Assessment of long term stability of aqueous nanofluids using different experimental techniques

Abstract

The long term stability of nanofluids is of paramount importance for their practical technological applications. Though several experimental techniques are available for the assessment of stability of nanofluids, interpretation of results from these techniques is complex and hence often becomes erroneous. In this paper, the long term stability of three water based metal oxide nanofluids: α-Al2O3, TiO2 and γ-Al2O3 nanofluids- has been investigated using UV–visible spectroscopy, dynamic light scattering, zeta potential measurements, phase contrast microscopic and visual observation. The time-dependent changes in absorbance, hydrodynamic diameter and zeta potential results of α-Al2O3 and TiO2 nanofluids, showed significant aggregation of nanoparticles, suggesting poor nanofluid stability. On the contrary, γ-Al2O3 nanofluids showed a time independent behavior, suggesting a good stability without significant aggregation of particles. The phase contrast optical microscopy images confirm the macroscopic clustering (aggregates in micron scales) in α-Al2O3 and TiO2 nanofluids and an aggregate free γ-Al2O3 nanofluid. The visual observation showed a clear phase separation in α-Al2O3 and TiO2 nanofluids, especially at higher particle loading, while no phase separation is observed in γ-Al2O3 nanofluids. All five measurements independently confirm that γ-Al2O3 nanofluid is stable and α-Al2O3 and TiO2 nanofluids are unstable. These results provide the merits of each techniques and a guideline for interpreting the experimental results with regard to stability of nanofluids using different experimental techniques, along with their strengths and weakness.