Experimental investigation of thermophysical properties of Al2O3 TiO2 CuO Fe3O4 water ethylene glycol quadri hybrid nanofluid.

Multi-component nanofluids have attracted increasing attention for advanced heat transfer and energy systems; however, experimental data on oxide-based quadri-hybrid nanofluids, particularly regarding their coupled rheological and interfacial behavior, remain scarce. In this study, the thermophysical properties of a novel Al₂O₃-TiO₂-CuO-Fe₃O₄/water-EG quadri-hybrid nanofluid were experimentally investigated. The suspension was stabilized using oleic acid and sodium dodecyl sulfonate, and the nanoparticles were characterized by SEM and XRD analyses. Dynamic viscosity, electrical conductivity, and surface tension were measured in the temperature range of 298-340 K at nanoparticle volume fractions of 0.1-1%. The electrical conductivity increased monotonically with concentration, reaching a maximum enhancement of approximately 170% at 1% loading. Surface tension exhibited a non-monotonic U-shaped trend, with a maximum reduction of 29% at 0.1% concentration. Rheological analysis revealed two distinct regimes: a low-shear Newtonian plateau and a high-shear shear-thickening behavior, where viscosity increased by nearly one order of magnitude under combined high-temperature and high-shear conditions. To model this nonlinear behavior, a high-accuracy RBF-based correlation was developed, reproducing the experimental viscosity data with a deviation below 3%. The results highlight the strong coupling between temperature, shear rate, and nanoparticle concentration, providing new insights into the design of quadri-hybrid nanofluids with tailored flow and interfacial properties.