Foaming and Foam Stability
Several conditions are needed to produce foam: there must be mechanical work, surface active components (surfactants) that reduce the surface tension, and the formation of foam faster than its breakdown. To create foam, work (W) is needed to increase the surface area (ΔA):
where γ is the surface tension.
Stabilisation of foam is caused by van der Waals forces between the molecules in the foam, electrical double layers created by dipolar surfactants, and the Marangoni effect, which acts as a restoring force to the lamellas.
Several destabilising effects can break foam down. (i) Gravitation causes drainage of liquid to the foam base, (ii)osmotic pressure causes drainage from the lamellas to the Plateau borders due to internal concentration differences in the foam, while (iii)Laplace pressure causes diffusion of gas from small to large bubbles due to pressure difference. Films can break under disjoining pressure, These effects can lead to rearrangement of the foam structure at scales larger than the bubbles, which may be individual (T1 process) or collective (even of the "avalanche" type).
Read more about this topic: Foams
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