Nanofluids are suspension of metallic or metal-oxide solid nanoparticles with size varying generally from 1 to 100 nm dispersed in heat transfer liquids such as water, ethylene glycol and engine oil. It has been found that important heat transfer enhancement may be achieved using nanofluids instead of conventional fluids. This is of major importance for electronics and microelectronics where liquids cooling systems are necessary and miniaturizing is needed. In this regard, determination of the nanofluids viscosity is essential to establishing adequate pumping power as well as the convective heat transfer coefficient, because the prandtl number and Reynolds number are functions of viscosity. The conventional correlations such as Einstein model [1] consider only the nanoparticles volume concentration on nanofluid viscosity prediction. Nguyen et al. [2, 3] during their experimental investigation studied the influence of temperature and the particle size on dynamic viscosities of water/ Al2O3 and water/CuO suspension. In the present study the various models for nanofluids viscosity determination is studied and the advantage and disadvantage of each models discussed.

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