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View Full Version : Nucleation points in fizzy drinks...

04-18-1999, 12:02 AM
Dear Cecil,

The theory on the nucleation points that cause bubbling from certain spots in drinks evoked an interesting question:

Hypothetically speaking, if a 100% scratch-free vessel were to be made, where would the bubbles arise from?

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Smile, it keeps people wondering what you're upto!

04-19-1999, 03:49 PM
There's a mathematical relation between the size of a bubble (actually, the radius of curvature) and the pressure inside the bubble: basically, the smaller the radius of curvature, the higher the pressure.

This is why scratches are nucleation sites:
a bubble forming inside a small scratch has a much greater radius of curvature for the gas/liquid interface than a spherical bubble of the same volume would have, thereby decreasing the pressure necessary to form the bubble.

In your hypothetical case where there are no good nucleation sites, eventully a bubble will form on a "bad" one: turbulence, a shock wave caused by you walking past the flask, or even just a momentary defect in the liquid itself caused by the random motion of molecules.

Once a bubble does form, a large fraction of the liquid will decide that this is the chance it has been waiting for (it's much easier to increase the size of an existing bubble than to form one in the first place) and join the party, whereupon the bubble grows very quickly and splashes the rest of the liquid all over the lab (not that I would have ever done anything like this myself; someone, uh, told me about it, yeah, that's it...). This is called "bumping".

To avoid this, it's common practice to add boiling chips (generally small pieces of inert porous material; sometimes glass beads are used instead) to liquids before heating them. This provides lots of nice nucleation sites and avoids bumping, provided of course that you remember to put in the boiling chips
first and don't try to add them when the liquid's already at the boiling point.

04-19-1999, 04:04 PM
Actually, I just realized that in the title you specifically mentioned 'fizzy drinks' while what I was talking about was more connected with boiling.

For a fizzy drink in a perfectly smooth container, you probably won't get bumping OR bubbles unless you shake it; you'll get some outgassing from the surface of the liquid itself.

05-14-1999, 08:21 PM
guess this is an old topic, but I just got here!

You wrote that nucleation sites provide a haven for bubbles to form. LIkewise, rain and
snow form around dust motes in the air.

So... If I toss a handful of dust in the air on a humid day, will I generate a mini- rainstorm?

If not, why not?
<HR>

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Pal ;-)

05-15-1999, 01:31 PM
So... If I toss a handful of dust in the air on a humid day, will I generate a mini- rainstorm?

Pal,
The day would have to be more than just humid. The air would have to be supersaturated with water (i.e. >100% humidity). Chances are, at your level of the atmosphere, these conditions don't happen very often. However, this process is similar to seeding the clouds. It is possible to fly high into the atmosphere where the air is supersaturated and to drop nucleation devices into the clouds and start a storm. I'm not sure what it is that they usually use to do this, though.

TheDude

05-15-1999, 02:01 PM
Hi guys,
Thanks for the good response I got on my question... just wanted to share some of my knowledge too...

As far as I know, it's SILVER IODIDE they use to seed clouds. Since cloud-seeding can prove expensive at times, at times they just burn the silver iodide - it goes up with the smoke and gives almost the same effect - of course, not AS good an effect, most of it gets wasted in the bargain.

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Smile, it keeps people wondering what you're upto!