Effect of temperature, nanoparticle concentration, and volume on the nucleation characteristics of graphene oxide nanofluids freezing

The effects of cooling temperature, nanofluid concentration, and droplet volume on the supercooling and nucleation behavior of graphene oxide (GO) nanofluid droplets were investigated. Instead of conventional droplet suspension methods, including acoustic or magnetic levitation, a thermocouple-based droplet suspension technique was adopted to eliminate the influence of external acoustic pressure or magnetic fields on droplet supercooling. Results indicate that the novel testing method effectively reduces the influence of various external factors on supercooling. Theoretical studies are conducted to calibrate the contact angle and surface tension to account for the effects of ambient temperature and nanofluid concentration within the classical nucleation theory. It is observed that the degree of supercooling increases as the cooling temperature decreases for both water and nanofluids. However, nanofluid droplets exhibit significantly lower supercooling compared to pure water. Specifically, the degree of supercooling decreases as the nanofluid concentration increases from 0 to 0.03 wt%, with a maximum reduction of 45.83%. The 0.03 wt% concentration is identified as the optimal concentration for supercooling suppression under the present apparatus parameters and tested operating range. In addition, supercooling decreases as droplet volume increases, while crystallization time is prolonged. Morphological observations reveal that droplets remained transparent during the supercooling stage. Upon reaching the nucleation temperature, transparency vanished instantaneously, followed by rapid crystallization characterized by internal bubble formation and volumetric expansion. The process concluded with complete freezing and the formation of a distinct protrusion at the apex.