# What are the principles behind foam formation?

We know that while pouring water in a glass, there is close to no foam, but if pouring beer, there is much foam (unless pouring "slowly"). So, can the most prominent factors that contribute to foam formation be listed and explained?

I've found an article that explains some concepts about foams and has some curious images. It also states that foams in general are not well understood. But I'd like to hear a qualitative explanation of why certain properties of liquids such as viscosity and volumetric flux make it foam or prevent foaming.

As I had a MSc thesis that had to regard this problem within it, did look into this extensivelly. For Beer formation it would be good for you to read http://edepot.wur.nl/202245, tho it's an older PhD thesis, it was checked and backed by experimental data. Alot about foam formation and breakdown was explained. Found also that many newer works also used much of his work, offcourse upgradeing it and finding new things, mainly about Plaetu's border and surfaces forming specific geometries to save energy, mostly those research was about soap films.

Four physical processes determine foam formation and breakdown:

1. Bubble formation and growth
2. Creaming and drainage
3. Coalescence
4. Disproportionation

In general, bubbles can be produced in a liquid by:

1. Agitating or whipping
2. By sparging or diffusing gas through a porous material
3. By decreasing the pressure of a with gas saturated liquid

The creaming of bubbles is the rise of the bubbles to the top of the system. Drainage is the liquid flow from a foam to the liquid underneath. One could also argue that creaming becomes drainage as soon as the bubbles start to interfere and to influence each other in their motion. The creaming process may be described with Stokes law. However, this law can only be applied if the bubble surface is immobile and the Reynolds number is low.

Drainage occurs if the bubbles become more densely packed. The foam becomes dryer and the bubbles become deformed. During drainage, the foam evolves gradually from a foam with spherical bubbles to a foam with polyhedral bubbles. In a polyhedral foam the Plateau border suction contributes as a driving force for drainage, in addition to gravity. As a consequence of the curvature of a Plateau border, the pressure inside the Plateau border is lower than inside the bubble and in the plane film. Therefore, liquid will flow from the film to the Plateau border. Through the Plateau borders this liquid will drain from the foam as a result of gravity.

Coalescence in foams is the merge of two bubbles caused by the rupture of the film between the bubbles. Two smaller bubbles become one larger bubble. Coalescence is often related to drainage. Films can drain to a certain equilibrium thickness. When this equilibrium thickness is reached, the film may persist over a very long period of time. Equilibrium films only rupture when the film liquid evaporates, or when disturbances occur.

Disproportionation is a coarsening process, that is the result of inter-bubble gas diffusion, caused by a gas pressure difference between bubbles. If a single gas is present, pressure difference corresponds to a difference in Laplace pressure. This pressure difference may be a result of a difference in size. According to the law of Laplace the pressure in a smaller bubble is higher than the pressure in a larger bubble, assuming that the surface tensions $(\sigma)$ of both bubbles are equal.

Taken from Ronteltap 1989.