Experimental investigation and mechanism analysis: Effect of nanoparticle size on viscosity of nanofluids

Viscosity is important for evaluating thermal properties of nanofluids and easily influenced by many factors such as solid concentration, temperature, size & shape of nanoparticle. As so far, among all of them, the effect of particle size on viscosity is still unclear. For such reason, a series of experiments were performed by dispersing two different nanoparticles (Al2O3 and ZnO) respectively into engine oil in different particle size (20–100 nm) with volume fraction ranging from 1% to 15%. For the particle size and volume fraction investigated, viscosity is found to be different functions of particle size under different background concentration of nanoparticles; In addition, ANOVA analysis provides a comprehensive data interpretation for the effect of all factors contributing to viscosity. Moreover, mechanism analysis on particle size effect, which relies on DLVO theory and SEM microstructure of the samples was made. Consequently, the dependence of particle size effect on background concentration of nanoparticles was proposed and three corresponding stages of downtrend, constant trend and uptrend (viscosity varies with the increase of particle size) were presented. •Viscosity was found to be different functions of particle size under different particle concentration.•Distinct interactive effect of particle size & volume fraction was verified by ANOVA.•Internal mechanism of particle size effect on viscosity was revealed by DLVO theory.