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How to prevent large bubbles from eating small ones ?

Mix 1% liquid and 99% air and you will obtain a foam, a complex material that has completely different properties from the two main components that make it up. It is an assembly of air bubbles surrounded by liquid whose mechanical and insulating properties are used in many industrial applications from food and cosmetics to fire fighting. One important question for the reliable application of foams is the control of their stability over time.

One of the mechanisms of foam aging, called coarsening, is due to the pressure difference between bubbles. On average, smaller bubbles contain a gas at higher pressure than larger ones, so that small bubbles tend to empty into larger ones. In the last fifteen years, researchers have studied Pickering foams in which bubbles are stabilized by solid particles instead of soap molecules. These foams are known to have very good stability and to resist coarsening but the underlying mechanisms are still ill understood.

The team Soft Interfaces at the Laboratoire de Physique de Solides (CNRS-Univ. Paris Sud) has designed a model experiment to better understand these mechanisms. Two syringes are immersed in a stabilising solution and are used to create two bubbles (see figure). The bubbles can be connected through a pipe. The relative pressure inside the bubble is given by the Laplace pressure P = 2 γ / R where γ is the surface tension and R the radius of the bubble. The smaller bubble that contains gas at the highest pressure will then quickly empty into the larger bubble.

(Left) Experimental setup. (Right) Image of the bubbles before coarsening (initial state), and after coarsening stops (final state).

We used as a stabilising solution a mixture of oppositely charged silica particles and surfactants. The surfactants adsorb onto the particles and render them partially hydrophobic. The particles thus tend to adsorb at the liquid/air interface. Due to their large size, the particles are nearly irreversibly anchored in the interfaces. The direct implication is that when the small bubble decreases in size as it empties into the large one, the particles are more and more concentrated at the surface and thus the surface tension decreases. The pressure decreases and a balance is reached between the pressure in the large and in the small bubble. The ripening can thus be completely stopped (see figure). This phenomenon is quantified by a parameter called the surface elasticity. In collaboration with Cyprien Gay, from the MSC laboratory (CNRS – Paris 7), we have shown that quantitative criteria on surface elasticity could predict the arrest of coarsening as a function of the initial size of the two bubbles.

Reference :

Arresting bubble coarsening : A two-bubble experiment to investigate grain growth in the presence of surface elasticity
A. Salonen, C. Gay, A. Maestro, W. Drenckhan and E. Rio
EPL 116, 46005 (2016), doi:10.1209/0295-5075/116/46005.

Contact :

Emmanuelle Rio