Effect of surfactants on the stability and thermophysical properties of Al₂O₃+TiO₂ hybrid nanofluids

This study investigates the influence of eight distinct surfactants on the stability, rheological characteristics, and thermophysical properties of hybrid nanofluids (NFs) containing Aluminum oxide (Al₂O₃) and Titanium dioxide (TiO₂) in a Water-Ethylene Glycol (EG) mixture (60:40). The objective and novelty of this research is to choose the best surfactant for long run applications in terms of stability, viscosity and thermal conductivity. The results reveal that polyvinylpyrrolidone (PVP) exhibited the highest stability among all surfactants, surpassing others in zeta potential and visual stability over 60 days. Additionally, all the nanofluids exhibited Newtonian behavior. Surfactant addition increased viscosity, with polyethylene glycol (PEG) showing the highest enhancement of 37.2 % at 30 °C. Thermal conductivity increased with temperature, maximum TC enhancement of 8.3 % at 80 °C was given by PEG and oleic acid (OA), but these surfactants were unstable to be used in any application. In conclusion, PVP emerged as the most suitable surfactant for Al₂O₃-TiO₂ hybrid nanofluids due to its exceptional stability, moderate viscosity, and TC enhancement. It enhanced the TC by 3.6% at 80°C and contributed 28.9% in viscosity increment at 30°C. This research contributes to the understanding of surfactant effects on nanofluid properties, particularly highlighting PVP's potential for long-term heat transfer applications.