One of the mysteries of stout beers is the sinking bubbles observed
while a recently poured pint is settling. Our
completes the explanation of this phenomenon. We show that in fact
stout beer bubbles may sink or rise: which they do depends entirely on
the shape of the glass!
One of the distinctive features of stout beers is the sinking bubbles
observed while the head forms in a recently poured pint
experimental and simulation work has demonstrated that this is due to
a downwards current close to the edge of the pint glass (the current
and bubbles move up in the centre of the glass). The small size of the
bubbles found in stout beer allows them to be dragged downwards. Our
research explains why this current forms and the role played by the
shape of the glass in promoting it. This resolves the final mystery
associated with this puzzling phenomenon.
(a,b,c,d) Glass slopes inwards: sinking bubbles. (a) Buoyant bubbles begin to rise. (b) This leaves denser, bubble free, beer at the wall. (c) The heavier beer flows downwards under gravity. (d) Bubbles are dragged downwards by the current.
(e,f,g,h) Glass slopes outwards: rising bubbles. (e) Buoyant
bubbles rise. (f) Bubbles accumulate at the wall, reducing the density
of the beer. (g) The lighter beer rises due to its buoyancy. (h)
Bubbles are observed to rise.
Simulation of bubbly flow in a pint of
stout beer. Bluish colours indicate low bubble concentration and
reddish colours indicate high bubble concentration. Note that a region
of low bubble concentration forms at the wall of the glass.
A neat way to confirm our theory is to
pour stout beer into a tilted measuring cylinder. Sinking bubbles can
be seen on the lower surface and rising bubbles on the upper