An approach for tailoring the interfacial thermal conductance of copper-water nanofluids through ion additions and the underlying mechanism

Smaller Kapitza resistance is known to be the most crucial factor responsible for the larger thermal conductivity of nanofluids. But what the underlying mechanism is remains ambiguous. Non-equilibrium molecular dynamics simulations were performed in the present work to examine the effect of ionic addition on the Kapitza resistance of Cu-water nanofluids. The temperature profile indicates that the concentration and type of the additive slightly affects the thickness of the interfacial layer, but the temperature drops significantly. For concentrations of 1 M, the interfacial thermal conductance (ITC, the reciprocal of the Kapitza resistance) of Cu-water with NaCl, NaOH and HCl increases by 48.1 %, 29.2 % and 22.6 % respectively, compared to that without additives. The greatest ITC improvement of 123.7 % can be achieved by adding 3 M NaCl. The density profiles of Cu-water nanofluids with various additives suggest that the changes in the interfacial structure are primarily due to the positions of the added cations and anions. It is important to note, however, that the ITC is closely related to the orderliness of the interfacial layer and the matching of the phonon density of states, since the addition of NaCl can increase the PDOS at lower frequencies 1–5 THz, where that for the Cu nanoparticle appears. The findings provide a practical approach to tailor the thermal conductivity of nanofluids by the addition of appropriate ions.