Molar Selleck Wortmannin excess volumes vs. molar fraction for different EG nanofluids at 303.15 K and 20 MPa. Filled circle, A-TiO2/EG; filled triangle, R-TiO2/EG; empty triangle, Fe3O4/EG [38]; empty diamond, Fe2O3/EG [38]; empty circle, (48-nm ZnO)/EG [39]; empty square, (4.6-nm ZnO)/EG [39]. Rheological behavior As pointed out, only a reduced number of studies about the rheological behavior of nanofluids can be found in the literature, and there are inconsistencies such as Newtonian and non-Newtonian behaviors reported for the same nanofluid as well as discrepancies in the effects of temperature, particle size, and shape, and high shear viscosity values [40–44]. In this context,

a key issue is to obtain nanofluid structural information, and one of the feasible methods is through detailed rheological analyses [45]. In this work, two types of studies have been carried out. Viscosity as a function of shear rate, the so-called flow curve, was determined for both nanofluids at 303.15 K and at five different mass concentrations (5, 10, 15, 20, and 25 wt.%). LY333531 The applied torques started from 0.1 μNm, covering

shear rate ranges from 0.1 to 1,000 s−1. Figure 6a,b shows these flow curves for both nanofluids at different concentrations. Unlike the base fluid, both sets of nanofluids present a clear shear thinning (pseudoplastic) non-Newtonian behavior. In the lowest shear rate region, Newtonian plateaus are easily identified as the concentration rises. This non-Newtonian behavior opposes that reported previously by Chen et al. [14] that studied EG-based nanofluids containing 0.5 to 8.0 wt.% spherical TiO2 nanoparticles. Chen et al. [14] affirmed that a Newtonian behavior is found at a shear rate higher than 0.05 s−1. It should be taken into account

that our viscosity results for Newtonian EG agree with those of Chen et al. [14] within an average deviation of 1.5% [32]. The controversies found in the literature either on rheological studies indicate that the specific properties of the nanoparticles such as shape, structure, and size, and the interaction between the base liquid and nanoparticles can play an essential role in determining the rheological behavior of nanofluids. However, in this case, the main reasons of the different rheological behavior on TiO2/EG nanofluids may be attributed to the following: (1) the range of nanoparticle concentration studied by Chen et al. [14] (<8 wt.%) is lower than those analyzed in this work (<25 wt.%), (2) the range of shear stress studied in this work covers a wider area, and it is here where shear thinning appears, (3) the minimum shear rate which the equipment can reach is decisive to determine the first Newtonian plateau, especially at low nanoparticle concentration, and finally (4) the different stability and aggregation of particles affect flow conditions because the effective mass concentration can be higher than the actual solid mass. Figure 6 Viscosity ( η ) vs. shear rate ( ) of EG/TiO 2 nanofluids at different concentrations.