Experimental Studies of the Sedimentation, Stability and Thermal Conductivity of Two Different Nanofluids ^†

Fluids containing nanometer-sized particles (nanofluids, NFs) are potential candidates to improve the performance and efficiency of several thermal devices at micro- and macro-scale levels. However, the problem of sedimentation and instability of these colloidal dispersions has been the biggest obstacle for industrial-scale applications. In this work, two different NFs were tested using distilled water (DI-Water) as the base fluid. The first is a traditional NF formed by Al₂O₃ nanoparticles (NPs) with 50 nm diameter, and the second is a novel NF formed by poly (acrylic acid)-coated iron oxide NPs (Fe₃O₄@PAA) with ~10 nm diameter, obtained through a hydrothermal synthesis process. The main objective of this study was to evaluate the colloidal stability of these NFs over time using different volume fractions and compare it with DI-Water. Results involving sedimentation studies and zeta potential measurements showed that the proposed Fe₃O₄@PAA NF presents a higher colloidal stability compared to that of the Al₂O₃ NF. Additionally, thermal conductivity measurements were performed in both Fe₃O₄@PAA and Al₂O₃ NFs at different NP concentrations, using the transient plane source technique. Results showed higher thermal conductivity values for the Fe₃O₄@PAA NFs compared to those of Al₂O₃ NFs. However, a linear enhancement of thermal conductivity with increasing NPs concentration was observed for the Al₂O₃ NF over the whole range of NP concentrations tested, whereas two different regimes were observed for the Fe₃O₄@PAA NF.