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Foam under the scrutiny of scientists
Foams are omnipresent in our daily lives in forms as diverse as shampoo foam, beaten egg whites, or the furnishing of chairs. Each time, air bubbles dispersed in a matrix of water, polymers, and even glass or metal… Understanding the physico-chemistry of these foams is essential in order to better control their production and develop new applications !
Making more or less foam
Water alone does not foam. "Surfactants" must be added to it, which are able to stabilise the bubbles, like soap or proteins (in egg white). By contrast, to prevent the washing machine from overflowing or your stomach from burning, foaming must be averted thanks to specific molecules.
Foam deteriorates
Bubbles are not all the same size : the gas, under more pressure in smaller bubbles, diffuses through their walls and makes them disappear. The remedy : finding a gas that diffuses more slowly than air, or better yet, rigidifying the bubble walls (e.g. using nanoparticles as a "surfactant", so that the foams no longer mature). The film between two bubbles sometimes ruptures. This coalescence leads to a cascade of ruptures, producing a characteristic sound (heard in the bath !). Because of gravity, water flows to the bottom of the foam, which dries out ! To slow down this phenomenon, it would be advisable to increase viscosity using additives (e.g. thickening agents in food).
Studying the movement of foam
Under the effect of a stress, foam can maintain the properties of an elastic solid, and beyond a certain limit, it will run like a liquid. This is the principle of shaving foam.
Industrial applications as varied as shape memory materials, foams metal for the automotive and aerospace sectors, light food products… already benefit from this upstream research.
Laboratoire de physique des solides, CNRS-Université Paris 11
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