Synthesis, thermophysical behavior, and environmental implications of nanofluids: A comprehensive review

• Reviews nanofluid synthesis strategies and key thermophysical mechanisms. • Explains how nanolayer formation and Brownian motion enhance heat transfer. • Reports up to 48% higher conductivity and 123% improved heat transfer with HNFs. • Summarizes nanofluid use in CSP, PVT, geothermal, and electronic cooling systems. • Evaluates nanoparticle toxicity, LCA, and sustainable nanofluid development. . Nanofluids engineered colloidal suspensions containing nanoparticles uniformly dispersed in base fluids offer substantial advantages for heat transfer enhancement across a range of engineering systems. This review provides a comprehensive synthesis of recent developments in nanofluid preparation strategies, emphasizing the influence of nanoparticle characteristics on thermophysical behavior. Key mechanisms such as interfacial nanolayer formation, Brownian motion, and aggregation are examined for their roles in modifying viscosity, thermal conductivity, and heat transfer performance. Reported studies show that hybrid nanofluids (HNFs) can achieve up to 48% higher thermal conductivity, 123% greater convective heat transfer coefficients, and 34% improvement in system energy efficiency compared with conventional fluids. However, these benefits are often accompanied by increased viscosity and higher pumping power demands. The review also discusses applications in heat exchangers, electronic cooling, concentrated solar power (CSP), photovoltaic/thermal (PVT) systems, and geothermal energy recovery. Environmental considerations, including nanoparticle toxicity, life-cycle assessment, and sustainable synthesis, are critically evaluated. This work integrates recent progress in synthesis, performance optimization, and environmental safety, providing direction for developing next-generation, eco-efficient nanofluids for advanced thermal management.