In this paper a critical investigation of layering phenomenon has been carried out, by means of experimental and numerical analyses, to explain the differences in thermal conductivity between nanofluids based on metal (Cu) and metal oxide (CuO) nanoparticles. Particularly, molecular dynamics simulations have been developed to investigate the adsorption of water molecules surrounding Cu and CuO nanoparticles of various sizes. Furthermore, different volume concentrations of nanoparticles in water have been analyzed. The numerical results revealed two shell-like formations of water molecules (layers) close to the Cu nanoparticle surface, differently from CuO nanoparticle, where no significant layering phenomenon has been observed. This result can explain the higher thermal conductivity of Cu-based nanofluid with respect to CuO-based one, which has been experimentally measured. The numerical and experimental results lead to the conclusion that layers of ordered water molecules surrounding metal nanoparticles play an important role in explaining experimental data of nanofluid thermal conductivity. (C) 2016 Elsevier Ltd. All rights reserved.