Image: STEPHAN KOEHLER/Harvard University

Light and fluffy foam is actually serious business. To ensure that canned draughts have predictable, lasting heads, for example, Guinness developed the Widget, a plastic ring that releases nitrogen into the beer as it is opened; it won the Queens Award for Technological Advancement in 1991. Coca-Cola, on the other hand, has sought ways to create more fleeting foams from soda fountains. And all the while, physicists have chased after a theory to explain how foam behaves. Now they may have one, reported in this week's issue of Physical Review Letters by Howard Stone and his colleagues at Harvard University.

The real trick in understanding foams was figuring out just how they change over time. Researchers knew that two distinct processes took place. First, the foam coarsens¿meaning simply that its bubbles enlarge¿and second, the liquid in the foam drains away. What they didn't know was how these two events influenced each other. To find out, Stone and company developed computer simulations and then compared them to the actual events in two types of foam. In one concoction, the researchers used CO₂ gas, and in another, C₂F₆, which diffuses from bubble to bubble less readily.

As expected, they found that drainage in the latter foam occurred 10 times more slowly than in the former because the C₂F₆ gas eliminated coarsening. But whereas adding more liquid to the C₂F₆ foam made it drain faster, the same was not true for the CO₂ foam. Turning back to their model, the team concluded that the extra liquid actually inhibited coarsening in the CO₂ foam because it made it harder for gas to diffuse. In the foam with the slower diffusion rate, though, the smaller bubbles continued to drain, thinning the bubble walls and accelerating coarsening and further drainage. In the end, the scientists reconciled the model with the data, thus offering the first demonstration of how coarsening affects drainage.

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