The effect of aggregation on the viscosity was experimentally investigated in the graphite water-based nanofluids. The shear thinning non-Newtonian behavior is observed in the effective viscosity measurement with the shear rate in the nanofluids. On the basis of the best fitting of the experimental data, the viscosities at zero shear rate and at infinite shear rate are determined. As a function of the particle volume concentration and holding period of the nanofluids after preparation, the effective viscosity the increases. The maximum enhancement of the viscosity at infinite rate of shear is over 24 times higher in the 4 vol % 3-day nanofluids compared with the base fluid. A scanning electron microscope was applied to reveal the morphology of aggregated nanoparticles in the nanofluids. The large and irregular aggregation of the particles is found if the nanofluids were kept for three days. Experimental results indicate that the abnormal effective viscosity increase of nanofluids is related to the nanoparticle aggregation in their base fluids.

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