What is causing the foam
How is foam created and why does it disappear again?
For example, foam is a collection of soap bubbles in dishwashing water or bath water. For a soap bubble, we need soap, or surfactants, as they are also contained in washing-up liquid. What is special about surfactants? We know that there are two different types of liquids: those that mix well with water - alcohol, for example - and those that do not mix with water at all - these include fat and cooking oil.
The specialty of soaps or surfactants is that they have two sides: a “water-loving” side and a rather “greasy” side that turns away from the water.
If you add a drop of detergent to the water, it immediately spreads as a very thin film over the surface of the water. With this soap film, the water-loving side of the individual molecules now points downwards towards the water, and the fatty side points away from the water, i.e. upwards. When we mix up this surface, the foam is created in the form of many soap bubbles.
This can be illustrated by thinking of the soap bubbles that children create: They dip the "blow ring" into the soapy water, so that a thin skin of soapy water forms in the ring. The difference to the surface of the rinse water is that the soap film on the skin in the blow ring surrounds the water on both sides - like a sandwich. Outside is the soap film, inside is the water. And when you blow in, the soap bubble is created.
So the skin of a soap bubble consists of three layers: inside and outside the soap, i.e. the surfactant, with the water in between. The fact that this becomes an almost perfect sphere is due to the fact that a sphere is the most economical form: With a certain amount of trapped air, a sphere requires the smallest surface area.
But when I run into a bubble bath or wash the dishes, I don't blow.
No, but if you let in additional water you disturb the soapy water surface, and air gets under the water surface. This air does not stay there for long, but rises in the form of bubbles. When these air bubbles penetrate the surface of the water, they do the same thing as when blowing soap bubbles: they take the film of soapy water with them on the surface and form it into a bubble.
But then the foam does not consist of beautiful balls ...
This is due to the fact that the many soap bubbles that initially appear in the foam immediately merge. And then nature goes back to being economical. Economical means that the many soap bubbles try to create common boundaries. And so there are many irregularly shaped bubbles in the foam. The fact that the foam disappears is because the skin of the blisters is made of soapy water. This soapy water slowly flows down, causing the bubbles to disintegrate.
Is that the same with beer foam?
The physical principle is similar, but the foam is created not only by the unrest on the beer surface in the glass, but also by the rising air bubbles. Beer contains carbonic acid, and these air bubbles carry protein-containing substances that are in the beer with them when they rise. That is the second difference: The skin of the bubbles in the beer foam does not consist of soapy water, but of a protein-containing substance. This is almost always the case with food. Whenever we cook rice or beat eggs - almost always when it foams while cooking, proteins are involved. But the physics is the same.
The text and audio of this post are licensed under the CC BY-NC-ND 4.0 Creative Commons license.
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