## 3 Results and Discussion
### Influence of Aging Time on Microfluidized Nanoemulsions  
Figure 3a-e shows the influence of aging time on droplet size distributions for emulsions with HLB numbers of 11, 12, 13, 14 and 15, respectively. All subfigures except 3b show an increase in droplet size with aging time. However, while in Figure 3a DSDs are shifted to the right along the \(X\)-axis, Figure 3c-e show increased second-peak heights and lower first-peak heights, indicating smaller droplets. These results point to different destabilization processes [25; 26]. Specifically, the multimodal final distributions shown in Figure 3c-e are related to the phenomenon of coalescence [27]. However, Ostwald ripening is the predominant mechanism illustrated in Figure 3a because there is a shift along the \(x\)-axis and no second peak is evident [28]. Hence, we may say that the HLB number is a determining factor not only for droplet size distribution in emulsions after preparation, but also for the predominant destabilization mechanism which they undergo. In addition, those emulsions that exhibit bimodal distributions just after preparation due to recoalescence also undergo coalescence with aging time. Hence, the HLB number can induce one of two \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**HLB** & **11** & **12** & **13** & **14** & **15** \\ \hline D\({}_{3,2}\) (\(\mu\)m) & 0.224 \(\pm\) 0.010 & 0.203 \(\pm\) 0.012 & 0.257 \(\pm\) 0.034 & 0.246 \(\pm\) 0.013 & 0.341 \(\pm\) 0.011 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Sauter diameter values for emulsions formulated with 5 wt% lemon oil as a function of surfactant-mixture HLBs. Figure 2: Flow behavior of emulsions formulated with lemon oil as a function of surfactant-mixture HLBs. destabilization mechanisms due to the initial DSD produced. However, no change was observed in the emulsion prepared with an HLB of 12 (Figure 3b). Indeed, the emulsion with an HLB number of 12 (monomodal DSD after preparation) appeared to be stable with aging time in terms of droplet size, while the other systems exhibited destabilization. To gain a deeper insight into physical stability, backscattering (BS) measurements for the emulsion with an HLB number of 11 are shown in Figure 4. Decreases in BS can be observed, particularly in the lower and higher zones of the measuring cell. These are related to a clarification process in the lower zone and to the migration of droplets in the higher zone. These observations clearly indicate the destabilization mechanism of cream, which is promoted by the low viscosity value exhibited by the sample. This is confirmed by the Newtonian behavior of the systems developed, which indicate the total lack of structure of the sample. In addition, there is little decrease in BS in the middle zone of the measuring cell. This is related to an increase in droplet size with aging time, which is corroborated by the laser diffraction results presented above. A similar tendency in BS was observed for the other emulsions developed, but to lesser or greater extents. Figure 3: Droplet size distributions for emulsions with HLBs of (**a**) 11, (**b**) 12, (**c**) 13, (**d**) 14 and (**e**) 15 as a function of aging time. Finally, in order to characterize physical stability, and to quantify the destabilization processes affecting these nanoemulsions (creaming, Ostwald ripening or coalescence), we calculated the Turbiscan Stability Index using Equation (3) in the Materials and Methods section above. Figure 5 illustrates the Turbiscan Stability Index (TSI) for increasing aging times as a function of the HLB number. It is important to note that every destabilization mechanism contributes to the TSI, i.e., not only the growth of droplets but also creaming. Hence, the TSI values are a method to quantify all of the destabilization mechanisms involved in the dispersed systems. Interestingly, emulsions with an HLB number higher than 12, which exhibited coalescence, had higher TSI values than emulsions with HLB numbers of 12 or 11.