The Physics of Guinness

Today is St Patrick’s Day.

To any person with close relatives that are Irish, you probably know of this as a semi-religious holiday. To any of the rest of you, it is probably associated with the Cheltenham festival, various exaggerated Irish stereotypes, wearing green, and drinking green cocktails and Guinness.

When you inevitably partake in this drinking ‘tradition’, it is customary to see many strange things along your way to the pot of gold at the end of the rainbow. However, not all of this is due to the effects of alcohol.

You stand at the bar, and as you watch your pint of Guinness settle after being poured, you think to yourself. “How much have I had? My drink is frothing downwards.”

To the seasoned Guinness drinker, this perhaps, is no uncommon sight. There will, however, be some of you that have never noticed this. Let me tell you this isn’t Irish trickery, there is a scientific reason that this happens.

There are several things going on here, but what you see is in short down to the flow created by nitrogen bubbles specifically, and the glass in which the Guinness is being poured.

Some curious scientists deemed that the study of the air particles within stout was worth having a look at. As a result scientific studies were conducted, offering insight into the mechanisms, and another five years later provided further insight into unanswered questions from this and other papers.

These papers explain how the specific gas used to nucleate the bubbles under changing pressure (make the drink fizzy when it opens) alters the size and properties of the bubbles.

When any pint is poured and it begins to settle, the bubbles rise, as you would expect, as the gas is less dense than the fluid in which they are maintained. This happens as a function of the buoyancy of the gas. However, what is so special about Guinness is due to the properties of the Nitrogen and Carbon dioxide foam within the stout, which differs from Carbon dioxide alone in lagers. The use of nitrogen as the foam means that the stout does not have the same potential for spontaneous bubble formation, as a lager; and as such would be much flatter without interference, within a can or bottle as an example. As a result, these methods of drink distribution have within them a ‘widget’ responsible for the release of gas into the Guinness, which allows the formation of the head and a fizzier drink in comparison.

However, nitrogen bubbles, once in the head of the pint, are forced to the sides of the glass. At this point that the foam reaches the edges, and the flow of bubbles from the centre of the glass continues. As the flow in the head of the pint will not counter the majority flow from the glass, the remaining foam is forced down, continuing the current that is observed as downwards bubbles.

Later papers carried out to investigate the physics behind this current formation determined that this only occurs in certain pint glasses. By this I mean glasses that are curved and become narrower from the top of the pint to the bottom.

In these glasses there are areas of lower bubble density around the edges of the glass, and this creates a drag on the liquid that causes it to be moved in a current.

It is describe how a uniform distribution of bubbles in this shape of glass would cause an area of low pressure at the edges, which then causes the flow of liquid to follow this.

Guinness flow in essence is loosely comparable to telling all of the people in the local that there is cheaper pints at the end of the bar at which you are standing. When they all move in that direction, their seats are empty, and you can fill them (inverse flow into low pressure). But if there are people already at that side of the bar (a straight glass, or one that is wider at the bottom), no empty seat for you, and no inverted liquid flow.

Written by Benjamin Dias-Dougan