Sinking Bubbles in Stout Beers

Anyone who has ever tried Guinness or another stout beer knows that the bubbles in the glass appear to sink. This suggests that they are driven by a downward flow, the velocity of which exceeds the upward velocity of the bubble due to the Archimedean force. The existence of such a flow near the wall of the glass implies that there must be an upward flow somewhere in the interior. The mechanism of such a circulation is, however, unclear. In this work, we demonstrate that the circulation in a glass of stout—or any other container with a bubbly liquid—is determined by the container's shape. If it narrows downwards (as the stout glass does), the circulation is directed downwards near the wall and upwards in the interior. If the container widens downwards, the circulation is opposite to that described above.