BioEd Online

Not all cutting-edge physics requires a particle accelerator. Using a straw and some soap, researchers have shown how a popping bubble can produce more bubbles. The work is published in this week's issue of Nature¹.

James Bird, now a postdoc at Massachusetts Institute of Technology in Cambridge, first noticed the effect while working on another experiment for his PhD at nearby Harvard University. When a bubble on a fluid's surface popped, Bird noticed that a ring of small bubbles formed where the bubble's edge had once stood.

"We saw this effect and couldn't really explain it," he recalls. So he and his colleagues decided to investigate. They began blowing bubbles through a straw and filming them with a high-speed camera as they popped.

Previous work had suggested that a ruptured bubble might vanish or break up and fall to the surface, but Bird saw something different. Rather than exploding, the walls of the bubble actually fold back on themselves. As they do so, they trap a small doughnut-shaped ring of air that breaks up when it hits the surface of the fluid. The result is a ring of smaller bubbles (see movie 1).

Bird and his co-workers spent the next three years developing the perfect theory of bubble bursting. They determined that only three factors affect bubble break-up: the surface tension of the bubble, the inertia of the fluid and the pressure of the gas trapped inside. Depending on these factors, a collapsing bubble might fold into two pockets of air instead of one — generating even more bubbles — or it might not form any offspring bubbles at all.

Party trick

The work is impressive in its scope, says Jens Egger, a mathematician at the University of Bristol, UK. "What's nice about this paper is that it's taken a small problem and found this beautiful structure," he says. Even daughter bubbles can burst and create more, even smaller bubbles, notes Eggers. "Self-similar structures just seem to be nature's way of making small things."

"Bubbles can be really useful in certain contexts and detrimental in others," Bird says. For example, bubbles are a constant worry in glass manufacturing because they can weaken material. In the oceans, however, bubbles can have a useful role by helping to convert salt into an aerosol, a crucial step in creating clouds. Small bubbles in particular seem able to project tiny particles into the air (see movie 2).

Bird says the group's calculations might improve climate models and industrial processes. "Other than that, it's a cool party trick," he says, admitting that more than once he has rolled up a paper straw to demonstrate his PhD work to friends over a beer.

Meanwhile, Bird's work with bubbles is not yet done. He has just been awarded a one-year fellowship from the US National Science Foundation to continue his research. "I thought I was going to be done with bubbles, but no, it keeps going," he says.