Re: [APD] CO2 Experiment #2

["David Ozenne" <dozenne at tenfold_com>]

Stuart Halliday wrote:
 
> BTW, in my setup the bubbles get a lot smaller very quickly. But I
did 
> notice that by two thirds up they're just tiny bubbles and not
getting any 
> smaller.
> So they don't dissolve completely which seems odd to me.
> Perhaps it's to do with their surface area?
> We need a CO2 expert here! ;-)

I'll take a shot at it, though I'm just a computer programmer with a
physics background.
 
The rate of diffusion from the bubble into the surrounding liquid is
dependent on the surface area as well as the relative concentrations
of
gas in the bubble and the water around it. Small bubbles have a higher
ratio of surface area to volume than large bubbles.
 
Therefore, given identical concentrations of gas in the water and the 
bubble, a small bubble will shrink faster.
 
This appears at odds with the observational data that bubbles shrink
somewhat and then apparently stop shrinking. I've observed this
myself.
So we are led to the conclusion that the relative concentrations of gas
in the
water and the bubble must shift.
 
Undoubtedly some of this is due to other gases (O2 and N2) diffusing
into the bubble. But there's another possibility, which is that the CO2

diffusing out of the bubble is not going very far, and is in fact
remaining
in the boundary layer and travelling along with the bubble, increasing
the local concentration and slowing the dissolution.
 
One interesting fact is that a smaller bubble will move through the
water
with a lower Reynolds number. Essentially, this means that it won't
generate as much turbulence around as it rises. This could account for
a significant difference in the diffusion rate around the bubble.
 
To figure out which is the more important effect, I'd propose
eliminating
the air. Fill a container with water, and then pull a strong vacuum to
purge the air. Then bubble some CO2 in. If the bubbles shrink away to
nothing, this supports the air theory. If they still shrink a bit and
then
stabilize, this supports the boundary theory. (Incidentally, if they
shrink
to some much smaller size than they did when air was present and then
stabilize, we're back to the air theory, at least as the dominant
effect.)
 
Another interesting experiment might be to observe the behavior of
different sizes of bubbles. If the boundary layer theory is correct,
I'd
expect bubbles to shrink to some characteristic size, mostly
independent
of their original volume. But if the air theory is correct, the larger
initial
bubble should take longer to dissolve and therefore take up more air,
and stabilize at a larger size.
 
Now if we could only answer the question of why anybody cares about
the content of these tiny bubbles, we could get back to talking about
aquatic plants.
 
David Ozenne
West Jordan, UT

http://www.tenfold.com

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