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

In the present work, the stability and rheological properties of a water-based hybridaluminum oxide (gamma-Al2O3) nanofluid containing polyethylenimine (PEI), a cationic polyelectrolyte additive, were investigated. The gamma-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 gamma-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 degrees 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 N-H bond from each NH2 groups interact with Al-O bond. The Al-O bond length of alumina was found to diminish upon interaction with PEI (i.e. Al-O bond length of bare alumina is 1.746 angstrom and is reduced to 1.654 angstrom in the PEI-alumina complex) and the N-H bonds of PEI is stretched to a bond length of 1.040 angstrom from 1.018 angstrom, indicating the interaction of N-H bonds with Al-O 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.