Emulsions are homogenous mixtures, usually oil in water "colloids" for beverages. Emulsifiers/surfactants are often necessary functional ingredients, important whenever using multiple phases. The colloid particle size are held stable by two repulsive forces, steric and electrostatic. The colloid particle is wrapped in a micelle, the particle size determines steric repulsion, milk oil particles are considered stable between 2 and 5um for reference. pH and other present ionic functional groups play a role in the electrostatic repulsive force.
Emulsions are analyzed using particle characterization studies, the most common of which is laser diffracted particle sizing. Less understood is zeta-potential analysis, which determine electrostatic repulsion. Both of these characterization techniques can suffer from competing data in complex beverage solutions, you may have a particle that overlaps the oil particle sizes obscuring your results for example.
When choosing your emulsifier:
Smaller oil particle sizes are better.
Emulsifiers can be artificial or natural.
Usage is based on oil% in solution.
One common mistake I see is confusing a surfactant with a suspension agent. If those oil particles are on material that migrate in solution then you can still get uneven distribution.
Emulsions are a specialty of mine. I was lucky enough to have experience on the frontier of complex emulsion beverages and would be happy to share my knowledge with you. Please reach out if you need help.