Emulsions: Thickening with Droplets

Thanks to their very different structures and properties, water molecules and oil molecules don’t mix evenly with each other. Neither can dissolve in the other. If we use a whisk or blender to force a small portion of oil to mix into a larger one of water, the two form a milky, thick fluid. Both the milkiness and the thickness are caused by small droplets of oil, which block light rays and the free movement of water molecules. The oil droplets thus behave much as the solid particles in a suspension do. Such a mixture of two incompatible liquids, with droplets of one liquid dispersed in a continuous phase of the other, is called an emulsion. The term comes from the Latin word for “milk,” which is just such a mixture.