Bubbles clustering while pouring stout beers?

The texture of a stout beer and why it appears to descend as it's poured into a glass has now been explored by fluid dynamists, revealing what has, until now, been a bit of a mystery.

During the 70th annual meeting of the American Physical Society's Division of Fluid Dynamics, being held Nov. 19-21, 2017, in Denver, Colorado, researchers from Osaka University will present their work exploring the fluid dynamics behind this type of bubble clustering in stout and nitrogenized stout beers and carbonated drinks.

The researchers studied how long it took to develop the "void fraction," which is the fraction of the channel volume occupied by the gas phase, while also tracking the velocity of the bubbles. They were particularly interested in the propagation of the number density distribution, which describes the degree of concentration, of the bubbles and their texture near the inclined wall of a pint glass.

"Our velocity measurements are based on optical visualizations," said Kazuyasu Sugiyama, a professor at Osaka University. "We measured the velocity of each bubble, descending wave of texture, and the liquid velocity."

They used a microscope to zero in on the individual movement of bubbles. Then, using the brightness of the images, the researchers were able to use a method called "particle-tracking velocimetry" to measure the velocity of each bubble.

"There is a velocity difference between bubbles and the texture of bubble distribution, and our findings suggest that there is some hydrodynamic instability that results in wave formation," Sugiyama said. "We can observe the texture of bubbles for just 119.5 seconds after the pouring. This iconic texture is also called a 'cascade.'"

Intriguingly, the group's measurements revealed that the local void fraction and individual movement of the bubbles' velocity increase and decrease repeatedly with a time delay, according to Sugiyama. This implies that the texture pattern is composed of a viscous (thick) fluid that contains fewer bubbles, which causes thrust and suction. The bubble cascade splashes back and forth within the interior of its container (pint glass) and ends up forming in front of and behind the viscous fluid with fewer bubbles.

These findings may help "control multiphase flows for both general industries and beer production, such as those in fermenting vessels, mixing of colloids, and aeration in chamber," Sugiyama said.