Effect of particle size on SiO₂ nanofluid viscosity determined by a two-step method

According to review of the literature, the influence of nanoparticle diameter with irregular shapes on viscosity requires further research since there is no relation between particle size and nanofluid stability. In this study, SiO₂/EG–water-based nanofluid samples were prepared, and their viscosities were experimentally determined. SiO₂ nanoparticles had sizes of 7, 15, and 40 nm, and the base fluid was a 50% ethylene glycol and 50% water mixture. Nanofluid samples were prepared using a two-step technique. Viscosity change was measured every 10 °C from 20 to 60 °C. The maximum viscosity values were observed for 7, 15, and 40 nm particles over an entire concentration range. Considering all measurements, the highest viscosity increase was 60.51% for 3% SiO₂ (7 nm) at 60 °C, and the lowest viscosity change was 7.72% for 1% SiO₂ (40 nm) at 40 °C. The most stable sample of the current study was 1% SiO₂ (15 nm), and its Zeta potential was − 35.6 mV. Finally, a new empirical equation that included temperature, particle diameter, and concentration terms is suggested to predict dynamic viscosity, with R _adj ²  = 0.98. It was also compared with previous correlations.