Figure 5 demonstrates the changes of zeta potential for GNP 750 suspensions as a function of pH values. In the GNP suspension, while using water as a base fluid, the GNPs tend to be positively charged before pH 3 and negatively charged within the entire pH ranges after pH 3. At approximately pH 10, the absolute value of zeta potential will be at maximum, while the maximum excess is 50 mV. The nanofluids which have a measured zeta potential above +30 mV or below −30 mV are having good stability [29]. It implies that the force of electrostatic repulsion between GNPs is sufficient to get over the attraction force between particles. Higher electrostatic force may also cause to form much more
free particles by improving particle-particle distance, in order that the distance exceeds the hydrogen bonding range between particles and further decreases the chance of particle coagulation and settling. The pH value of prepared nanofluids SAHA HDAC molecular weight was measured at about pH 8 while zeta potential
value appears to be 31.8, 40.9, and 45.7 mV for GNPs at 300, 500, and 750 m2/g, respectively. The inclination is that the zeta potential values demonstrate an enhancement for higher specific surface areas KU-57788 datasheet of GNPs. This phenomenon suggests that the GNPs nanofluid with higher specific surface areas might have better stability. Figure 5 Zeta potential values of GNP (750 m 2 /g) nanofluids as a function of pH value. Rheological behavior of GNPs Viscosity of nanofluids is one of the most critical parameters, which determines the quality of heat transfer fluid. Similar to simple fluids, temperature is the main effective parameter on viscosity of nanofluids. As expected, distilled water exhibits a Newtonian behavior within the shear rate range investigated. The viscosity value of distilled water was 1.034, which closely matches with its theoretical values at 20°C. The relative deviation is less than 2.5%. This is of the same order of magnitude as the experimental uncertainty.
Figure 6 reports the viscosity at a high shear rate of 500/s for different concentrations and specific surface areas as a function of all tested temperatures. While nanofluids and base fluids are C59 clinical trial strongly dependent on temperature, it is also observed in Figure 6 that the viscosity was decreased for higher temperatures. This is expected due to the weakening of the interparticle and intermolecular adhesion forces, and similar trends have also been observed in almost all other varieties of nanofluids. It can be clearly seen that viscosity increased for higher concentrations of GNPs and that the viscosity of nanofluid improved by 44% compare to the viscosity of the base fluid for 0.1 wt.% of GNPs. This can be realized in such a way that once the concentration increases, the nanoparticles make an agglomeration within the suspension.