When Guinness beer is poured into a pint glass, small-diameter bubbles (roughly 1/10 the size of those in carbonated drinks) disperse throughout the entire glass and the texture motion of the bubble swarm moves downwards.

Although some models have been proposed to explain how the downward movement of a bubble swarm as waves are caused in Guinness beer, the mechanism underlying the texture-formation had remained an open question.

Researchers working at Osaka University said that texture-formation in a pint glass of Guinness beer is induced by flow of a bubble-free fluid film flowing down along the wall of the glass. They said this phenomenon is similar to roll waves commonly observed in water sliding downhill on a rainy day. The research results were published in Scientific Reports.

The question is of interest in realms outside of the pub. Researchers working at Osaka University said there are a large number of small objects in nature (such as fine rock particles transported from rivers to the sea and microorganisms living in lakes and ponds) as well as small objects in industrial processes as well whose movement is important to understand. They said that their research results may be useful in understanding and controlling flows of bubbles and particles used in industrial processes as well as protein crystallization and cell cultivation used in the life sciences.

The researchers said that because the opaque and dark-colored beer obstructs physical observation in a glass and computation using supercomputers is necessary to conduct numerical simulation of flows including a vast number of small bubbles in the beer, the team produced transparent "pseudo-Guinness fluid" by using light particles and tap water. They filmed the liquid with a high-speed video camera applying laser-induced-fluorescence to measure its movement. In addition, using molecular tags, they visualized the fluid's irregular movement.

With these methods, the team poured their pseudo-beer into an inclined container to observe how the texture formed. They said that texture-formation appeared only in the region roughly 1 mm away from the inclined wall and didn't appear at all in the vertical wall vicinity.

They also observed a clear-fluid (bubble-free) film flow down along the inclined wall, capturing velocity and thickness of bubble-free film. While the texture appeared when the glass inclination angles were small, no texture appeared when the angles were large. The researchers said this demonstrated that the texture-formation in a glass of Guinness beer was caused by the roll-wave instability of the gravity current.