Biomass-derived carbon nanoparticles yield stable bio-nanofluids with high zeta potential, enhancing thermal conductivity for efficient heat transfer in engineering applications.

Ensuring the long-term stability of nanofluids (NFs) remains a challenge due to nanoparticle aggregation, precipitation, and poor dispersion. Zeta potential (ZP) plays a crucial role in preventing agglomeration and enhancing stability. This study investigates, for the first time, the combined effect of stability and thermal conductivity (TC) enhancement in nanofluids based on biomass-derived carbon nanospheres (CNSs). CNSs synthesized from eight different biowaste sources exhibited ZP values ranging from −17.0 to −45.6 mV, influencing dispersion and fluid behavior. These NFs demonstrated exceptional stability for up to 40 days without surfactants and achieved a TC enhancement of up to 111.8%. The research also explores the influence of ZP on TC, dynamic viscosity (V), and thermal diffusivity. The NFs displayed shear-thinning, non-Newtonian behavior, with viscosity values depending on CNS concentration, reaching 0.0000000302 Pa·s. The effect of pH (3–12) on stability and TC revealed maximum performance at pH 8, while optimal TC enhancement was achieved at 0.1 wt% CNS concentration. This study bridges the gap between laboratory research and industrial applications, offering sustainable, low-cost, and high-efficiency coolant solutions for the automotive sector. It supports seven Sustainable Development Goals (SDGs) through an innovative waste-to-wealth approach.