3.5 Change in emulsion viscosity with time
The emulsion viscosity changes were calculated using the viscosity index, a ratio of apparent emulsion viscosities on day 7 to day 0 (Figure 8). A value of viscosity index less than one indicates that the viscosity decreased upon storage, which could be due to the water droplet networks breakdown and lowering of emulsion stability against phase separation. All CO-emulsions reported viscosity indices less than one, except the emulsions with S-CA, where they remained close to one (Figure 8A). Conversely, all MO-emulsions with additives in the aqueous phase reported a viscosity index above one, except the S-CA emulsions, which can strengthen the water droplet network structure with time, leading to higher stability of MO-emulsions.
With LMP, all CO-emulsions showed viscosity indexes lower than one, except by LMP2-Ca0.045, which had the highest viscosity index (1.48 ± 0.02), which could be associated with the strongest network formation after storage (Figure 7B). On the contrary, all MO emulsions with LMP demonstrated a viscosity index very close to one (Figure 7B). Overall, the MO bulk phase might allow more interaction of the aqueous phase additives with GMO at the interface due to the less interaction of GMO with MO hydrocarbon. This way, higher droplet stability and a more robust droplet network could be developed during storage, leading to better viscosity protection for MO emulsions than CO-emulsions.
3.6 Emulsion viscoelasticity
All the emulsions reported higher storage (G’) than loss (G”) moduli in the low-strain regime (Figures 9 and 10). No emulsions showed true linear viscoelastic region (LVR) except fresh S0 (Figures 9 A1 day 0), indicating weak gels. All emulsions showed a steady decline in G’ with an increase in strain, also indicating a weak gel structure.
After storage, all CO-emulsions reported lower range G’ than the fresh emulsions (Figure 9, A2), implying that the initial structure on day 0 has diminished in strength, similar to the lower viscosity index reported in Figure 8. Most CO-emulsions showed on day 0 showed a peak in G” at the crossover, which indicates relaxation in droplet network structure, leading to gel breakdown. However, such peak in G” mostly disappeared after 7-das storage, except for S-CA (both concentrations), indicating loss of network structure with time. All MO-emulsions (Figure 9, B) exhibited a faster drop in G’ with strain and lower crossover stain than CO-emulsions, which could explain their higher sedimentation velocity under accelerated gravitational force reported in Figure 7. After 7 days, most MO-emulsions, except S-CA, showed unchanged or some increase in G’ (Figure 9, B1), which was also similar to the viscosity index data (Figure 8)
Unlike CO-emulsions with S and AA or CA, no peak in G” was observed with Ca and LMP. After 7 days, all CO-emulsions showed mostly unchanged weak viscoelastic behaviour (Figure 10, A2). Similar to what was observed in Figure 9, in Figure 10, MO emulsion showed a faster drop in G’ as a function of strain and a crossover at a lower strain compared to CO-emulsions. After storage, MO-emulsions with Ca and LMP1.5 demonstrated an increase in gel strength (Figure 10, B2). Overall, CO and MO emulsions’ viscoelastic behaviour showed different levels of weak gel strength, with MO emulsions appearing weaker than the CO emulsions. The presence of S, AA, CA or their combination in CO-emulsion created gels that can withstand more shear force than the CO-emulsions with Ca and LMP.