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.