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May 13, 2013

Pinning down the physics of bubbles

Bubbles are a serious business. While they’re beloved as a childhood pastime and a bathtub luxury, the physics behind the delicate, iridescent clusters remains remarkably complex.

Now mathematicians have pinned down the ephemeral physical processes that mark the life, and death, of these suds. Their findings, published this past week in the journal Science, could prove useful to chemical engineers seeking to better understand all kinds of foams, from shaving cream to plastic insulation.

Bubble clusters, known as foams, appear in a variety of forms and play an important role in everyday life: in the washing machine, in ocean froth, on the head of a happy-hour beer. And they form the basis of products such as fire retardants, bicycle helmets and refrigerator insulation.

These deceptively commonplace materials are difficult to understand because their relevant parts work on different scales of space and time.

The fragile, filmy wall of a bubble is just micrometers thick, thinner than a human hair. The gas within that transparent shell is vast by comparison.

And while it takes tens or hundreds of seconds for gravity to drain the liquid out of bubbles, they burst at speeds of hundreds of centimeters per second and rearrange themselves in a series of steps that takes less than a second, according to UC Berkeley mathematicians Robert Saye and James Sethian.

Generally, when trying to understand and model such a system, researchers look at the smallest scales to get a detailed understanding of the forces at play.

But here, with more than one scale to consider, focusing on only the small details could have caused researchers to miss the forest for the trees.

Fortunately, the large and small scales involved in foams usually can be dealt with independently, Saye and Sethian explained in their report.

“The details at one space or time scale are not necessarily important at another scale,” they explained.

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