It is a mystery that has confounded many a pub-goer. Locals testify to the fact that bubbles in their booze always go up, with one exception.
The Royal Society of Chemistry has finally solved the puzzle why Guinness bubbles break the rule by going down - yet still end up at the head.
In a lunchtime recreation, members of the RSC followed the perfect method of pouring a can of Guinness and watched it begin to settle while noting the reaction. When the bubbles touch the glass they experience drag, as happens when a finger is slid along the surface. At the centre of the glass, the bubbles are free to rise rapidly, as they naturally do in a liquid, creating a circulation.
The bubbles push and pull the surrounding liquid with them by the time they reach the creamy head. At the top, this liquid flowing upwards hits the surface and flows outwards towards the glass edge, which pushes the current down. As the flow moves downwards in waves, it pushes and pulls the bubbles that are hanging around at the glass edge pulling them down. More bubbles flow up at the centre and the circulation continues.
Thankfully, the thirsty drinker doesn't have to wait too long before the Guinness settles and the cycle loses momentum. Although there are perfect ways to pour a pint of Guinness from the tap or can, failure to do so is not crucial to how the bubbles behave.
Dr Andrew Alexander, senior lecturer in chemical physics at the University of Edinburgh who first carried out the experiment, said: "I'd wanted to try and capture the bubbles going down as I had obviously wondered whether it really did happen, having drunk a few Guinness during my time at university, or whether it was an optical illusion created by the waves in the drink that don't contain any bubbles. Nobody had carried out the experiment before.
"To capture the image, we had a camera which uses 4500 frames a second and a zoom lens of times 10. When we saw the bubbles really were going down, I was immeasurably happy. We then filmed it as a colleague pointed out that people might have said all we did was turn the photos upside down. But it's true. The circulation cells in the glass provide the same effect like you see in a tornado."
A spokesman for the RSC said: "Guinness is good for this experiment as the bubbles are small, due to being released at high pressure by the widget and therefore easily pushed around. The gas in the bubbles is also important. In lager beers, the gas is carbon dioxide which is more easily dissolved into the liquid. The gas in Guinness bubbles is nitrogen - not so easily dissolved and therefore not prone to grow larger.
"Finally, the contrast between the dark liquid and the light cream bubbles make the bubbles much easier to see. We're pleased to have finally solved this mystery in time for St Patrick's Day when many people will no doubt be enjoying a pint or two."