Why do bubbles in Guinness sink?

TL;DR

This paper investigates why bubbles in Guinness sink during settling, revealing that glass shape influences flow patterns and bubble behavior through simulations and experiments.

Contribution

It demonstrates how glass shape determines flow circulation and bubble sinking or rising in stout beers, clarifying the mechanism behind the phenomenon.

Findings

Narrower glasses induce downward flow near walls and sinking bubbles.

Wider glasses cause upward flow and rising bubbles.

Flow patterns depend on glass geometry, affecting bubble behavior.

Abstract

Stout beers show the counter-intuitive phenomena of sinking bubbles while the beer is settling. Previous research suggests that this phenomena is due the small size of the bubbles in these beers and the presence of a circulatory current, directed downwards near the side of the wall and upwards in the interior of the glass. The mechanism by which such a circulation is established and the conditions under which it will occur has not been clarified. In this paper, we demonstrate using simulations and experiment that the flow in a glass of stout depends on the shape of the glass. If it narrows downwards (as the traditional stout glass, the pint, does), the flow is directed downwards near the wall and upwards in the interior and sinking bubbles will be observed. If the container widens downwards, the flow is opposite to that described above and only rising bubbles will be seen.