With the addition of the egg yolk in the second mixing bowl, the oil and vinegar remained mixed. The egg yolk contains lecithin. Lecithin molecules surround the oil and prevent the oil molecules from coming together, so they stay in a solution for longer. This is how mayonnaise is made, but of course, to make real mayonnaise you would add powdered mustard and salt. Lecithin is a type of emulsifier, like detergents.
Detergents break up oil into smaller sizes so it can be more easily washed from clothes or dishes.
An emulsion is a type of colloid. The experiment Ooblick also results in a colloid mixture of cornstarch and water. Wear goggles at all times. Always wear gloves. This experiment is to be done in-class with teacher supervision at all times.