A gel–sol transition phenomenon of oxidation multi-walled carbon nanotubes–glycerol nanofluids induced by polyvinyl alcohol

Oxidation multi-walled carbon nanotubes (O-MWCNTs)–glycerol nanofluids were prepared, either with or without any polyvinyl alcohol (PVA), and their thermal conductivity (TC), stability, fluidity, and rheological properties were investigated. The results demonstrated that the TC enhancements of the nanofluids were almost linear with the nanotube concentration but showed little dependence on temperature. The largest TC enhancement was up to 21.0% when the volume concentration was only 1.4 vol%. The nanofluids exhibited high stability for more than 2 months and a very interesting gel–sol transition phenomenon induced by a low amount of PVA (about 0.06 wt%), which can be observed by an inverted and side tube method and supported by the rheological data. As the fluidity loss of carbon nanotube-based nanofluids has long been one of the major obstacles for their potential applications in energy transfer technologies, the novel gel–sol transition phenomenon introducing the fluidity recovery has great significance in both academic research and practical application fields of the nanofluids. Furthermore, a reasonably stable and disperse mechanism was also proposed to explain this phenomenon.