Foam, a set of “Siamese” bubbles

A soap bubble is a thin liquid film that encloses a certain quantity of gas, air in this case. The ratio is something in the order of 15% liquid to 85 % air. This explains the extreme lightness of the foam. A foam is made up of a large quantity of bubbles which form a set, a structure whose properties are very different from those of an isolated bubble, for example such as that produced by a child blowing into a ring to make soap bubbles. Imagine soap bubbles in a quantity of foam. They share a common boundary, an interface in the form of a thin film by which they are linked, a little like Siamese twins.

Just after the foam has been created by the action of the piston in the syringe, the antifoam particles are distributed more or less equally in the liquid zones that constitute these interfaces between the bubbles. But when the source of mechanical energy disappears (when the piston stops), the displacement of antifoam particles is not at all random. The liquid which makes up the films tends to drain off due to the effect of gravity causing the antifoam particles to slide down from the top to the bottom inside the wall that connects the two bubbles. The particles, which are denser than the liquid, fall quicker. This is a sedimentation phenomenon, like a pebble in water, or to make a better comparison in terms of density, like a marble in a jar of honey.

The result of this is that, under the effect of gravity, the film gets thinner and thinner. Below 4 nanometers, the wall is too thin and it becomes porous, allowing an exchange of air between the two bubbles. The smaller bubble, in which the pressure is stronger, will transfer its air to the bigger bubble. The film between the bubbles finally collapses. This is the same effect we see in a foam bath or the head on beer: after a few minutes, the foam becomes lighter. In fact there are fewer bubbles and they are bigger. Larger bubbles have a bigger contact surface with the outside air than the smaller ones which facilitates evaporation and therefore the final disappearance of the bubbles. If you don’t touch your beer for a period of ten minutes it goes completely flat. “Guinness is an exception, says a smiling Hervé Caps. Thanks to the small size of its bubbles, it keeps its head much better than the other beers”.

The antifoam molecules accelerate this process of bubble bursting because they do not “fall” straight into the liquid. Their inevitable trajectory becomes similar to that of a car on the motorway drawn towards the crash barriers on the side. The antifoam particles are therefore attracted towards the nearest wall and eventually come into contact with it which leads to the destruction of the film between the two bubbles.

Foam in orbit 

What becomes of the antifoam molecules once the bubble has burst and disappeared? They are redistributed throughout the rest of the foam and can therefore contribute to breaking the film between two other bubbles and so on “This repeated mechanism explains why the antifoams are effective in very small doses”, explains Hervé Caps. “We consider that a particle under the effect of gravity takes 50 times less time to attack a wall between two bubbles than in a process of displacement by diffusion which would be random”. Gravity therefore plays an essential role in the bubble bursting process. On the other hand, absence of gravity also plays an essential role in this…

This research, published in NPJ Microgravity makes a valuable contribution to improving theoretical knowledge about the behaviour of foam. “In previous research, explains Hervé Caps, conducted on board the International Space Station with the assistance of Frank Dewinne (see article: Mission OasISS), we demonstrated that an antifoam is inefficient when a set of bubbles are without a film. This is the case when the proportion of liquid is higher than 36 % and the proportion of gas is less than 64 %. In this case, while they are side by side, the bubbles remain spherical and do not have an interface which prevents the draining mechanism of the antifoam particles and therefore limits their contact with the film walls”.

Fundamental though it may be, this research on the behaviour of foam in microgravity conditions is being followed attentively by industry. The challenge is to improve the efficiency of foam or antifoam products in order to reduce the quantities used and therefore reap the resultant economic and ecological benefits. This has clear implications for the cosmetics industry but also for the food industry. The unctuousness of milk products, for example, is due to the addition of “Foaming” substances.