## 3 Results and Discussion
### Rheological and Thermophysical Properties  
The flow curves of the different-loaded nanofluids at two temperatures are presented in Figure 2. It evidences a Newtonian nature for all samples The dynamic viscosities of the samples were considered as the mean values in the 10 to 1000 s\({}^{-1}\) shear rate range. The experimental dynamic viscosities for the base fluid in the 283.15 to 323.15 K present deviations lower than 2.5% with respect to literature data [49; 50; 51; 52; 53]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Sample** & **pH** \\ \hline W & 6.2 \\
0.005 wt\% S-GnP/W & 4.3 \\
0.05 wt\% S-GnP/W & 3.3 \\
0.005 wt\% P-GnP/W & 5.2 \\
0.05 wt\% P-GnP/W & 5.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: pH values of the different-loaded nanofluids at room temperature. Figure 1: Size distribution by number for the S-GnP (**a**) and P-GnP (**b**) dispersions in water. Table 3 shows the dynamic viscosity values for water and the S-GnP and P-GnP nanofluids, evidencing a 59% decrease with the increasing temperature in the 40 K analysed range for all samples. This same temperature behaviour for the base fluid and the different nanofluids was also reported previously [42; 43; 54]. The increasing temperature over a fluid causes a decrease of the cohesive intermolecular forces among molecules, producing a viscosity reduction [43; 55]. The dispersion of nanoadditives does not alter these relative variations at the analysed concentrations. Increases with the rising nanoadditive concentration are observed for both nanofluid sets. Viscosity is usually defined as the internal friction of fluids caused by the molecular cohesion that provokes resistance to flow [56]. The dispersion of nanoadditives in a fluid leads to a higher level of friction, which implies a greater resistance to flow and consequently higher viscosities. These slight increases for the analysed concentrations show maximum values of 10% and 4.9% for S-GnP and P-GnP dispersions, respectively. Note that the reported increases for S-GnP are twice as high as those for P-GnP. Figure 3 shows the dynamic viscosity increases in relation to the base fluid for the different-loaded nanofluids. These increases appear almost temperature-independent in the whole analysed temperature range, as we also previously pointed out for other samples [43]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & & \multicolumn{3}{c}{\(\boldsymbol{\eta}\) [mPa-s]} \\ \cline{2-6}
**Temperature [K]** & **W** & **0.005 wt\%** & **0.05 wt\%** & **0.005 wt\%** & **0.05 wt\%** \\  & & **S-GnP/W** & **S-GnP/W** & **P-GnP/W** & **P-GnP/W** \\ \hline
283.15 & 1.298 & 1.350 & 1.429 & 1.315 & 1.356 \\
293.15 & 0.986 & 1.014 & 1.071 & 1.002 & 1.031 \\
303.15 & 0.787 & 0.821 & 0.860 & 0.796 & 0.816 \\
313.15 & 0.639 & 0.662 & 0.696 & 0.647 & 0.664 \\
323.15 & 0.535 & 0.553 & 0.587 & 0.539 & 0.561 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Dynamic viscosity values for the different loaded S-GnP and P-GnP nanofluids at temperatures from 283.15 to 323.15 K.  
Figure 2: Flow curves for the different loaded S-GnP and P-GnP nanofluids in the 10 to 1000 s\({}^{-1}\) shear rate range at 283.15 K (blue) and 293.15 K (orange). The experimental thermal conductivities for all the designed nanofluids at 293.15 K practically reproduce the same value of the base fluid, 598 mW-m-1-K-1, with deviations below 0.3% in all cases, much lower than the experimental uncertainty of the measurements. Therefore, it can be affirmed that the selected low concentrations do not entail effective thermal conductivity variations with respect to the pure base fluid. Figure 4 shows the temperature dependence of density for all the samples, compared with that measured in the case of pure water. The experimental density values for the base fluid at temperatures from 283.15 to 323.15 K agree well with literature, maintaining deviations below 0.08% [51].