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Physics

Foams are very much interested by both science and technology nowadays. To explore them requires real interdisciplinarity as means of chemistry, physics and mathematics are to be involved researching build-up, stability and destruction of foam structures.

Fig. 1. Exploring foams needs approaches of chemistry, physics and engineering sciencies too.
Credit: Dennis Weairie, Stefan Hutzler, Making, Measuring and Modelling Foams, Europhysics News 1999.

Everyday life is filled with foam-like objects of natural or artificial origin. Examples are the bread, cookies, shower bath, cleaning sponge and heat insulation of houses.

Fig. 2. Natural foam : must froth in a wine cellar of Miskolc

What circumstances make it possible to create foam?
From thermodynamic aspects these structures are instable as in this complex of gas and fluid they try to minimize their contacting surface measurement. Notonwithstanding, foams have very large surfaces – consider the high number of bubbles crowded side by side. How can you stabilize them? There are materials - called surface-active - acting as decreasing the surface stress and increasing surface elasticity (Gibbs-Marangoni effect). Also applicable is the use of tiny (in the range of thousands of mm) particles to create particle stabilized foam by spreaded over the surface of bubbles while forming mesh-like structures.

How are foams produced? A simple way is the blowing method pressing the gas into the fluid though a pipe. Other is the nucleation method: lowering pressure around a with gas supersaturated fluid to get bubbling (think of soda water). Another way is by whisking as you make whipped cream for coffee. Finally there is a system used by firemen with foam gun: very high speed fluid snatches and transports air with him.


What happens with the generated foam is another story written by principles of physics.
Fluid foams destruction begins with the weakening process mainly caused by gravitation and capillarity forces. Fluid of foam cell walls begins to flow into the channels formed on boundaries of interfacing cells and along them downwards owing to gravity. Consequently the upper part of the foam becomes „dry”, - with fluid content less than 1 – 2 % - featured by polyeder cell forms. Lower part is getting to be more and more „wet” with higher fluid content forming rounded shape cells instead of former polyeder types.

Fig. 3. Water base foam. Upper part with low fluid content and polyeder cells. In lower part (floating on the water surface) the closer to the surface the higher is the water content of the foam and the more rounded are the cells.

Another process along the foam life is getting coarser grained - owing to gas diffusion between bubbles the bigger ones are growing while the smallers gradually disappear. Third process is the bursting of cells. The cell walls getting thinner and thinner finally burst owing to a small vibration, mechanical disturbance or thermal fluctuation. Flowing of foam may be provoked by a force exerted by any mechanical impact, press differences or other environmental effect. Accordingly the bubbles start moving, sliding on each other and the foam structure transforms. Naturally all these processes are not independent from each other.

Fig. 4. The four basic interactions defining foam life

Foams can be simply characterized by their „foam readiness” and stability. Foaming readiness means the ability to produce foam from a given quantity of fluid and gas applying a given technology and is defined by the achieved maximum foam volume. Stability describes the life time of the foam before collapsing.