Scientists have long believed that viscous bubbles collapse due to gravity. Now, a new study from researchers at Boston University, Princeton University, and the Massachusetts Institute of Technology turns this accepted mechanism, as well as bubbles, upside down (Science 2020, DOI: 10.1126/science.aba0593). By flipping bubbles and watching them with high-speed cameras, the team demonstrates that how bubbles pop depends on surface tension, not gravity. This improved understanding could help scientists better control bubble formation, which could be useful in a number of applications, including spray painting and making ice cream.
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The following is the script for the video.
Tien Nguyen: This silicon oil bubble is about to be popped. As air escapes, the bubble collapses, and wrinkles form around its edges. This process is driven by gravity acting on the thin film—or so scientists thought.
Now, a new study proposes to upend this traditional understanding of how bubbles in viscous fluids collapse. By better understanding how bubbles burst, scientists can control their presence or absence, which may be useful in a number of industries.
For example, in applications like spray painting, it’s best to minimize bubbles. If you’re making ice cream, on the other hand, it’s better to maximize air bubbles to make it soft and scoopable, says James Bird, who led the recent study.
Bird began to question how viscous bubbles collapse more than a decade ago. He reasoned that if gravity were the main force behind a bubble’s collapse, then flipping it upside down should make the popped film deflate towards the ground.
Instead what happened is that the upside-down bubble collapses in the same inward manner as if it were upright. So Bird and his coauthors hypothesized that another force was responsible for viscous bubble collapse: surface tension. Yet investigating this theory wasn’t easy.
James Bird: The challenging part is visualizing it and getting the light just the right way that you can see wrinkles in what otherwise would be a transparent film.
Tien Nguyen: The authors estimate that they watched thousands of silicon bubbles, of varying viscosities, pop to collect enough data to measure how fast the bubbles collapsed. They found that the speed of collapse depended on viscosity and capillary forces, allowing them to confirm that surface tension was indeed the driving force behind the bubbles’ collapse.
The team also investigated why the films wrinkle as they collapse, which aside from its eye-catching appearance, has practical implications; these wrinkles can trap air and cause more bubbles to form. Next, Bird says they plan to study how more complex mixtures of fluids behave so they can better understand bubbles in the real world.