Discovering the secrets of bubbles

Have you ever wondered why bubbles in champagne and sparkling wine fizz up in a straight line while bubbles in other carbonated drinks, like beer or soda, don’t?

Researchers from Brown University and the University of Toulouse have now conducted a series of numerical and physical experiments to find out why. The findings, described in a Physical Review Fluids study, not only explain what gives sparkling wine its line of bubbles, but may also hold important implications for understanding bubbly flows in the field of fluid mechanics.

Roberto Zenit, Brown engineering professor and author of the study, said: “Most people have never seen an ocean seep or an aeration tank but most of them have had a soda, a beer or a glass of champagne. By talking about champagne and beer, our master plan is to make people understand that fluid mechanics is important in their daily lives.”

The team’s goal was to investigate the stability of bubble chains in carbonated drinks. In champagne and sparkling wine, the gas bubbles that continuously appear rise rapidly to the top in a single-file line and keep doing so for some time. This is known as a stable bubble chain.

In other carbonated drinks, like beer, many bubbles move off to the side, making it look like multiple bubbles are coming up at once. This means the chain isn’t stable.

The researchers set out to explore the mechanics of what makes bubble chains stable and if they could recreate them, making unstable chains as stable as they are in sparkling wines. They found that stable chains occur due to ingredients that act as soap-like compounds called surfactants. These molecules reduce the tensions between liquid and gas bubbles, making them rise smoothly.

Since bubbles are always small in beverages, surfactants are key to produce straight and stable chains. Beer contains surfactant-like molecules, but the stability of the chains is dependent on the type of beer. In contrast, bubbles in carbonated water are always unstable since there are no contaminants helping the bubbles move smoothly.

The experiments conducted by the research team were relatively straightforward. To observe the bubble chains, the researchers poured glasses of carbonated beverages. They then filled a small rectangular plexiglass container with liquid and inserted a needle at the bottom so they could pump in gas to create different kinds of bubble chains.

They then gradually added surfactants or increased bubble size. They found that when they made bubbles larger, they could stabilise chains without surfactants. When they kept a fixed bubble size and only added surfactants, they were also able to stabilise chains.

The two experiments indicate that there are two distinct possibilities to stabilise a bubble chain: adding surfactants and making bubbles bigger.

The researchers performed numerical simulations to explain some of the questions they couldn’t explain through the physical experiments, such as calculating how much of the surfactants go into the gas bubbles, the weight of the bubbles and their precise velocity.

They plan to keep looking into the mechanics of stable bubble chains in an effort to apply them to different aspects of fluid mechanics, especially in bubbly flows.

“We’re interested in how these bubbles move and their relationship to industrial applications and in nature,” Zenit said.

Image credit: iStock.com/DNY59