Synthesis, stability, transport properties, and surface wettability of reduced graphene oxide/water nanofluids

In this study, reduced graphene oxide (rGO) is synthesized from graphite using modified Hummer and chemical reduction methods. Various characterizations are done using X-ray diffraction, Raman's spectra, Fourier transform infrared, scanning electron microscopy and atomic force microscopy. Different concentrations of 0.01, 0.1, and 0.3 g/l of rGO/water nanofluids are then prepared by ultra sonic homogenizer and probe sonicator. Dynamic light scattering technique is used to identify the size of rGO flakes in DI water. The thermal conductivity, viscosity, and surface tension of rGO/water nanofluids reveal their dependency on concentrations and temperature. Due to the improved dispersion stability as evident from zeta potential, the thermal conductivity of 0.01 and 0.1 g/l concentrations exhibits negligible change whereas 0.3 g/l shows a minimal change for a period of five days. The enhancement in thermal conductivity of 0.3 g/l of rGO/water nanofluid at 75 °C is 10%. The rGO/water nanofluids exhibit Newtonian behavior at higher shear rates due to the weakening of intermolecular interactions. The enhancement in surface tension is mainly due to the increase in surface energy by the accumulation of rGO flakes at the liquid-gas interface. Studies on wettability indicate an increase in contact angle with concentrations. Though it is not favorable as it reduces the contact between solid and liquid surface, many research works explain the enhanced boiling heat transfer mainly with porous layer rather than contact angle. Results show that the rGO/water nanofluid can be used as a suitable replacement for the conventional fluids in heat transfer applications.