By Harold McGee
It takes work to make an emulsion. We all know from experience that when we pour water and oil into the same bowl, they form two separate layers: one doesn’t just turn into tiny droplets and invade the other. The reason for this behavior is that when liquids can’t mix for chemical reasons, they spontaneously arrange themselves in a way that minimizes their contact with each other. They form a single large mass, which exposes less surface area to the other liquid than does the same total mass broken into pieces. This tendency of liquids to minimize their surface area is an expression of the force called surface tension.