Mayonnaise is an emulsion , which is a mixture two liquids that normally can't be combined. Oil and water is the classic example. Emulsifying is done by slowly adding one ingredient to another while simultaneously mixing rapidly. This disperses and suspends tiny droplets of one liquid through another. However, the two liquids would quickly separate again if an emulsifier were not added.
Emulsifiers are liaisons between the two liquids and serve to stabilize the mixture.
Eggs and gelatin are among the foods that contain emulsifiers. In mayonnaise, the emulsifier is egg yolk, which contains lecithin , a fat emulsifier. Small particles like this do not settle out and will pass right through filter paper. The particles in a colloid can be solid, liquid or bubbles of gas. Emulsions are liquid-liquid colloids, tiny liquid droplets suspended in another liquid.