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.