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Experimental studies of the sedimentation, stability and thermal conductivity of two different nanofluids
* 1 , 1, 2 , 1, 3 , 4 , 3, 5 , 3 , 2 , 1, 4
1  Metrics, Mechanical Engineering Department, University of Minho, Campus de Azurém, 4800-058 Guimarães, Portugal.
2  International Iberian Nanotechnology Laboratory, Braga 4715-330, Portugal.
3  IN+, Center for Innovation, Technology and Policy Research, Instituto Superior Técnico, Universidade de Lisboa. Av. Rovisco Pais, 1049-001 Lisboa, Portugal.
4  CEFT, Faculdade de Engenharia da Universidade do Porto (FEUP), R. Dr. Roberto Frias, 4200-465 Porto, Portugal
5  CINAMIL, Military Academy Research Center, Portuguese Military Academy, R. Gomes Freire, 203, 1169-203 Lisbon, Portugal


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 Al2O3nanoparticles (NPs) with 50 nm of diameter and the second is a novel NF formed by poly (acrylic acid)-coated iron oxide NPs (Fe3O4@PAA) with ~10 nm of 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.

Keywords: Nanofluids; zeta potential; stability; thermal conductivity