A dispersion of a fairly large amount of insoluble gas (usually air) in a small amount of liquid is called a bubble. There is a large interface between the gas and liquid, and the liquid separates the gas from each other in the form of a thin layer to form a heterogeneous system.

According to its form, foam can be divided into foam and foam two kinds: independent separate individual bubble called bubble (generally in the high viscosity of solvent-based coatings will appear similar bubbles); Bubbles clustered together in different sizes are called foams (common in water-based coatings).

Containing sufficient liquid, the bubble has no significant deformation, called moist bubble or spherical bubble; As the liquid flows out of the aggregate foam, the film becomes thinner and the bubbles cluster closer together, forming a polyhedral structure called a wet dry bubble or polyhedral bubble.