Investigation of collective dynamics of solvent molecules in nanofluids by inelastic x-ray scattering

Inelastic x-ray scattering is conducted to investigate the reason why the thermal conductivity of nanofluids increases beyond a theoretical value by the addition of a small amount of nanoparticles (NPs). High-frequency sound velocity is obtained by analyzing inelastic x-ray scattering data of nanofluids. Since the high-frequency sound velocity reflects thermal conductivity and structural relaxation of solvent molecules, the effect of nanoparticles on heat transport and molecular dynamics of the medium of nanofluids could be clarified. Cu nanoparticles (Cu NPs) are found to be extremely effective to largely increase the high-frequency sound velocity of medium ethylene glycol, while the effect of alumina NPs on water is limited. The increase in high-frequency sound velocity agrees well with that in thermal conductivity. It is concluded that solvent molecules are highly restrained by Cu NPs in nanofluid, leading to high thermal conductivity.