3.4 Average water droplet size
The water droplet size was calculated using image analysis. Only the
water droplets outside the network structure were analyzed for this
calculation. Thus, emulsions with LMP and Ca in CO and MO were not
reported because the water droplets were aggregated, forming a network
spanning the whole microstructure. It was assumed that the free water
droplet size would represent the entire emulsions for comparison
purposes. The control CO emulsions (S0) showed a high
d4,3 (9.2 ± 0.1
µm),
which was similar to CA0.5, followed by CA0.125 (8.2 ± 0.0 µm) and
S0.125 (7.9 ± 0.0 µm) (Figure 6A). The emulsions formed with AA showed
considerably lower d4,3 (6.0 ± 0.1 µm) at both
concentrations. The addition of S to either AA or CA showed the smallest
droplet size of all CO emulsions, which could be related to their higher
emulsion height fraction reported in Figure 2. All MO emulsions reported
a much smaller droplet size (about 7 times lower) than CO emulsions
(Figure 6B). The MO emulsions prepared with S0 and S0.125 showed a
non-significant difference in d4,3 (1.2 ± 0.03 µm).
However, the incorporation of AA and CA and their mixture with S
reported a significant reduction of droplet size (< 0.72 ±
0.01 µm). The smaller droplet size obtained in MO emulsions compared to
CO-emulsions can be a factor for their higher emulsion stability against
phase separation (Figure 1 and 2).
3.5 Accelerated
sedimentation velocity of emulsions
Water droplets’ sedimentation velocity under accelerated gravitation
(2325xg) was used to identify how fast the emulsions would phase
separate (Figure 7), which was used as an indicator of long-term
stability. The CO emulsions with S0 and S0.125 had high sedimentation
velocity under accelerated gravitation (on average, 9.8 ± 0.2
µm/s),
which could be due to the larger water droplets and coalescence under
centrifugal force (Figure 7A). There was a sharp decrease in
sedimentation velocity with the addition of AA, CA, or their mixture
with S, which can be associated with the combined presence of smaller
water droplets and the droplet network formation.
The sedimentation velocity in MO emulsions significantly decreased with
S0 (6.8 ± 0.2 µm/s) and S0.125 (7.3 ± 0.1
µm/s)
compared to CO-emulsions, which could be due to the smaller droplet size
of the former (Figure 7A). However, the sedimentation velocity of all
other MO emulsions with AA, CA, S-AA, or S-CA demonstrated higher values
than CO emulsions, despite their visually higher emulsion height, under
earth gravitation after seven days (Figure 1 and 2). It can be said that
the water droplet network provided better emulsion kinetic stability
against earth gravity. However, accelerated gravitational force probably
broke the network structure, leading to a faster separation of large
water droplet aggregates in MO-emulsions. Erramreddy et al. (2017)
reported a correlation between gel yield point and centrifugal
separation. The minimum relative centrifugal force required for the
creaming of oil droplets increased with gel strength. In the present
case, the higher sedimentation velocity of MO-emulsions under
accelerated gravitation could be due to their lower-yielding behaviour
than CO emulsions, which will be discussed under the viscoelasticity
section.
The addition of Ca significantly reduced CO-emulsion sedimentation
velocity (7.4 ± 0.4 µm/s) compared to S0, indicating higher stability
against phase separation (Figure 7B). Emulsions with LMP (both
concentrations) exhibited a further reduced sedimentation velocity.
Similar behaviour was also observed for MO emulsions, except for the one
with Ca. Overall, CO-emulsions with AA, CA, their mixtures with S and
LMP exhibited lower sedimentation velocity than MO-emulsions. Lower
sedimentation velocity indicates that the presence of aqueous phase
ingredients could help to improve long-term W/O emulsion stability.