# In beer, why do bubbles travel faster upwards in the center?

This is a very odd question, but I just so happened to be observing a pint of beer and realized that the bubbles floating up seemed to travel faster towards the center of the glass than towards the edges of the glass.

My first assumption was that the sides of the glass cause friction which slows down the air from escaping. But I then noticed that there were 3 distinct layers of bubbles... The first layer being the bubbles hugging the glass. The second being bubbles slighly away from the glass and the third being bubbles directly in, or around the center, of the glass.

These layers seemed to travel slower dependent upon how close they were to the glass.

My second thought then was maybe some sort of differentiation between the layers. But I'm not very educated in fluid dynamics so I wasn't sure how accurate this assumption was...

Essentially, my question boils down to, why? Why did the center bubbles travel faster upwards than the edge bubbles?

• I feel an urge to replicate your experiment tonight. Perhaps several times just to get statistics… Aug 24, 2023 at 0:49
• This is an extremely fun question, and I feel obligated to mention the apocryphal tale that the cloud chambers so widely used in particle physics experiments were inspired by a physicist looking at the bubbles in his beer. Even more bold tellings suggest that the first bubble chamber used to track particles in the lab was actually filled with beer! Aug 24, 2023 at 1:55

Bubbles along or close to the axis of symmetry (middle) of the glass have more freedom to move upward than the ones closer to the inside edge, due to friction along the inside edge.

That is, the bubbles moving upward close to the edge of the glass experience drag and the further from the edges the bubbles are, the faster they are allowed to move. i.e., bubbles close to/along the axis move the fastest with slower flows the closer you get to the edge.

In fact, it is interesting to watch different types of fizzy drinks/beers and the patterns of motion the bubbles follow. For example, in Guiness beer (and perhaps others), people have noticed$$^1$$ that the bubbles sometimes move downward.

This sounds contradictory, but it is easily explained when you consider the types of currents moving bubbles can form$$^2$$. In this case, since the bubbles in the center flow upward a lot more freely than those near the edges (due to friction as explained), when they get to the top, they strike the surface and begin moving toward the edge. The net effect is a mild flow of liquid moving from the lower center to the surface, along to the edge, then back down and around again in a "circular" type flow.

$$^1$$ Standford university have a very nice illustration of this here.

$$^2$$ Bubbles are continually striking, pushing around, and dragging, the fluid molecules in their path which causes such currents.

• Interesting. So essentially, the bubbles arent just moving up but the current of the bubbles is moving in an almost oval pattern whereby the liquid moving towards the top flows more frequently in the center. But towards the edge, its actually moving downwards, assumable due to convection, which brings the "bubbles" back down to the bottom to restart the cycle? Aug 24, 2023 at 1:48
• If it was for the thermal convection, the layer near the glass would go upwards (assuming the beer is colder than the surrounding air and the glass). But the bubbles make everything complex. Aug 24, 2023 at 10:31