A bubble like this broke, leaving the ring of smaller bubbles seen on the right.
Whether it’s beer or soap, soup or suds, bubbles are a fact of life. These evanescent but ubiquitous structures posed a perfect challenge for James Bird, who has just received his PhD from Harvard University. “Bubbles are fun, but the dynamics are fast enough that you have to use high speed cameras, and the visual aspect appeals to me,” Bird says.
Bubbles matter in realms ranging from medicine to climate. But until now, nobody knew that when larger bubbles break, they spawn smaller bubbles. Even stranger, as Bird and his graduate advisor Howard Stone, who is now at Princeton University, have found, these “daughter” bubbles can even spawn “granddaughter” bubbles.
This chain reaction only occurs when many factors, like bubble size and liquid viscosity, are correct.
Suitably, Bird says the bubble study arose when he and co-author Laurent Courbin were “playing in the lab late one night, trying to get a bubble to spread on different surfaces, but instead this hemispheric bubble would pop to create a ring of daughter bubbles. We looked at each other and were not sure what was going on.”
As Bird recalls, “Howard gave me permission to have fun and see if I could figure it out.”
Tense at the surface
The major physical force at work in the bubbles is surface tension – the same phenomenon that causes water to creep up the side of a glass. Surface tension occurs because smaller surfaces have lower energy, so stretchy materials like films and balloons tend to adopt a shape with the smallest area.
Surface tension forms spherical bubbles because spheres have the smallest area for any given volume of fluid. But surface tension also exerts pressure on the trapped gas, which explains why bubbles pop rather than just break.
After a bubble breaks, the retracting liquid may form a lip that traps a donut of air. This donut is the origin of tiny “daughter bubbles” that can break and form yet another ring of “granddaughter” bubbles.
The formation daughter bubbles is seen in simultaneous movies shot from the side and bottom. Left: at about 2.5 milliseconds (ms), a lip forms that later traps air inside an unstable, donut-shaped structure. Right: by about 10 ms, the donut has sub-divided into air bubbles.
Using high-speed video, Stone, Bird and their colleagues popped a bubble and watched it break. As the trapped gas rushed out, surface tension retracted the bubble film, and a lip formed around at the top. “If you apply force to something, it tends to move in straight line,” says Stone. “When the soap film pops, surface tension pulls to open the ring, so the film moves in a horizontal line at first.”
Meanwhile, the absence of the internal pressure causes the rest of the bubble to implode. The combination of these two motions creates a tiny lip at the top of the bursting bubble. As the bubble retracts, the lip curls over and briefly traps a donut of air around the bubble.
From that point, says Bird, “It’s 19th century physics. The daughter bubbles form for the same reason that a faucet jet breaks up into little droplets.” Translated: The droplets have a lower energy state than the stream of water, and the daughter bubbles have a lower energy state than the donut of trapped air around the broken bubble.
A daughter bubble created by the rupture of a larger bubble breaks, forming a jet that propels micron-sized droplets (arrows) into the air.
The formation of “daughter bubbles” is not the end of the story, however: when they land on the liquid, they may also break up, forming a third generation of bubbles.
When a bubble pops, a jet of material may rise at the center. In a big bubble, these jets remain at the surface, but the higher pressure in a tiny bubble will squirt a smidgeon of liquid, together with any associated chemicals or particles, into the air.
The “bubble-begets-more-bubbles” phenomenon could matter, because these jets can carry pathogens and spread disease in hot tubs or swimming pools.
And a ridiculous number of bubbles — between 1018 and 1020 — supposedly break every second in the oceans. These bubbles can carry heat, chemicals and water vapor into the atmosphere, affecting weather and climate.
Computerized climate models must consider the interaction between ocean and atmosphere, which entails accounting for all these breaking bubbles, Stone says. “If you don’t know these parameters, your model is filled with ad hoc parameters [AKA wild guesses] and you don’t even know the order of magnitude. Computer models are only as good as the parameters that go into them.”
By showing how large bubbles create a cascade of smaller bubbles, the new study highlights the real-world effects of large bubbles, says Bird. “People have discounted bubbles bigger than a few millimeters because they did not create aerosols, but we think the impact is actually much greater because the bigger bubbles are a source of lots of little bubbles, which can make a lot of aerosols.”
And the answer to your inevitable last question is yes. Bird “absolutely” did play with soap bubbles as a kid.