Controlling the macroscopic liquid-like behaviour of halloysite-based solvent-free nanofluids via a facile core pretreatment

Despite the focus of many studies on the preparation, characterization, and potential applications of solvent-free nanofluids based on diverse nanomaterials, fewer studies have focused on how to control the macroscopic liquid-like behaviour of solvent-free nanofluids. The present study prepared a halloysite-based solvent-free nanofluid and developed a facile method of tuning the fluidity of this nanofluid by preheating halloysite (Hal) at different temperatures. The effects of preheating on the structure and surface functional groups of halloysite and further on the fluidity of the obtained nanofluids were explored. Heating Hal at 110 °C resulted in little changes to its surface functional moieties, and the obtained nanofluid (Hal110-F) did not exhibit a markedly different fluidity compared with Hal-F at room temperature, though Hal-F and Hal110-F had different melting temperatures and exhibited different fluidities at a lower temperature. While for Hal heated at 700 °C, the corresponding nanofluid (Hal700-F) exhibited a lower fluidity than Hal-F and Hal110-F due to the remarkable calcination-induced changes in the species/amount of surface functional moieties of Hal700. This method for controlling the fluidity of the nanofluids via a facile core pretreatment was believed to be versatile for most, if not all, solvent-free nanofluids.