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Let's start at the point where
you have just poured your pint of Guinness, and it is starting to
settle. At the inside surface of the glass, the bubbles are
touching the
walls of the glass and they experience drag -
just in the same way as
you can feel if you slide your finger along a glass surface. At
the center
of the glass, the bubbles are not touching the walls, and are free to
go up: this is what bubbles of gas
really want to do when they are in a liquid, as we are used to
seeing.
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The bubbles at the center rise
rapidly until they get
to the top, just below the head (the "froth"). In doing this, they have
pushed
and
pulled the surrounding liquid with them. At the top, this liquid flowing
upwards hits the surface and flows outwards towards the edges of the
glass.
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The current is directed downwards
by the edges of the glass. As the flow moves downwards in waves,
it pushes and pulls the bubbles that are hanging around at the edges of
the glass. The flow can be seen as the dark lines of liquid (no
bubbles) that wave quickly down the inside of the glass.
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What goes around comes
around. More bubbles flow up at the center, and the circulation
continues.
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Eventually the settling process
comes to an end. More and more bubbles have been deposited into
the head of the beer during the settling, and the cycle
loses momentum.
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In summary:
bubbles at the center
rise up and create a circulation
in the glass. The circulation
causes bubbles at the edge of the glass to be pushed downwards.
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