Effect of aggregation on the viscosity of copper oxide–gear oil nanofluids

Results on viscosity of the stable nanofluids, prepared by dispersing 40 nm diameter spherical CuO nanoparticles in gear oil are presented. Viscosity of the studied nanofluids displays strong dependence both on CuO loading in the base fluid, as well as, on temperature between 10 and 80 °C. Presence of aggregated CuO nanoparticles in the fluid, with average cluster size ∼7 times the primary diameter of CuO nanoparticles, have been confirmed by DLS data. Viscosity of the nanofluids is enhanced by ∼3 times of the base fluid with CuO volume fraction of 0.025, while it decreases significantly with the rise of temperature. Newtonian behavior of the gear oil changes to non-Newtonian with increase of CuO loading. Shear thinning is observed for nanofluids containing CuO volume fraction >0.005. CuO volume fraction dependence of the viscosity of CuO–gear oil nanofluids is predicted well using the modified Krieger–Dougherty equation derived taking into account the aggregation mechanism. Temperature variation of the nanofluid viscosity agrees very well with the modified Andrade equation, reported by Chen et al.