If you pour oil and water into a jar and shake it, they briefly mix. However, the oil drops quickly group back together and separate into a top layer. This happens because water molecules prefer to bond with other water molecules instead of oil.
To keep an emulsion mixed for a long time, chemists add a special emulsifier chemical. Emulsifier molecules have a unique dual structure that helps them bridge the two liquids. One end of the molecule strongly loves water, while the other end loves oil.
When added to the mixture, the emulsifier molecules surround the tiny suspended oil droplets. They bury their oil-loving tails into the oil drop and point their water-loving heads outward. This creates a protective chemical shell around each individual droplet in the mixture.
The droplets can no longer touch each other, so they cannot merge together anymore. Emulsions generally fall into two main chemical categories depending on which liquid is most abundant. Oil-in-water emulsions feature tiny oil drops floating throughout a much larger volume of water.
Water-in-oil emulsions feature tiny water drops suspended inside a much larger volume of oil. Without a good emulsifier, the mixture will eventually break and separate back into distinct layers.