## 4 Results and Discussion
### Discussion  
The starch nanoparticles stabilized Pickering O/W emulsions investigated in this work were relatively unstable as compared with the non-ionic surfactant stabilized O/W emulsions. If left unstirred or unagitated, the Pickering emulsions readily coalesce and separate into oil and water phases. Therefore, only in-situ measurements of viscosity under agitation conditions are reliable for Pickering emulsions. The surfactant-stabilized O/W emulsions, on the other hand, were stable with respect to coalescence and phase separation. Consequently, both in-situ and off-line measurements of viscosity are reliable and consistent.  
When comparison is made at the same oil concentration, the Pickering O/W emulsions were found to be more viscous that the surfactant-stabilized O/W emulsions. The consistency index \(K\) of Pickering emulsions is significantly higher than that of surfactant-stabilized emulsions, especially at high concentrations of dispersed-phase. Figure 11 shows the comparison of the relative viscosities of Pickering emulsions and surfactant-stabilized emulsions at a fixed rpm of 30 (shear rate of 36.78 s\({}^{-1}\)). Clearly, the Pickering emulsions were significantly more viscous than the surfactant-stabilized emulsions. The relative viscosity of emulsion, defined as the emulsion viscosity to matrix fluid viscosity, was consistently higher for the Pickering emulsion as compared with the surfactant-stabilized emulsion at the same volume fraction of oil.  
It is interesting to note the Pickering O/W emulsions were more viscous than the surfactant-stabilized O/W emulsions despite having a much larger average droplet size. As noted earlier, the average droplet size of Pickering emulsions was about six to seven times larger than that of the surfactant-stabilized emulsions. The effect of droplet size on the rheology of emulsions has been studied in detailed by [PERSON] (1998); [PERSON] (1999); [PERSON] (2000); [PERSON] (2001). The influence of particle size on the rheology of suspensions has also been investigated by many authors [PERSON] (1999); [PERSON] (2001); [PERSON] (2001); [PERSON] (2001). In general, the dispersions (emulsions or suspensions) become more viscous upon decreasing the size of the inclusions, especially at high concentrations of dispersed phase. In the present work, we observed the opposite behavior in that the viscosity of emulsions of larger droplet size was higher than the viscosity of emulsions of smaller droplet size. There are at least three reasons for the high viscosity of large droplet size Pickering emulsions investigated in this work: 1) The viscosity of the matrix (continuous-phase) of Pickering emulsions was higher than that of the surfactant-stabilized emulsions due to the presence of starch nanoparticles  
Figure 10: Images of fresh and aged surfactant-stabilized (left bottle in each box) and nanoparticles-stabilized (right bottle in each box) O/W emulsions.  
in the matrix phase. However, the higher viscosity of the matrix phase alone cannot explain the high viscosity of the Pickering emulsions, as the data plotted on a relative viscosity basis (see Figure 11) also show Pickering emulsions to be more viscous; 2) the effective volume fraction of Pickering emulsions was higher than that of the surfactant-stabilized emulsions due to the presence of a monolayer or multi-layers of starch nanoparticles at the surface of the droplets [22]; and 3) the transmission of stresses from the continuous phase to the dispersed-phase (oil) was inhibited due to the presence of starch nanoparticles at the surface of the droplets. Consequently, droplets behaved more like rigid particles without any internal circulation of fluid within the droplets.