Rheological behavior of carbon nanotube and graphite nanoparticle dispersions.

Dispersions containing nanoparticles (nanofluids) are mixtures with unique properties, and their transport properties depend on the three-dimensional network or microstructure of the nanoparticles, which can be affected by various factors including shear stress, particle loading, and temperature. In this research, we studied the rheological behaviors of dispersions containing two different carbon morphologies: multiwalled carbon nanotubes (rodlike nanoparticles with L/D = 30), and graphite particles (disklike nanoparticles with L/D = 0.025). All nanofluids showed shear thinning behavior in steady shear measurements and those containing nanotubes had lower power law indices than graphite dispersions. Shear stress broke down the microstructure network and oriented both rodlike and disklike nanoparticles in the dispersions. The presence of a modest amount of nanotubes in the graphite nanofluid affected the microstructure of the dispersion and caused a remarkable decrease in its power law index. Microstructures of nanofluids strongly depended on the dispersant chemistry used to stabilize the particles, and high temperature may cause dispersant failure. Mechanical methods for dispersing the particles affected the geometry of the nanoparticles and therefore the rheological properties of the nanofluids. In the creep recovery tests, the compliance of graphite nanofluids quickly returned to zero when the stress was removed, while nanotube dispersion with high nanotube loading showed an elastic response during recovery. These results suggest that the microstructure in the dispersions is affected by nanoparticle morphology, dispersant chemistry, and shear stress.