Unusually stable liquid foams

• Obtaining stable liquid foams is an important issue in view of their numerous applications.
• The origin of the stability is related to the slowing down of coarsening, drainage or coalescence, and eventually to their arrest.
• Drainage can be arrested if the liquid of the foam either gels or solidifies.
• Coalescence is arrested if the foam films become very thick and/or rigid.
• Coarsening is slowed down if films are small or very thick, it is seemingly arrested when the surface layer compression modulus is large and the layer resists collapse.

Obtaining stable liquid foams is an important issue in view of their numerous applications. In some of these, the liquid foam in itself is of interest, in others, the liquid foam acts as a precursor for the generation of solid foam. In this short review, we will make a survey of the existing results in the area. This will include foams stabilised by surfactants, proteins and particles. The origin of the stability is related to the slowing down of coarsening, drainage or coalescence, and eventually to their arrest. The three effects are frequently coupled and in many cases, they act simultaneously and enhance one another. Drainage can be arrested if the liquid of the foam either gels or solidifies. Coalescence is slowed down by gelified foam films, and it can be arrested if the films become very thick and/or rigid. These mechanisms are thus qualitatively easy to identify, but they are less easy to model in order to obtain quantitative predictions. The slowing down of coarsening requests either very thick or small films, and its arrest was observed in cases where the surface compression modulus was large. The detail of the mechanisms at play remains unclear.

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