Have you ever tried to add some drops of oil into a glass containing water? If you have, you probably realized that the water and the oil do not mix with each other. The same happens when you try to wash your oily hands with just water—your hands will remain very oily. Let us say that this happens because water and oil are enemies and they want to stay as far from each other as possible. However, making enemies is not a good thing, so we are going to explain a technique, called emulsification, which can make water and oil become good friends. When water and oil become friends, the mixture is called an emulsion.
Emulsions are classified according to many factors but most importantly according to (i) type of emulsion: water-in-oil and oil-in-water and (ii) the size of their particles. When the particles are very tiny, they are called nanoemulsion (particle size: 10–100 nm). Just so that you have an idea of how small the particles in a nanoemulsion are: imagine a 1-cm long ant and a 1-nm long emulsion particle. Ten million of these particles together would have the same length as the ant.

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Go to source: https://kids.frontiersin.org/article/10.3389/frym.2018.00048