In the present study a comprehensive review on rheological characteristics of nanofluids for their advanced heat transfer applications has been conducted and presented. The present article critically summarizes the recent research developments regarding the theoretical and experimental investigations about viscosity of different nanofluids. In addition, different reasonably attractive theoretical models and experimental correlations are explored and well discussed. Moreover, the current study

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... alyzes several factors those strongly influencing viscosity of nanofluids include solid volume fraction, temperature, particle size, particle shape, different base fluids, surfactants addition, ultrasonication, nanoclustering and pH value. Important theoretical and experimental results from many researchers and predictions from a number of viscosity models are compared and discussed with appropriate justification. Most results reveal that the viscosity of nanofluid upsurges due to an increase in particle concentration while that belittles with diminishing temperature. Augmentation of nano-additives size leads to decreasing/increasing of nanofluid fluid viscosity. For the most nanofluids, Newtonian behavior is observed for low volume fractions, shear rates, concentrations and viscosity while non-Newtonian behavior is visualized for high volume fractions, shear rates, concentrations and viscosity. Nanofluids used carbon nanotubes are almost non-Newtonian in nature while nanofluids not involving carbon nanotubes are mostly Newtonian. Finally, the research challenges and needs in this important area of nanofluids are also highlighted.