The physics of fizziness

When bubbles burst the thin liquid film at the surface, the "bubble cap" that separates the bubble from the atmosphere disintegrates and the resulting opened cavity collapses. This, in turn, causes an upward jet that releases a few tiny droplets as it breaks up.

During the American Physical Society's Division of Fluid Dynamics (DFD) Meeting, Nov. 23-25, 2014, in San Francisco, Calif., Thomas Seon and Elisabeth Ghabache, researchers working for CNRS & UPMC at the Institute d' Alembert, in France, and their colleagues will describe the intricate roles of bubble shape, capillary waves, gravity and liquid properties in bubble-bursting jet dynamics, which form the prelude to aerosols -- including unexpected results which should help pave the road to the control of bubble-bursting aerosols and may even find more novel applications such as the fine-tuning of aroma diffusion in champagne or wine.

"Bubble-bursting drops are ubiquitous in everyday life and provoke the pleasant fizzy sensation when savoring a glass of sparkling wine, champagne or any soda," said Seon. "But on a larger scale, they constitute sea spray aerosols or sea mist, which plays a huge role in the chemical exchanges between ocean and atmosphere."

During the past 60 years, while numerous laboratory studies have documented bubble-bursting drops' properties such as their ejection speed, maximum height or size, a comprehensive picture of the mechanisms at play is still lacking. "In particular, the sequence of violent events preluding jet formation and the roles of liquid properties remain elusive," explained Seon.

So what exactly are the basic underlying capillary fluidic effects explored by Seon and colleagues? Their work is based on what happens when the film separating the bubble from the atmosphere (surface) drains and bursts -- leaving an unstable opened cavity. This cavity is millimeter-sized, so the restoring force, which tends to return this hole to a flat equilibrium, is capillary -- not gravity -- driven.

"Capillary waves propagate along this cavity," pointed out Seon. "And the collapsing waves give rise to a high-speed vertical jet that shoots out above the free surface. The jet then fragments into droplets, generating an aerosol of one to 10 droplets."

Seon, Ghabache and colleagues demonstrated that droplet ejection depends not only on the bubble geometry, but also, critically, on the liquid properties. "We characterize the relation between all of these parameters," he said. "One of the most counterintuitive results is that bubbles bursting in more viscous [thicker] liquids produce smaller and faster droplets."

The researchers' unexpected results should help pave the road to the control of bubble-bursting aerosols and may even find more novel applications such as the fine-tuning of aroma diffusion in champagne or wine. "By changing the viscosity of champagne slightly, we could generate an aerosols-optimizing-the-diffusion aroma," Seon noted.