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there is one question related to this topic : What are the principles behind foam formation? but I'm not yet satisfied by the answer.

There is a law that show a correlation between the energy used by a bubble to maintain itself and the surface of it. So logically, when two bubbles are touching themselves there will be a phenomenon of fusion/merging in order to reduce the energy used.

But concerning foam that is not actually the case... there are thousand of bubbles and there are litterally touching each other, lying one on top of the other but they do not merge... why?

thank you and happy easter!

Marine Galantin
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All foams are energetically unstable i.e. the foam has more energy than it would have if it collapsed to a liquid with no entrained air. The reason for this is simply that an air-water interface has an interfacial energy associated with it. This energy is simply equal to the surface tension. A foam has a high surface area and collapsing the foam reduces the surface area, so collapsing the foam always decreases the total interfacial energy.

The reason foams do not immediately collapse is that the soap films that make up the foam are kinetically stable. I explain this in some detail in my answer to Why soap bubbles are made of soap? Even though it is energetically favourable to collapse the soap film there is a kinetic barrier that prevents it happening.

Any easy way to see this (and a favourite lab demonstration of my colloid science professor) is to add a hydrophobic powder to the foam, for example powdered PTFE or polyethylene. The hydrophobic particles act as nuclei for pores to open in the soap films and this allows the films to rearrange to reduce their surface area. When you add the hydrophobic powder to a foam the foam collapses completely in a few seconds.

John Rennie
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