Why does emulsification occur

Emulsification occurs when two or more liquids mix together, but do not blend. One liquid must remain in the dispersed phase, while another hangs in the continuous phase, with the first liquid resting inside of it. The most common emulsion is an oil and water emulsion. A good example of this is dietary fat. Other examples of emulsions include butter, milk, cream, and vinaigrettes. In both butter and dietary fats, emulsification occurs when fat surrounds droplets of water. This is known as a water in oil emulsion. In substances such as milk and cream, the water droplets surround fat, making it an oil in water emulsion.

Because of the process of emulsification, many emulsions have an almost cloudy appearance, due to the many phases of emulsification. Emulsions are unstable and therefore energy is needed to create them, be it stirring, shaking, homogenizing, or spraying. There are many kinds of emulsions which can be formed, and once the emulsion is formed, over time it reverts back to its stable state. Because of this, active substances known as surfactants are sometimes added to emulsions so that they do not change during their years of storage. A prime example of this is a vinaigrette dressing, which will quickly disperse unless it is shaken continuously. This phenomenon in the emulsification process is known as coalescence, and it occurs when small droplets within the emulsion recombine to form larger ones.

In order for emulsification to occur, a substance known as an emulsifier must be present. An emulsifier is what stabilizes an emulsion, and frequently, many emulsifiers are also surfactants. An example of a good emulsifier in the food industry is egg yolk, which contains the chemical known as lecithin, a known surfactant. In addition to egg yolk, honey and mustard are also emulsifiers.
In many cases, when particles which work to stabilize emulsions can be worked through a mechanism known as the Pickering stabilization. Mayonnaise is a prime example of an emulsion which uses this method, with stabilized egg yolk lecithin being the surfactant in the emulsion.
Sometimes, during the process of emulsification, the inner substance can act as an emulsifier for its self, and this results in a nanoemulsion. A nanoemulsion is when the inner state of an emulsion has dispensed into nano-sized droplets while still inside the outer phase. This is a very rare occurrence and there are only a few substances known to man which react this way when they are combined. Perhaps one of the most well known nanoemulsions occurs when bleach is poured onto any surface in order to disinfect it. Bleach works by wrapping its self around the offending partically and actually getting inside, emulsing again, which results in the nanoemulsion. This is why bleach is able to kill a host of different bacteria and viruses, as it literally works from the inside out in order to kill the organisms.

Emulsification occurs in many different places, not just in the foods we eat and the substances we use. Our own body has a process of emulsification which helps break down fatty compounds as well.