▎ 摘　　要

The practical application of thermally enhanced nanofluids in automotive industries for higher energy efficiency requires that nanoparticles be buoyant, leading to Brownian motion in the base fluid without aggregation. Numerous studies have reported the long-term stability of dispersions resulting from steric hindrance and an affinity for the surrounding base fluid via modification of nanoparticles with functional groups. In this study, we performed molecular dynamics simulations of nanofluids containing a single graphene sheet with various functional groups to investigate the influence of affinity, as well as particle-size, on the behavior of the graphene sheet. Using the concept of the speed of nanoparticle, we quantitatively evaluated the dependence of behavior on affinity to investigate whether having more functional groups with more attractive interactions has an impact on the stable dispersion in nanofluid. In addition, the simulation results for the particle-size effect revealed that the larger nanoparticles produced more stable dispersion in nanofluid. We concluded that the behavior of the graphene sheet depends on a combination of two factors: different charge assigned to atoms due to a nitrogen atom, and the difference in mass, which influences the speed of the nanoparticles. (C) 2017 Elsevier B.V. All rights reserved.