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> > > But it ain't so simple. The question begs: what's "gross"?
> >
> > If I understand your question correctly, I believe we are talking
> > something on the order of 20,000 atmospheres.
> >
> Actually, you've mentioned the 20K ATA figure several times.
> I'm curious where this came from. Surface tension of
> five gas atoms in a huddle? :)
I saw a presentation by David Yount (bubble physics guy) in 1989. The
gist of what he said was that there *MUST* be micronuclei present prior
to the dive, because it would take a pressure supersaturation of more
than 20K atm to spontaneously form stable gaseous voids in fluid. In
fact, I remember him alluding to an experiment whereby gel exposed to
something like 10 atm and immediately decompressed to 1atm gets full of
bubbles that grow. The same gel compressed to 20K ata and brought
straight to the surface gets no bubble formation. The theroy being that
the 20K compression crushes nuclei into oblivion, and without nuclei,
bubbles do not form.
I've been reluctant to say it outright, because I only have my memory to
go on (I did not take notes). I think I might have one of Yount's
references to this effect. In my CFT, I will try to find the reference.
> Clearly, even from staring at a champagne glass, it's obvious
> that some chaotic agent besides surface tension is at work in
> seeding bubble formation - like, for instance, a microscopic
> span that only requires X gas molecules to fill, causing
> just enough of a gas accumulation to act as a seed
> under conditions of supersaturation. So long before surface
> tension comes into the picture, there's a seed. Actually, as I
> recall, there's been some recent work that sheds light on the
> pure-physics end of bubble seeding, and as I also recall, it had
> to do with exactly that - cracks or spans in molecular-width
> ranges - I'll have to go look it up.
If you find anything, please post it.
> Right, here's the thing, and it assumes that bubbles don't
> just pre-exist, and niggles into the nucleation (seeding) issue,
> which is admittedly squirrely territory.
I'll try to brush up on this topic. I know what I want to say, but I
don't trust my memory (I read all this stuff almost ten years ago).
> When we assume that bubble growth at the seed level is a factor
> of absolute pressure
There are other factors besides ambient pressure, namely bubble size (which
determines proportional effect of skin tension on internal bubble pressure).
>, we are supposing that bubble
> seeds are comprised of a composite of the representative
> dissolved gasses, and that supersaturation is acheived
> for the whole of the composite mix, rather than looking
> at the possibility that supersaturation occurs
> for each gas independantly, based upon the relative
> partial pressure of the dissolved versus inspired gasses.
>
> Since we allow on-gassing to be independant for each gas, why don't
> we make equivalent consideration for off-gassing?
>
> On the acuteness issue, I don't think so. It would seem
> to be more a matter of crossing a single supersaturation
> threshold for each gas.
Again, I'm not sure I follow you here.
> > Exactly what aspect of the steepeness of the gradient in the former case
> > (He in blood, N2 in lungs) would cause bubbles to sponatneoulsy form?
>
> The lack of He in the lungs increases the partial pressure gradient
> for *helium* (N2 has nothing to do with it, directly).
And how does this steep gradient cause bubble formation? Seems to me
that He would be encouraged to leave the blood into the lungs, rather
than enter the bubbles and cause them to grow. What I'm asking is, if
you eliminate He in the lungs, how does this encourage bubble growth?
> > Would it be the rate of diffusion across the alveolar membrane? If so,
> > where do the voids (bubbles) come from, exactly?
> >
> I'd have to point back to the seeding issue on this one.
I promise you, the seeds are already there.
Rich
Richard Pyle
Ichthyology, Bishop Museum deepreef@bi*.bi*.ha*.or*
1525 Bernice St. PH: (808) 848-4115
Honolulu, HI 96817-0916 FAX: (808) 841-8968
"The views are those of the sender and not of Bishop Museum"
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