Challenges and difficulties in developing hybrid nanofluids and way forward

Hybrid nanofluids are a novel category of nanofluids, achieved by combining two or more nanoparticles consisting of metal-based or metal oxide particles in various base fluids, displaying exceptional improvement in thermophysical chArceristics such as thermal conductivity. Scholars and researchers have worked on the potential properties of hybrid nanofluids, such as thermal conductivity, viscosity, density, and heat transfer coefficient in recent years. Still, some critical problems may occur in the commercialization of nanofluids. Several key challenges such as foam formation, stability, high cost, increased friction factor, pressure drop, and pumping power; degradation of original properties; predicting models for thermophysical properties, safety, and environmental concerns; and selection of suitable hybrid materials are also discussed.