The properties of nanoparticles (including shape, size, material, and volume concentration) may significantly influence the thermal properties of nanofluids. Through molecular dynamics simulations, the aim of this study is to investigate the influence of nanoparticle properties on the thermal conductivity of nanofluids and find an effective criterion for predicting thermal conductivity enhancement. By establishing a series of simulation models, thermal conductivities of nanofluids were calculated on the basis of the Green-Kubo formula. It was found that all the nanoparticle properties that have been considered in this work influence the thermal conductivity of nanofluids, and the influence rules were discussed. Furthermore, there is a positive correlation between the distribution of atomic potential energy and the thermal conductivity of nanofluids. Therefore, the ratio of energetic atoms in nanoparticles is proposed to be the criterion for predicting enhancement of the apparent thermal conductivity of nanofluids.

Impact and friction model of nanofluid for molecular dynamics simulation was built which consists of two Cu plates and Cu-Ar nanofluid. The Cu-Ar nanofluid model consisted of eight spherical copper nanoparticles with each particle diameter of 4 nm and argon atoms as base liquid. The Lennard-Jones potential function was adopted to deal with the interactions between atoms. Thus motion states and interaction of nanoparticles at different time through impact and friction process could be obtained and friction mechanism of nanofluids could be analyzed. In the friction process, nanoparticles showed motions of rotation and translation, but effected by the interactions of nanoparticles, the rotation of nanoparticles was trapped during the compression process. In this process, agglomeration of nanoparticles was very apparent, with the pressure increasing, the phenomenon became more prominent. The reunited nanoparticles would provide supporting efforts for the whole channel, and in the meantime reduced the contact between two friction surfaces, therefore, strengthened lubrication and decreased friction. In the condition of overlarge positive pressure, the nanoparticles would be crashed and formed particles on atomic level and strayed in base liquid.

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Molecular dynamic model of nanofluid between flat plates under shear flow conditions was built.The nanofluid model consisted of 12 spherical copper nanoparticles with each particle diameter of 4 nm and argon atoms as base liquid. The Lennard-Jones (LJ) potential function was adopted to deal with the interactions between atoms. Thus, the motion states of nanoparticles during the process of flowing were obtained and the flow behaviors of nanofluid between flat plates at different moments could be analyzed. The simulation results showed that an absorption layer of argon atoms existed surrounding each nanoparticle and would accompany with the particle to move. The absorption layer contributed little to the flow of nanoparticles but much to the heat transferring in nanofluids. Another phenomenon observed during shear flowing process was that the nanoparticles would vibrate and rotate besides main flowing with liquid argon and these micro-motions could strengthen partial flowing in nanofluids.