## 4 Results and Discussion  
### Rheological Behavior of Emulsions  
Figure 4 shows the transient plots of viscometer dial reading for surfactant-stabilized and nanoparticle-stabilized oil-in-water emulsions at different rpms. The plots indicate that the steady state was reached within 1 min. Also, the dial reading versus time data obtained from three separate runs at the same rpm (see Figure 4a) nearly overlapped with each other, indicating excellent reproducibility. The variations of temperature and electrical conductivity with the increase in the dispersed phase (oil) concentration of emulsion are shown in Figure 5 for both surfactant-stabilized and nanoparticle-stabilized emulsions. The temperature was nearly constant from one concentration to another, but the electrical conductivity of emulsion decreased with the increase in the nonconductive oil concentration of the emulsion as expected. No phase inversion of O/W emulsion to W/O emulsion was observed in surfactant-stabilized and nanoparticle-stabilized Pickering emulsions. Note that phase  
Figure 4: Transient plots of viscometer dial reading for (**a**) surfactant-stabilized and (**b**) nanoparticle-stabilized (Pickering) O/W emulsions at different rpms. inversion of emulsion is characterized by sharp changes in the electrical conductivity [12]. When an O/W emulsion inverts to W/O emulsion, a sharp drop in the electrical conductivity is expected as the nonconductive oil phase forms the continuous phase in W/O emulsion  
Figure 6 shows the rheograms of surfactant-stabilized and nanoparticle-stabilized O/W emulsions for different concentrations of dispersed-phase (oil). For the surfactant-stabilized O/W emulsions (Figure 6a), the data at low shear rates (\(\leq\)73.56 s\({}^{-1}\)) were obtained from the in-situ device (Brookfield UL adapter connected to a Brookfield model LVT viscometer), whereas the data at high shear rates were obtained using an off-line device (Fann co-axial cylinder viscometer). The data obtained at high shear rates using the off-line device are consistent with the data obtained at low shear rates using the in-situ device in that they fall on the same line. For the nanoparticle-stabilized Pickering O/W emulsions (Figure 6b), all the data were obtained using the in-situ device (Brookfield UL adapter) while maintaining the homogenizer mixing. These emulsions were not sufficiently stable to carry out the off-line measurements. Figure 5: Variations of temperature and electrical conductivity of (**a**) surfactant-stabilized and (**b**) nanoparticle-stabilized O/W emulsions with the increase in oil concentration. The rheograms are linear on a log-log scale, indicating that the emulsions followed the power-law behavior:  
\[\tau=K\dot{\gamma}^{n} \tag{3}\]  
where \(\tau\) is the shear stress, \(\dot{\gamma}\) is the shear rate, \(K\) is the consistency index, and \(n\) is the flow behavior index. At low concentrations of oil, the emulsions are Newtonian in nature as the flow behavior index \(n\) is unity. At high concentrations of oil, \(n<1\) indicates non-Newtonian pseudoplastic (shear-thinning) behavior. Figure 6: Rheograms of (**a**) surfactant-stabilized and (**b**) nanoparticle-stabilized O/W emulsions at different oil concentrations. Figure 7 shows the plots of consistency index (\(K\)) and flow behavior index (\(n\)) as functions of oil concentration for both surfactant-stabilized and nanoparticle-stabilized O/W emulsions. The consistency index \(K\) increased with the increase in oil concentration, whereas the flow behavior index \(n\) decreased with the increase in oil concentration. Thus, the O/W emulsions became more viscous and shear-thinning with the increase in oil concentration. The consistency index \(K\) rose slowly with the initial increase in oil concentration. A sharp increase in \(K\) occurred when the oil concentration was increased above about 40% by vol.