London: American engineers have uncovered the physics that works behind the bursting of bubbles.
The study appears in the journal Nature.
Lead author James C Bird, a graduate student at the Harvard School of Engineering and Applied Sciences (SEAS), said: "In order to minimize surface area, a bubble will be nearly hemispherical when it is in contact with a solid or liquid interface.
"We found that when these hemispherical bubbles pop, there is a two step process that can create a ring of smaller bubbles. While the resulting smaller bubbles have long been seen, until now the `how` has never been reported in the literature."
The curved nature of the bubble plays a critical role, as the shape leads to higher pressure on the inside than on the outside of the bubble.
Once the bubble opens up (i.e., bursts) it equilibrates, resulting in an inward net force due to the surface tension.
Bird added: "In the first step, the forces acting on the bubble cause the film to fold in on itself as it retracts and therefore trap a pocket of air in the shape of a torus, or donut. In the second step, surface tension breaks this torus of air into a ring of smaller bubbles just like surface tension breaks a thin stream of water from a faucet into individual droplets."
The cascade effect is short lived, occurring no more than twice in experiments to date.
Bird said: "The smallest bubbles no longer form a spherical cap and reintegrate into the liquid; this is the end of the cascade."
Since the popping process happens too rapidly to be seen with the naked eye, the team used high-speed cameras to film the collapse.
Based upon observing the video, they then constructed a numerical model to test and replicate their experimental assumptions.
The physics behind bursting appears to be independent of the material of the bubble.
The investigators were surprised to find that the ring effect is still seen with fairly viscous liquids like oil and even in solutions up to 5,000 times as viscous as water.