the nanoparticle contributes to the nanolayer viscosityenhancement. Finally, we observe a failure of the Stokes-Einstein relationbetween viscosity and diffusion close to the wall, depending on theliquid-solid interaction strength, which we rationalize in terms ofhydrodynamic slip.

Enhanced local viscosity around colloidal nanoparticles probed by Equilibrium Molecular Dynamics Simulations