Effect of particle concentration on the flocculation and sedimentation of unstable Al2O3-SiO2/water hybrid nanofluid

Hybrid nanofluids are considered to have better application prospects because of their superior thermal properties than traditional heat transfer media and single nanofluids. However, the instability problem caused by particles settling in fluids has always been hard to solve. Al2O3 and SiO2 particles with average particle sizes of 30 nm and 20 nm, respectively, were used to create Al2O3-SiO2/water hybrid nanofluids in this study. Following that, sedimentation observation, zeta potential, particle size distribution, viscosity, and absorbance were used to assess the sedimentation properties of the unstable hybrid nanofluids at various particle concentrations. The results showed that the flocculation and sedimentation of particles were cyclical and the sedimentation rate was positively correlated with the concentration. In addition, further analysis revealed that the ratios of SiO2 and Al2O3 particle concentrations in the supernatant of 0.05 wt%, 0.10 wt%, and 0.15 wt% nanofluids converged to 1:1, 1:1, and 1:3.5, respectively. The analyses indicated that at low concentrations, Al2O3 and SiO2 particles will flocculate with each other and settle together. When the concentration is higher, the Al2O3 nanoparticles with larger particle sizes will flocculate and settle first. This paper elucidates the sedimentation characteristics and mechanism of unstable hybrid nanofluids, which is expected to provide more references for practical applications.