High thermal conductivity of nanoparticles not necessarily contributing more to nanofluids

Nanofluids with high thermal conductivity represent a promising means of enhancing heat transfer. It is generally believed that nanoparticles with higher thermal conductivity offer greater improvements in the thermal conductivity of the associated nanofluid. However, some experimental results are inconsistent with this opinion, which seems surprising. In this letter, an effective medium model based on phonon theory is used to show that the phonon frequencies of the liquid and the nanoparticles play a key role. Nanoparticles with higher thermal conductivity that match the phonon frequency of the liquid can be more effective at very small sizes.