Synergistically enhanced stability and thermal properties of oil-based nanofluids through multi-scale surface grafting of HDTMS on Al₂O₃ nanoparticles

While nanofluids act as highly efficient heat transfer media, insufficient stability restricts their practical use in specific thermal management scenarios. In this study, hexadecyl trimethoxy silane (HDTMS) was used as a surface modifier. It was chemically grafted onto nano-Al₂O₃ particles, producing HDTMS-functionalized Al₂O₃ particles and the corresponding oil-based immersion composite nanofluids. The influence of HDTMS on the suspension stability of oil-based nanofluids was systematically evaluated, along with a comprehensive analysis of their thermophysical properties. Furthermore, the binding mechanisms between nanoparticles and mineral oil, both pre- and post-modification, were elucidated at the microscopic level via molecular dynamics simulations. Results indicated that HDTMS modification successfully introduced long carbon chains onto the nanoparticle surfaces, imparting enhanced lipophilicity. The 0.4 wt% modified nanofluids (HT-O) maintained a transmittance of 59.89% after 30 days of static settling. At the microscopic scale, the interaction energy between HDTMS-Al₂O₃ and mineral oil EC-160 increased by 51.92% at 343 K, and the secondary peak in the radial distribution function markedly decreased to 26.27 with stabilized fluctuations, indicating that the grafted layer effectively suppressed particle aggregation and promoted dispersion uniformity through steric hindrance. This study first applied modified alumina nanofluids to the immersion thermal management of batteries, providing key technical insights for developing immersion cooling media that integrate high thermal conductivity, long-term stability, and electrical insulation for battery thermal management. Meanwhile, molecular dynamics simulation was employed to reveal the intrinsic molecular-level mechanism behind the enhanced stability of the nanofluids.