Stability and rheological properties of hybrid γ-Al2O3 nanofluids with cationic polyelectrolyte additives

Abstract

In the present work, the stability and rheological properties of a water-based hybridaluminum oxide (γ-Al2O3) nanofluid containing polyethylenimine (PEI), a cationic polyelectrolyte additive, were investigated. The γ-Al2O3 nanoparticles used in the suspension was characterized using X-ray diffraction, TEM and FTIR analysis. Rheological results confirmed a non-Newtonian behavior in the shear range 0.01 s−1–1000 s−1 for hybrid γ-Al2O3 nanofluid with PEI as an additive. Interestingly, these nanofluids form gel or paste above 0.5 wt.% of PEI due to strong interaction among alumina nanoparticles as a result of surface charge screening brought about by protonation of PEI. Nearly temperature independent rheological behavior was observed in the temperature range 20–60 °C. The oscillatory rheological measurements indicate the elastic nature of the hybrid nanofluids at low strains due to the extended network structure formed by alumina nanoparticles. The preferred mode of interaction between PEI and alumina, different conformations and orientations were studied using DFT calculation, which shows that one NH bond from each NH2 groups interact with AlO bond. The AlO bond length of alumina was found to diminish upon interaction with PEI (i.e. AlO bond length of bare alumina is 1.746 Å and is reduced to 1.654 Å in the PEI-alumina complex) and the NH bonds of PEI is stretched to a bond length of 1.040 Å from 1.018 Å, indicating the interaction of NH bonds with AlO bond. The calculated interaction energy of PEI and alumina, is found to be −8.08 kJ/mol. These results offer possibilities of strongly altering the rheological behavior and stability of hybrid nanofluids with a very small amount of appropriate polyelectrolyte.