## Vacuum bubbles

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### Vacuum bubbles

This is an interesting challenge I saw on another forum. Nobody could answer it, and I can't explain it, but it's fun speculation.

If you have a balloon about 1/2 meter under water, and burst it, you get one or more bubbles rising to the surface.

If you had a thin glass sphere with a fairly hard vacuum inside, and you broke that sphere about 1/2 meter under water, would you get vacuum bubbles rising to the surface?
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### Re: Vacuum bubbles

Hint: bubbles are gas.

Heh.

TheVat

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### Re: Vacuum bubbles

BiV and Canady, cavitation might be a relevant effect to consider
https://en.wikipedia.org/wiki/Cavitation

"would you get vacuum bubbles rising to the surface?"
liquid water suddenly exposed to vacuum will vaporize to some extent, tending to fill the void with water vapor
so if there are some bubbles shot towards the surface they would not strictly speaking be *vacuum* bubbles
they would contain some vapor

I imagine that the problem is not well-posed because to know what happens you might need to know the temperature of the water and how large the glass sphere is compared with the half meter depth, and the *way* it is broken.

There would likely be shock waves produced---they might be asymmetric---there might be very dynamic processes akin to small explosions, also one needs to know how much energy is being released, I guess.

I find it hard to picture.

Marshall
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### Re: Vacuum bubbles

Marshall,
You are very correct, and astute. Assume STP for the water. Let the bubbles be about 50 mm in diameter, and they are broken by a sharp, quick blow to one side, using a spring loaded hardener plunger.

You are also right about the shock waves. Someone, I can't remember who, was doing research off Indonesia a few years back, and they were bursting vacuum spheres at some depth in order to produce shock waves, for some purpose that I cannot recall.
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### Re: Vacuum bubbles

Even if you could achieve a perfect vaccum in the glass sphere, there would still be tiny bubbles induced upon collapse due to a combination of cavitation and pressure shock causing dissolved gasses already present in the water to come out of suspension. I imagine the effect witnessed by a diver nearby would be a loud underwater bang, followed by the rise of some extremely tiny fizzy bubbles from dissolved gasses.

BTW, the term 'vaccum bubble' is misleading ... it'd be more correct to call it a void.

Darby
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### Re: Vacuum bubbles

So....there would be bubbles, right? Bubbles of water vapor, vaporized by contact with an area of vacuum, plus cavitation bubbles, plus dissolved gases nearby knocked out of suspension by shock? All of the above, or two out of three?

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### Re: Vacuum bubbles

Braininvat » September 9th, 2015, 12:41 pm wrote:So....there would be bubbles, right? Bubbles of water vapor, vaporized by contact with an area of vacuum, plus cavitation bubbles, plus dissolved gases nearby knocked out of suspension by shock? All of the above, or two out of three?

I'm leaning towards all of the above.

{burp}

Sorry ... gas.

Darby
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### Re: Vacuum bubbles

Darby » Wed Sep 09, 2015 12:36 pm wrote:Even if you could achieve a perfect vaccum in the glass sphere, there would still be tiny bubbles induced upon collapse due to a combination of cavitation and pressure shock causing dissolved gasses already present in the water to come out of suspension. I imagine the effect witnessed by a diver nearby would be a loud underwater bang, followed by the rise of some extremely tiny fizzy bubbles from dissolved gasses.

BTW, the term 'vaccum bubble' is misleading ... it'd be more correct to call it a void.

OK, how about, "Extremely low density air bubble?"
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### Re: Vacuum bubbles

What happens to room-temperature water at very low pressures? It boils.

Below is a phase diagram for water. Phase diagrams show you what phase (e.g. solid, liquid, vapor) that matter (e.g. water) exists in at equilibrium under particular conditions (e.g. at a given temperature/pressure combination).

To find "normal" conditions:

If you find the normal temperature/pressure on the graph, you'll see that water's in the liquid phase. By moving downward from that point on the graph, you can see what happens as the pressure is dropped. Once pressure is dropped enough, the water will vaporize. This is flash evaporation, and in this case it effects the cavitation that Marshall pointed out.

As water floods into the now-available void at very low pressure, some of it will vaporize, forming a gas (water vapor). As static conditions are reasserted and the system comes to equilibrium, this water vapor would condense back into the liquid phase, eliminating the bubbles. So it's a dynamics question: will the bubbles rise to the surface before the vapor that they're composed of condenses back into the surrounding liquid?

Complicating factors:
1. Water on Earth tends to contain dissolved stuff.
• Dissolved salts affect vaporization energy, temperature, and pressure.
• Dissolved gases will be liberated and in the newly formed bubbles as well. These dissolved gases will be a minority component, but since the surrounding water that's reabsorbing the bubbles presumably already contains saturation concentrations of gases like N2 (and potentially O2, depending on local aquatic life, which consumes O2 for respiration), then the kinetics for reabsorbing these dissolved gases may be slower. It's possible that most of the water vapor bubbles won't make it to the surface, but the trace, tiny (potentially microscopic) bubbles formed by inert gases will.
2. Bubbles tend to travel upwards, but smaller bubbles travel slower than larger ones. This is because the gravitational force directs them upwards in proportion to volume while drag forces act in proportion to surface area. This favors larger bubbles, which have far higher volume-to-surface-area ratio. More complicated, dynamic effects are also in play, e.g. the water isn't still, so very small bubbles may even be swept downward in mild currents.
• Bubble formation will depend on exactly how the void is experienced. For example, if the void suddenly appears, then the pressure drop will be encountered by a large amount of the containing body's surface area, distributing the evaporation more than if a smaller amount of water surface area encounters the void first.
• Depending on how the bubbles are formed, there may be one relatively large bubble, or very many super-micro bubbles.
• Super-micro bubbles may not even be observable before ultimately being reabsorbed by the body of water.
• Larger bubbles are more likely to make it to the surface before being absorbed into the body of water.
• If the void isn't a perfect vacuum, but rather actually just normal atmospheric gases under very low pressure, then these gases would form their own bubble(s). Such bubbles may reach the surface of the body of water if not absorbed by it sufficiently quickly. However, this effect can probably be ignored as I suspect that the original spirit of this problem wanted to focus on a true vacuum.
• Smaller bubbles not only travel more slowly, but they are also absorbed at a relatively faster rate since absorption is pseudo-proportional to surface area while a bubble's mass content is proportional to the bubble's volume.
• Since larger bubbles travel faster and are absorbed more slowly, they're the ones most likely to ultimately reach the surface of the body of water.
Natural ChemE
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### Re: Vacuum bubbles

Also, just to add it, the relevant energy quantity in this problem is probably best described as enthalpy, $H$,
$H{\equiv}U+p{\times}V$, where
• $U$ is internal energy;
• $p$ is pressure;
• $V$ is volume.

In this case, the volume of the void is filled in at, let's say, constant atmospheric pressure. So the enthalpy change is roughly
${\del}H{\approx}{p}_{\text{atm}}{\times}{V}_{\text{void}}$.

This enthalpy change can effect vaporization to an extent with a limit estimable in terms of the enthalpy of vaporization.

Naively (lazily) estimating the enthalpy of vaporization for water to be constant at the STP value over the domain of interest, e.g.
${{\del}H}_{\text{vap}}{\approx}40.65{\frac{\text{kJ}}{\text{mol}}}$,
then for the sphere given in the problem, there's enough enthalpy to, theoretically, drive
${n}_{\text{vaporized water}}{\approx}{\frac{{p}_{\text{atm}}{\times}{V}_{\text{void}}}{40.65{\frac{\text{kJ}}{\text{mol}}}}}$.

Assuming
• ${p}_{\text{atm}}{\approx}{1{\text{bar}}}$;
• ${v}_{\text{void}}={\frac{4}{3}}{\pi}{r}^{3}{\approx}{\frac{4}{3}}{\pi}{\left({0.1{\text{m}}}\right)}^{3}$;
• ${{\del}H}_{\text{vap}}{\approx}40.65{\frac{\text{kJ}}{\text{mol}}}$;
then that's about 0.0103 moles of water that theoretically could be evaporated, as calculated by WolframAlpha. That's roughly 0.19 grams of water. Using Wikipedia's value for the density of water vapor at STP as $0.804\frac{\text{g}}{\text{L}}$, that'd be a total potential bubble volume of about 0.23L, which is about 5.5% (WolframAlpha) of the original void's volume.

In short, the theoretical maximum bubble size under these simplifying assumptions is about 5.5% of the void's original volume. However this assumes that all of the enthalpy generated goes entirely into vaporizing water, which is at the dynamic extreme limit. At equilibrium, the enthalpy will be dispersed throughout the body of water as a slight increase in overall temperature. Any vapor generated will condense, releasing heat, until equilibrium is reached. So, will any bubbles reach the surface of the water before fully recondensing? That's the dynamics problem that the above post concluded with.

This post was mostly just to show how we might naively estimate the generated bubbles' total volume, which appears to be thermodynamically limited to 5.5% of the void's original volume. It wouldn't be too difficult to perform this logic more rigorously (e.g. consider pressure, enthalpy, vapor density, etc. using more detailed correlations than just the simple constant estimates), though if I had to solve this problem in a professional setting, I'd likely have to make a simulation program to attack is numerically. It's really a dynamics problem, so these simple, assume-pseudo-equilibrium calculations that I've been showing here are insufficient for good estimation.

In summation:
• Bubbles will be generated.
• How many bubbles, and how large they are, depends largely on the void's original size and how water floods into it at the start time that it "bursts".
• Total bubble volume is thermodynamically limited to about 5.5% of the void's original volume.
• If I had to guess, the actual initial bubble volume would probably be much smaller.
• After the bubble(s) are generated, they will begin to recodense back into the body of water while also generally moving upward.
• Bubbles may be observed at the surface of the body of water if they get there before recondensing back into the water.
• There are many complicating factors involved, including stuff like what gases and salts are already in the water.
• A good solution to this problem would require numerical simulations or extremely rigorous/advanced analytical calculations.
Natural ChemE
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### Re: Vacuum bubbles

NaturalChemE,
I really like your answers. I was unable to test this question as I could not obtain glass spheres containing a hard vacuum.
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### Re: Vacuum bubbles

NCE: I suppose depth (pressure) would play a role too.

Darby
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### Re: Vacuum bubbles

CP,

Have any old TV picture tubes laying about? That's a pretty good Vacuum Bubble.
A high velocity bullet would probably shatter it nicely from a few meters away, even under water. Or a long steel rod to replace the bullet under similar dynamics. Trying not to get an explosive's bubbles mixed with the Bottle.

or..

Take it deep enough that it implodes.

Regards,
Dave :^)

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### Re: Vacuum bubbles

Darby,

Yes, definitely, depth would play a huge role. In general I'd think that greater depths would result in a diminished chance to see bubbles since the bubbles would have to travel that much further, however more heat would be released when the void collapses $\left({\del}H{\approx}{p}_{\text{at that depth}}{\times}{V}_{\text{void}}\right)$, likely leading to greater vaporization. Additionally, if a large enough bubble is formed, then it might get a current or something going to help propel itself to the surface.

I guess that this would likely be a scale-dependent process. For example, if a massive, mountain-sized void suddenly appeared deep within the ocean, the huge bubble it'd make from vaporization would have a huge volume-to-surface-area ratio, causing it to rapidly rise while losing total mass at a relatively slow rate (since its surface area would be relatively minor to its volume). Plus it might get its own current going in what we might describe as a dynamic viscosity phenomena.

At greater depths, recondensation should happen relatively faster due to the greater pressure. High pressures tend to drive condensation.

Some elements within the bubbles may form their own subphases before redissolving. For example, vaporized CO2 might turn into supercritical CO2, which actually sinks, before rejoining the bulk water phase.

I kinda doubt that any of us will have enough time to explore this question in great detail, though it really is an interesting through experiment.
Natural ChemE
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### Re: Vacuum bubbles

This really is fun to think about!

I guess that a good way to test it would be with a syringe.
1. Get a syringe.
2. Fully evacuate it by pushing the plunger all the way in.
3. Cap it.
4. Pull the plunger back out all the way to form a decent vacuum.
5. Place its tip downward in a small body of water (like a cup of water).
6. Suddenly remove/puncture/whatever the cap.
7. Watch to see what kinds of bubbles, if any, are present at the top of the cylinder after water floods it.
The margin of error would be about how much air is actually contained within the syringe when it's supposed to be a vacuum, which we could estimate using a pressure gauge built into the side of the syringe. Then we could subtract out that already-contained gas volume from the observed total bubble volume.

Or to avoid needing a pressure gauge, we could estimate the pressure inside by just measuring the force that it takes to hold the plunger steady once it's been pulled out. Then the pressure differential between atmospheric and inside the syringe would be the force that it takes to hold the plunger steady times the area of the plunger. Then the pressure inside is atmospheric pressure (measurable by a barometer) minus the differential. Then the amount of gas inside can be calculated via the ideal gas law for that pressure and the volume of the syringe. Then the ideal gas law can be used to predict the volume that that amount of gas contributes to the apparent bubbles after the water floods in. Then the total bubble volume is subtracted by this contribution to get the experimental bubble volume.

Ideally the syringe will be of sufficient quality to generate a strong enough vacuum to get meaningful results. But, if not.. then I guess we'd just do the normal thing in science - report that the signal is too slight to be picked up by our current equipment and request funding for better instruments.
Natural ChemE
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### Re: Vacuum bubbles

Dave_Oblad » Wed Sep 09, 2015 9:47 pm wrote:CP,

Have any old TV picture tubes laying about? That's a pretty good Vacuum Bubble.
A high velocity bullet would probably shatter it nicely from a few meters away, even under water. Or a long steel rod to replace the bullet under similar dynamics. Trying not to get an explosive's bubbles mixed with the Bottle.

or..

Take it deep enough that it implodes.

Regards,
Dave :^)

Good idea. It's not a sphere and it has some gas in it, but you can see what NaturalChemE was talking about. The (OK, sigh) voids don't live very long,certainly not long enough to make it to the surface. Now I need a good way to visualize this. You know, a decent high speed, immersible camera runs almost 20 grand! I've got about \$1.78 to blow on this experiment.

I almost bought a 1/6 interest in a B/W picture tube factory once. I screwed up and used the money for grad school instead, otherwise I could try making tubes with some kind of dye. What do you suppose you would use to dye a vacuum?

BTW, if any one of you readers think busting a picture tube underwater might be fun, and you're not old guys like Dave and I, it can be very dangerous. Those things really go off and blow glass everywhere.

It would be interesting to try breaking a couple to see if you can duplicate the well-known "whale footprints" on the surface of the water.
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### Re: Vacuum bubbles

I don't think so, it will create a void in the water unlike the air pressure in case of balloon. Balloon has air filled inside it and vacuum sphere lacks this air. though you may see a wave formed, which would be due to displaced water. It will form circular waves similar to the one which you get when you throw stone in water.
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### Re: Vacuum bubbles

Did you read any of the posts after the OP? Answer truthfully, then I will offer you the merciful option of deletion.

TheVat