Nanoparticle mixing through rapid expansion of high pressure and supercritical suspensions

Mixing of binary mixtures of nanopowders afforded by rapid expansion of high pressure and supercritical suspensions (REHPS) is investigated to examine the roles of two previously reported deagglomeration mechanisms. The quality of mixing was characterized through intensity and scale of segregation using concentration data obtained through energy dispersive X-ray spectroscopy; the corresponding deagglomeration was quantified using differential mobility and image analyses in conjunction with electron microscopy. Increasing the pressure from which expansion was carried out, and decreasing the nozzle diameter led to improved deagglomeration. However, increased pressure alone did not influence the mixture quality, which was found to also depend on the scale of mixedness of the constituents before transport through the nozzle, establishing that the REHPS mixing is significantly improved by improving the quality of the premix. The scale of segregation correlated with the size of the most energetic eddies present during flow through the nozzle, both of which increased with nozzle diameter, corroborating the importance of previously reported shear-induced deagglomeration mechanism. Finally, REHPS was also shown to be capable of deagglomerating carbon nanotube bundles and mix them well with alumina, silica, and titania at submicron scale.