One-step synthesis of Cu/Therminol VP-1 nanofluids by phase transfer method and their thermal stability and thermophysical properties

Heat transfer oil-based nanofluids with metal nanoparticles can enhance energy conversion system thermal efficiency. However, high-surface-energy nanoparticles tend to aggregate, causing nanofluid instability. This study employed a one-step phase transfer synthesis method to prepare Therminol VP-1-based nanofluids containing copper nanoparticles. These nanofluids had uniform particle sizes of approximately 10.6 nm and excellent dispersion. The impact of four surfactants (Span-80, Tween-40, oleic acid, and dodecanethiol) on nanofluid stability was investigated, determining the optimal surfactant concentration range for superior dispersion stability. Nanofluid stability was also evaluated considering nanoparticle concentration, temperature variations, and heating–cooling cycles. Additionally, the influence of temperature and nanoparticle concentration on nanofluid thermal conductivity and viscosity was examined. A theoretical model based on Brownian motion has been used to predict thermal conductivity. The predicted values agreed well with experimental results when considering the variation in nanoparticle size caused by particle aggregation during the experiments.