Evaporation behavior of PEGylated graphene oxide nanofluid droplets on heated substrate

In this study, the evaporation behavior of graphene nanofluid droplets deposited on sapphire substrate was investigated in the temperature range from ambient to 106 degrees C. The evaporation process and the thermal mapping on the air/liquid interface of the droplets were recorded by side-view CCD camera and top-view infrared camera respectively. Graphene oxide (GO, 0.25 mg/ml), PEGylated reduced graphene oxide (rGO-PEG, 0.25 and 0.8 mg/ml) nanofluids, and water were tested at ambient conditions. The PEGylation was proved to improve the nanoparticles dispersion and the evaporation performance. Preparation without PEG-functionalization, or with an increased nanoparticle concentration, decreases the evaporation rate due to the poor dispersion of graphene nanoparticles and large accumulation. Because of extremely high intrinsic thermal conductivity, the addition of graphene nanoparticles in water is assumed to significantly improve evaporation rate, which is in accordance with thermal performance of water and 0.25 mg/ml rGO-PEG nanofluid at ambient temperature. However, on heated substrates, the evaporation rate of water is dramatically higher than 0.25 mg/ml rGO-PEG nanofluid at high substrate temperature despite the exceptional thermal conductivity of graphene nanofluid. The convective flows induced by thermal Marangoni effect are responsible for the increased evaporation rate of water at high substrate temperature and the application of graphene nanofluid to enhance the heat transfer efficiency is more effective at low substrate temperature. The calculations of thermal Marangoni number show that thermal Marangoni effect is always more important on water droplet evaporation than that of rGO-PEG nanofluid.