Thread: The bubble and the Boiling Point

I was quite unsure about whether this belongs to the physics community or of chemistry. Butas a student of "Physical Chemistry" i knew, I had to make a good choice. (lest I explode the entire physics community by pouring hot water on quark soup!)

Recently, I attended alecture on the thermodynamics of curved surfaces that had the lecturer quite convinced that bubbles cannot form in completely degassed water placed in a scratch-free and perfect in all smoothness-container, at the boiling point of water (whatever that is).

Now, the lecturer was so confused by my innumerable doubts that he had to tell me to go to hell (but, nevertheless I had to come back with this unanswered doubt).

I hope you forum wizards have a comprehensive answer to this issue.
(by the way, I'll haunt that guy later )

I think he is referring to nucleation sites, however since the surface he describes does not exist the water will boil.
Another mind bender suppose you have a microwave oven in space and float a ball of water in it and and heat it, technically it touches no srface except air .
Now what will happen??

Well, you can force water cavitation where it doesn't touch an impeller. That's kinda boiling.

2003, Don Pettit spent his six months aboard ISS toying with fluids especially water. Too bad for progress he didn't try and blow up the microwave. We did learn that boiling works similarly in space. Despite the lack of gravity-buoyancy, bubbles nucleate & mature amid fierce convections, at the heat source, before currents carry them off and new water moves in... as on Earth.

I think the lecturer gave two impossible conditions: a perfectly featureless container, and a perfectly even yet external energy source.

EDIT: Funny how little we know about water of all things. For example urine discharged from space shuttles was expected to vapourize in the vacuum of space... instead it formed droplets.

wel suppose you DO have a microwave oven in space and a floating ball of water in it.
ASSUME:
1. microwave oven is vacuum sealed
2. water does not touch any surface
3. microwave is initially at 30C and heating causes further temp rise

Water, in the vacuum of the isolated system(microwave), will keep vaporising, untill microwave is saturated with water vapour at vap press corresponding to the temp inside it. (therefore, water DOES infact, touch the surface)

then itgets heated.

I think you mean

1) Water floated in oven at normal atmosphere.
2) Apply vacuum pump.
3) ....

Everything goes crazy before you finish stage 2. Lowering the pressure will boil room temperature water. However, in boiling the water also cools dramatically, so it freezes.


Anyway I guess agitation - especially furious agitation like above - provides nucleation "sites". Is this true?

nucleation ites can be provided by a plethora of ways.

there can be homogenous nucl. a also heterogenous nucl.

turbulence or agitation - homogenous

porous material, scratched vessel surface, suspended thread, etc. - heterogenous

Originally Posted by fizzlooney

I think he is referring to nucleation sites, however since the surface he describes does not exist the water will boil.
Another mind bender suppose you have a microwave oven in space and float a ball of water in it and and heat it, technically it touches no srface except air .
Now what will happen??

If it touches air, then you are referring to a microwave inside a spacecraft with an atmosphere, not in space as such, and the only significant difference from being in your kitchen is the absence of gravity. In this scenario I think the water would evaporate from the surface of the sphere at an increasing rate as the water heated up. The details would depend on the specifics.

In that case you'd drive convections in the sphere because it can't possibly gain heat or shed heat evenly throughout. Then as PritishKamat said, you'd have nucleation sites... I'd guess around the core... maybe in a pretty pattern.

Convection won't occur in the absence of gravity.

Because there's no buoyancy right? There's a rise in pressure though, where you heat a fluid. That pressure drives convections. Then bubbles bursting through a surface perturb the fluid even more.

But don't mind me, just watch the video.

There was an earlier NASA experiment, with much larger volume and submerged, slow heater. That one resolved as single bubble covering the heater. In that case convections didnt' take over.

I think the violence of boiling depends not just on temperature difference but also higher surface tension in a small volume. That's kinda testable on Earth.

I can’t watch the video at the office – it seems to be blocked. I guess they want me to do some work instead.

Without benefit of the video, here’s what I think.

Because there's no buoyancy right?

Right. The local acceleration due to gravity is right there in the formula for convection and if g is zero then so is convection.

There's a rise in pressure though, where you heat a fluid. That pressure drives convections. Then bubbles bursting through a surface perturb the fluid even more.

There’s only a rise in pressure if the fluid is contained within a vessel that prevents it from expanding and in that case the pressure rise is equalized throughout the fluid so there’s no pressure differential to drive anything. Perhaps this is a bit too much of a generalization. I suppose there can be local pressure differentials for an instant before it equalizes. If the microwave heating is perfectly uniform then there should still be no local differences, but maybe the sphere of liquid acts as a lens to focus the microwaves … jes’ speculatin’.

I think the violence of boiling depends not just on temperature difference but also higher surface tension in a small volume. That's kinda testable on Earth.

I know what you mean, but actually the surface tension isn’t dependent on the volume. The pressure inside the sphere will be higher than the atmospheric pressure outside it, and this differential does depend on the radius and the surface tension. This would mean that the sphere would expand as the water heats up, and at the same time the surface tension drops so it gets pretty complicated.

I’ll watch the video tonight and see how wrong I am.

Good video. If convection is defined as the movement of masses of liquid regardless of the cause then OK, there is convection. As the commentator says convection in microgravity is "by other means". It's not what I generally understand by the term, because such a loose definition could encompass stirring with a spoon, but never mind, I'll accept that convection doesn'r necessarily have to have anything to do with gravity.

The nucleate boiling occurs at the metal surface of the soldering iron. What's driving the convection? Surely it's the reaction of the bubbles against the metal as they form and expand. Each bubble pushes off dragging some liquid water with it and producing circulation. There are several differences between this and fizzlooney's scenario so doesn't really seem to address his question.

Originally Posted by Bunbury

I suppose there can be local pressure differentials for an instant before it equalizes.

Wouldn't that be persistent though, so long as you keep applying heat? And then you have persistent cooling at the outer surface. So a nice loop...or loops. I'm not so sure that boiling is necessary for weightless convection. Real world geometry is never balanced.

Originally Posted by Bunbury

I suppose there can be local pressure differentials for an instant before it equalizes.

won't there always be local pressure/temperature differentials!?...................equal values would be found in an 'ideal' state - but no situation is ever 'perfect' in that sense.

In a microwave, things heat-up by absorbing microwaves.

Now consider the situation of a cube microwave in which a large drop floats in its own vapor.

The vapor pressure(p') and the drop radius (r) obey the Kelvin Equation

{ln(p'/p0)}= Vx2x(S.T.)/rRT

Now, the drop (perfect sphere) absorbs radiation evenly. The temp gradient may decay exponentially from outer surface of drop towards its centre.

Where there is no gravity, there is no 'light' or 'heavy'. hence there can be no convection current. And pressure will remain same throughout the bulk of the drop. (But this value will keep increasing = vp + 2x ST/r)

However, as microwave temp increases, vp will rise for that temp. So the drop will go shrinking as time progresses.

After sufficient time, the drop will be no more, and the saturated vapor will get heated into superheated steam, soon crossing the critical point of water to become a indistinct phase (gas).

(Assuming, microwave temp is higher than critical pt of water)[/tex]

There are pressure differentials in the video example because there is boiling at the metal surface and the bubble formation creates a pressure gradient that drives flow.

The question asked was what happens when there is no metal surface, and heating is by microwaves i.e. there are no nucleation sites. In theory water can he heated to 90% of its critical temperature, around 309C before homogeneous nucleation occurs. In practice, dissolved air might provide nucleation sites and initiate boiling at a much lower temperature. Maybe even the microwave vibrations act as nucleation sites. And in any case the water will have all evaporated from the surface before the superheat reaches 309C.

I'm unsatisfied. I still think there will be some convection without gravity, only due to uneven heating and cooling. Fluids expand when heated. Consider the ocean currents, or wind (yes I know the strongest currents are buoyancy driven).

Balance just doesn't happen in real life without feedback. There's no feedback in a stable water sphere. Sure in theory if you stack opposing magnets the top magnet will balance levitated. The prediction falls short! I think this gets to the heart of our argument with that lecturer.

Well, I’m not satisfied either, if that makes you feel better. First I’m reluctant to accept that eddies in a pure liquid can act as nucleation sites. I think rather that you have to have bubbles first, from crevices or surface roughness or possibly from specs of dirt acting as nucleation sites. Turbulent eddies can then be created by bubble movement and maybe can enhance bubble formation, but not initiate it. I have googled a bit for evidence of eddies alone acting as nucleation sites and haven’t found anything so far.

So in my opinion if you have a pure liquid (no dissolved gases, no dirt) and no hard surface to provide nucleation sites, then even if there are eddies caused by heating I doubt that these can initiate nucleation.

I think that uneven heating resulting from how the microwaves are focused can cause some convection, even in a zero gravity environment. This would perhaps occur near the edges of the sphere where surface tension at the surface provides a sort of wall against which a warm cell could push and start moving away into the bulk of the sphere. Perhaps this results in a warm center and a cool outside as heat is lost at the sphere’s edges and warm packets bounce off the edges into the middle.

So I’m sticking to my belief that the sphere will lose mass by evaporation from the surface and won’t start to boil unless it gets very close to the critical temperature.

Of course scientific beliefs are always subject to revision in the face of new information. :P

So where’s fizzlooney?

Playing in the ISS kitchen I hope. I'd bet on your prediction (evaporation before nucleation) if that's what he's up to.

Maybe it depends on volume, or severity of gradient, or an equal degree of surface cooling. Relative velocity between convective cells increases with cell size.

For larger bodies I'm picturing the segments of an orange, though some other cute arrangements are possible.