An emulsion is a heterogeneous mixture of two or more liquids, in which one ends up as
very tiny droplets inside the other. Very often, the liquids involved are not mutually
soluble — like adding some water to a bottle of cooking oil. You will notice that, even after
some shaking and agitation, it does not dissolve in each other, but appear as bits and
pools in the main liquid. Emulsions behave this way.
Emulsions are colloidal systems too. (Check out the difference here)
Here is an illustration of what happens to an oil-water mixture:
Emulsions are more viscous than the oil or water that they hold. Examples of emulsions are
ice cream, salad dressings and paints.
'Because a large number of emulsions contain water as one of the two phases, emulsions
are classified into two categories: 1) Oil-in-water and 2) Water-in-oil. Oil-in-water
emulsions consist a dispersed phase of oil droplets in a water medium. Water-in-oil
emulsions consist of a dispersed phase of oil in a water medium. One can distinguish
between an emulsion types based on the volume fraction of the two phases.'
Colloids and Emulsions, By Jonathan Collins, Ha Dihn, Heather Gonzales, Ursula Koniges, and Andrew Nordmeier