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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?

Qmechanic
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Corey Mitchell
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1 Answers1

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Your initial assumption is correct.

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.

joseph h
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