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Changing foam stability with salt

Anniina Salonen Laboratoire de Physique des Solides, Université Paris Sud, Orsay

Foams are used in food products, for cleaning purposes, to extract petrol and in many other industrial processes and products. This makes the control of their properties and stability such important questions. Aqueous foams are collections of gas bubbles in water and they are intrinsically unstable, which means that they will always eventually disappear. The lifetime of the foams depends on the efficiency of the used stabilisers in slowing down the different mechanisms of ageing.
We are most familiar with surfactant-stabilised foams. In general such foams are rather unstable and disappear within hours. However, we have shown that the presence of salt can change foam stability dramatically. The addition of salt can lead to the precipitation of surfactant, where the surfactant crystals can behave as particles at interfaces (as seen in the photograph). The stability of these foams depends on the properties of the precipitate, which in turn can be controlled by both the concentration, and the type of salt used. Therefore we can create rather versatile superstable foams, which destabilize upon command, making simple surfactant foams surprisingly smart.