Re: Soda Bubbles

[Leigh Palmer <palmer@SFU.CA>]

> Here's a fun one that a student brought to me recently:  After you pour a cup
> of soda, watch the bubbles that rise to the top (you have to wait until the
> initial foam dies down).  The individual bubbles that make it to the top will
> often times race toward oneanother.  I had never noticed this before but it
> really does happen.  There appears to be something at work other than
> random-chance collisions.    Why are the bubbles drawn to oneanother?   If
> this leads to some giant scientific discovery, my student has dibs on naming
> it the Geoff Phenomenon.  Thanks.
>
> Andy O'Neil
> C. K. McClatchy H. S.

Shooting entirely from the hip let me make some observations which may
help your budding scientist to answer his own question.

The dominant force acting in such phenomena (if I understand correctly
what was observed - I didn't do the experiment, but I will if it will
help) is almost always surface tension. Two bubbles coalesce because
the combination has a lower surface tension energy than the two have
when separated. The perimeter of the larger bubble will be less than
twice the perimeter of each single bubble because the double bubble
can be higher than the single bubble. The shorter the perimeter the
lower the interfacial energy, and systems tend to evolve spontaneously
toward lower (free) energies. The colescence requires the rupture of
the surface between the bubbles. There is usually a small energetic
barrier to be overcome before this happens; sometimes that barrier is
sufficiently high that coalescence doesn't occur.

The bubbles will distort the surface in their vicinity and thus feel a
mutual attraction. I'm not sure that it is the dominant mechanism in
the Geoff effect (my guess is that it is unlikely), but there is a
gravitational term to be considered. It might be interesting to look
at the phenomenon at high magnification in reflected light using a
camcorder in macro mode at a shutter speed as high as you can manage,
perhaps shooting through a magnifier as well. By playing with the
illuminant (say a slide projector with a grid focussed on the surface)
you may be able to discern the shape of the surface during the
phenomenon.

The movement of the bubbles toward one another is analogous to the
movement of two bowling balls toward one another when placed on a
taut elastic sheet, but in that case the energy term which is
minimized is the sum of gravitational and elastic potential energy.
(A different mechanism seems to be at work when two attractive people
of opposite sex are placed on a cheap mattress, but the result is
somewhat similar, though this system is somewhat external-observer
averse.)

Is Geoff looking for a good university science program after
McClatchy? We're always looking for good observers at SFU.

Leigh
Sacramento H. S.
Sacramento J. C. (now C. C.)
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Simon Fraser University