Science Quiz #3: Do bubbles in a pint of Guinness go up or down?

You may be wondering what bubbles in a pint of Guinness have to do with science. It turns out to be more than you might expect.

According to the Irish physicist in the video below it took until 2004 to figure out the answer to this question (which he implies was debated for maybe a century before that), so don't feel bad if you don't know the answer.

Here is the quiz:

What direction do the bubbles in a pint of Guinness move?
A. Up.
B. Down.
C. Neither up nor down.
D. Some move up and some move down.

This is one topic where a picture, or in this case a video is really more valuable than many words, because it shows in detail how the bubbles move. You'll find the answer to the quiz at around 2 minutes and 50 seconds in the video, and for those who can't watch the video, I'll wait until page 2 to post the answer so as to not prejudice your responses, though you can also find the answer in the scientific paper "Waves in Guinness" linked below if you don't want to wait.

Video with the answer at 2:40:
Guinness Science - Sixty Symbols


Just in case you think this isn't really a science topic, here is a link to a 15 page long scientific paper called Waves in Guinness

Our theory involves a physically based regularization of the basic equations of the two-phase flow, using interphasic pressure difference and virtual mass terms, together with bulk or eddy viscosity terms. We show that waves can occur through an instability analogous to that which forms roll waves in inclined fluid flows, and we provide a description of the form of these waves, and compare them to observations. Our theory provides a platform for the description of waves in more general bubbly two-phase flows, and the way in which the flow breaks down to form slug flow.

The Guardian even published a small article about this topic with a cute play on words for the title:
The Fizzics of Guinness

Look closely at a pint of Guinness and tell me: do the bubbles go up, or do the bubbles go down? Why is the head coloured the way it is? Is foam a gas, liquid or solid? An Irish physicist discusses

A little bit of a spoiler alert: the physicist in the video doesn't even drink, and just buys the Guinness to study and explain the physics, though I imagine other physicists visiting the same pub probably drank and studied their Guinness. Lord Kelvin is a famous physicist mentioned in the video, who was interested in this topic.

Here's a hint: The bubbles in Guinness contain nitrogen, so don't automatically assume they will all go up like the bubbles in regular beer which are not made of nitrogen. It turns out there's also some interesting physics related to that which explains the unexpected color of the head. There is really interesting physics almost everywhere we look, but I have to wonder how many people have downed their share of Guinness without even wondering about the physics of it?


Links to prior threads in this science quiz series, where the answers are not always what you might expect:
Science Quiz #1: Is the Earth's North Magnetic pole a North Magnetic Pole?
Science Quiz #2: Is E=mc² right or wrong?

I've drank many a pint honestly never considered I'll check the video when I get to an area with a decent connection.

Star and flag

I cheated, the answer is ........

I knew only because I've seen the video before because I'm a big Sixty Symbols fan, as well as Periodic Videos, nottinghamscience, Objectivity and DeepSkyVideos. Basically all of the Brady videos He really asks some great questions sometimes, and the professors he gets are all wonderful teachers. Very nicely put together op by the way, Quality.

originally posted by: Qumulys
I cheated, the answer is ........

I knew only because I've seen the video before because I'm a big Sixty Symbols fan.

Nice to meet another sixty symbols fan. Another series I like which you didn't mention is called Veritasium. I found one of those particularly interesting where even the physicists needed some help to figure out the physics, so I may do a future thread on that topic. When the physics stumps physicists regarding mundane objects we see every day, it's interesting.

Very nicely put together op by the way, Quality.

Thanks for the feedback.

I muvhprefer staring into a Black and Tan than a Stout-- hiccup.

a reply to: Arbitrageur

When they go up they are up, and when they go down they are down, and when they go only halfway up, they are neither up nor down.....

Was an interesting watch mind.....thanks.

a reply to: Arbitrageur

I was just listing the video channels on youtube made by Brady. But I'm also a big Veritasium fan (Derek) as well, another great science channel is SmarterEveryDay (Destin) who covers a wide range of topics and never ceases to share and explain something cool. I think actually they both did a collab video on the coriolis(sp?) effect where you watch both videos at once. It was very cool actually and worth checking out if you can get them to both play at the same time. Then there's the original VSauce channel, CPG Grey and Thunderfoot (only when he's on about science though). There is actually some really great quality available on YouTube these days! It's a shame that this stuff isn't on tv more during after school hours for the kids.

Oh, I better give a shoutout to Astronomy Live by an ATSer!

a reply to: Qumulys

To be honest after 3 pints im more interested in what they are doing inside me and how they are escaping

a reply to: Arbitrageur

Before watching the video, I would like to posit a possible answer.

I believe that bubbles in a pint of Guinness travel both upward, and downward, depending on several factors. First, when a pint is freshly poured, the liquid within the glass is still bleeding off the motive force applied to it by gravity and pumping action in tandem, when it falls from the pumps nozzle, into the glass. Then, when an amount of time has passed, one can say that within a certain framework of thought, the movement of gas in the glass stops being attributable to the pumping process, and starts to operate on its own dynamic.

I believe the gas bubbles in the glass move both up, and down after the settling period, because when they rise from the bottom of the glass, they release some gasses, and become unable to sustain their buoyancy, falling back down into the bottom of the glass, where they may gain buoyancy again and then return to the surface or near to it.

originally posted by: TrueBrit
I believe the gas bubbles in the glass move both up, and down after the settling period, because when they rise from the bottom of the glass, they release some gasses, and become unable to sustain their buoyancy, falling back down into the bottom of the glass

An excellent guess my friend, but I'm not sure if he discusses buoyancy in the video, however it's discussed in the scientific paper which maintains that the bubbles never lose their buoyancy, specifically it says:

Guinness can be considered a mixture of a viscous (black) liquid and (buoyant) nitrogen bubbles.

If we define buoyancy as:

"b : the power of a fluid to exert an upward force on a body placed in it; also : the upward force exerted "

Then we might conclude that if buoyancy is the only force at work, the bubbles must rise.
However, there are other things taking place besides buoyancy in this case, related to waves and fluid dynamics.

a reply to: Arbitrageur

Having watched the video now, I understand that the issue is somewhat more complex than I had imagined it, although I had considered certain elements of surfactant action with regard to the process. But the video was very interesting indeed.

Incidentally, I do not think that buoyancy issues would produce an up only action, since a bubble could loose buoyancy upon reaching the upper part of the glass, where the pressure is somewhat lower than at the bottom of the glass, then fall again until they gain buoyancy again by transfer of gasses from the liquid to the bubble itself... Bah, tis a thorny conundrum either way!

I have not watched the video yet and may not even watch it depending on how my day goes.
But I will give my guess, educated by watching Guinness in a glass.
The bubbles go down around the outside of the glass and rise in the center of the glass. That is until the head stabilizes and then all the bubbles rise.

originally posted by: TrueBrit
Incidentally, I do not think that buoyancy issues would produce an up only action, since a bubble could loose buoyancy upon reaching the upper part of the glass, where the pressure is somewhat lower than at the bottom of the glass, then fall again until they gain buoyancy again by transfer of gasses from the liquid to the bubble itself... Bah, tis a thorny conundrum either way!

Depends on what you mean by "loose buoyancy", you're right the pressure of the fluid is lower at the top than the bottom, but this lower pressure also allows the same bubble to occupy more volume, which is why they tell scuba divers to never hold their breath as they ascend...if they do the air in their lungs can expand.

No matter how much less the pressure is at the top than at the bottom, the fluid is still denser than the gas bubbles so they will always be buoyant.

a reply to: butcherguy
Good guess, but if it's that obvious that's what's going on I have to wonder why this was such a matter for debate for a century as the physicist suggests.