In this study, various nanofluids were synthesized and investigated in order to evaluate the density and viscosity of carbon nanoparticles suspension-based diesel oil (DO). The experiments were performed to measure the density and viscosity of several nanofluids containing nanoparticles of multi-walled carbon nanotubes (MWCNT) and graphene nanoplatelets (GNP) in the form of non-covalent with surfactant oleic acid (OA) and covalent with hexylamine (HA). These particles were individually functionalized with hexylamine. Then, five different nanofluids including graphene-HA/DO, multi-walled carbon nanotubes-HA/DO, graphene-OA/DO, carbon nanotubes-OA/DO, and a hybrid of nanographene plates and multi-walled carbon nanotubes-OA/DO at concentrations of 0.05, 0.1, 0.2, and 0.5 mass% were synthesized. The density and viscosity of these nanofluids were studied at temperatures of 5–100 °C. The results show that by increasing temperature, viscosity, shear stress, and density decreased for all mass concentrations. Also, an increase in mass concentration led to an increase in viscosity and shear stress, density, and viscosity index of all nanofluids compared with the pure DO at the constant temperature. The maximum increases in viscosity and density are, respectively, for HA-MWCNT/DO and OA-MWCNT at 0.5 mass% and 5 °C that is equal to 1.474 Pa. s for viscosity (82% more than viscosity of pure oil at 5 °C) and is 0.871 g cm−3 for density (12% more than density of pure oil at 5 °C). A comparison between MWCNT and GNP revealed that in all nanofluids, the viscosity of OA-MWCNT/DO is more than that of OA-GNP/DO and in most of the nanofluids the density of OA-GNP/DO is more than that of OA-MWCNT/DO. For example, at 0.5 mass% and 5 °C, the viscosity of OA-MWCNT/DO is 2.1% higher than that of OA-GNP/DO while the density of OA-GNP/DO is 0.34% higher than OA-MWCNT/DO.

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