Q: At one of my local pubs there is a juke box. In the display on the front
are columns of liquid containing rising bubbles. As the bubbles rise they get
smaller 鈥� some of the smallest even vanish before they reach the top of the
column. Why do the bubbles get smaller rather than larger as they rise, as one
would expect?
A: The bubbles are of water vapour, rather than air. They are created by a
small heater at the bottom of the tube, and flow upwards. As they rise, some
of the vapour condenses back into the liquid, so the bubbles get smaller. The
liquid in the tube will quickly rise to a stable temperature where the heat
from the vapour and the loss to the surroundings is equal. This system has a
major advantage in that there are no moving parts or pumps, and the liquid is
kept in a sealed system.
A: Each column has an electric heating coil in the base, which vaporises
some of the fluid which is surrounding it. By pulsing the heating current on
and off, discrete bubbles are formed rather than a continuous stream. As these
bubbles rise up the column the vapour condenses back into the liquid, causing
the bubbles to shrink. The heating period is so arranged that the bubbles
completely condense by the time they reach the top of the column.
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