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Re: [APD] CO2 Experiment #2

> 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! ;-)

David Ozenne replied:
> I'll take a shot at it <snip>,
> 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. <snip, contrasts with observation>

Yes -- It's a surface area and relative gas
composition problem.  CO2 diffuses into the water,
and O2 and N2 diffuse into the bubble *from* the
water (because of partial pressure) so the bubbles
will *never* completely dissolve.

Fluid dynamics are terribly complicated -- we build
$500K cell sorters and have *lots* of issues with
micro-bubbles, turbulence, and other funny things
dealing with partial pressures of fluids and flow
(both liquids and gasses are mathematically treated
as fluids).  (We're looking for a good Fluidics
Engineer, so contact me offline if you're
interested.)  Fluidics engineers do math, and lots of
it, at or above the level of rocket scientists and
astronomers, so they're fairly hard to find.

This technique is famously used by water beetles
and the water spider, Argyroneta aquatica:  Go to
the surface, and a water bubble is "captured" over
the hairy abdomen.  Then, submerge:  The critter
consumes O2 from the bubble, and exhausts CO2 into
the bubble.  However, the CO2 dissolves *very well*
into the water, so the critter can stay submerged
a *lot* longer than it should have been able from
the original bubble -- as more CO2 builds up in
the bubble, it diffuses into the water, and as
the relative O2 in the bubble decreases, more O2
diffuses into the bubble from the surrounding water.
Even in some oxygen-depleted waters, the critter
may be able to stay underwater *indefinately*
(although more typically the bubble shrinks to
a steady-state, and the critter goes up to replenish
a more efficient gas mix, because N2 typically
accumulates in the bubble).

Take-away message, and much of the bane of our
instrument's existence:  The bubble will *never*
fully dissolve.  To force it to "dissolve", you'd
need to dramatically increase the fluid pressure
(and most of the time that's constant, or even
decreasing, as you move to the surface).

(Our instrument is on sale this week, so contact me
offline if you want to buy a $500K sorting flow
cytometer -- it's the size of a pool table.)

BTW, Paul Krumbholz mentioned this in the past
on the APD (28-Jul-1995, 


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