Despite several research efforts, the detailed mechanism that dominates the thermal conductivity of nanofluids has not been explained. We investigated the effect of the chemical surface design of NPs (hydrophobic–hydrophilic balance of the NPs containing Janus surface) and verified the influence of their self–assembled structures on the thermal conductivity of a nanofluid using molecular simulation. When hydrophobic (HO) NPs are added to a nanofluid, they only form one large cluster because they do not prefer to be in contact with water molecules. Hence, an increment in the thermal conductivity of the nanofluid added with HO NPs was observed. On the contrary, the mean cluster size of Janus NPs was limited because of their limited HO superficial areas (attractive domains). Hence, an increment in the enhancement ratio of thermal conductivity was not observed even when the volume fraction of the NPs (φNP) was increased. Thus, our results demonstrate that the mean cluster size of NPs is the key factor for thermal conductivity enhancement. In addition, this study reveals that it is possible to control the thermal conductivity of nanofluids (or the mean cluster size of Janus NPs) using Janus NPs with surfaces designed using anisotropic chemical interactions.
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