Do bubbles in Guinness go down?

Frequently Asked Questions (FAQ)

Here is a list of questions that we have been asked.  If you don't see an answer elsewhere on the site, it may be here.

  1. Why is this effect important?
  2. Is the shape of the glass important?
  3. Is the gas important?
  4. Method of pouring?
  5. What about viscosity?
  6. How did you get draught Guinness in the laboratory?
  7. What did you do with all the Guinness after filming?
  8. What's a widget?
  9. Do the model simulations and experiments agree?

Why is this effect important?

The effects are important because they tell us about how liquids flow, for example, in industrial processes and manufacture.  Liquid flows are important in a wide range of circumstances ranging from medicine to oceanography.  Drinks and food manufacturers also spend a considerable amount of effort and money to make their products appealing: easy pouring and a nice head is one of the aspects that we consumers look for in our beer.

Is the shape of the glass important?

The shape of the glass is not that important.  We obtained similar results in curved and straight glasses.  The pattern of flow will be different in the different shaped glasses, but not to the extent that would remove the effect.

Is the gas important?

Yes, it is.  See detailed answer to "Does it happen in any liquid?".

Method of pouring?

The way the Guinness is poured is not crucial.  Despite the angle/position of pouring, it is the upward flow of bubbles in the center of the glass that causes the circulation: see "Why do the bubbles go down?".  If you pour it so that there are hardly any bubbles produced, then you will not see the effect.

What about viscosity?

More viscous liquids flow more slowly (e.g., syrup).  The viscosity of Guinness will be only slightly different than that of water, but not much.  Certainly not enough to affect the movement of the bubbles that can be seen.

Guinness in the lab?

You might wonder how we got a good supply of Guinness in the lab.  The experiments took many days, and it would have been difficult to set up all the equipment in the local bar!  Fortunately, the Guinness company makes their draught stout available in cans, using a special patented pouring device (the "widget") that creates all those creamy bubbles.  Although we tried refrigeration before using them, condensation on the glass caused difficulties, so we used the cans at room temperature.

What happened to the Guinness you used?

Alas, we poured it away.  Don't be sad - you might never have seen any of the pictures if we had actually drunk any of it during filming!

What's a widget?

The widget is the patented device (also called a "smoothifier") in the special draught-flow can or bottle that contains a small amount of Guinness and nitrogen gas at very high pressure.  If you shake one of the cans you can hear it rattle.  Why not open a can up and have a look?  When the can/bottle is opened, the gas is suddenly released through one (or more) pin-sized holes.  This creates a lot of little bubbles.

Do the simulations and experiments agree?

Our experiments and conclusions agree very well with the simulations carried out by Prof. Clive Fletcher and his group.  The sinking bubbles in the experiments seem to be slightly larger than in the simulations. The experiments also suggest that the bubbles inside (that go up) seem to be nearer to the walls in the experiments.  An interesting detailed comparison would come from looking just below the head, where we see almost equal numbers of bubbles going up and down.

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(c) 2004 Alexander & Zare