# Bouyancy and liquid separation

hubble_bubble
Would it be true to say that boyancy, as in helium rising and liquids of different densities separating, with the densest at the lowest point, is an indication of gravity's natural tendency to attract the most dense mater to its centre point? If so is the Schwarzchild radius a natural consequence of this property?

Mentor
No, gravity attracts all matter in equal proportion to its mass.

hubble_bubble
I am not disagreeing with that. However surely that is stating the obvious without giving an answer to the original question. Ignoring the Schwarzchild radius, what is the cause of this density filtering, for want of a better description?

Mentor
However surely that is stating the obvious without giving an answer to the original question.
The original question was:
Would it be true to say that boyancy, as in helium rising and liquids of different densities separating, with the densest at the lowest point, is an indication of gravity's natural tendency to attract the most dense mater to its centre point?
The answer: [still] no, with the caveat that the question is poorly worded. Yes, it is obvious.

Ask yourself the opposite question: why don't stacked solid objects rearrange themselves according to their densities?
And also ignoring the equally irrelevant chartreuse moose....
...what is the cause of this density filtering, for want of a better description?
Fluid buoyancy. (to paraphrase the title of the thread....)

This is so obvious, it is confusing.

hubble_bubble
Ok so let's put this another way. Take two identical hermetically sealed spheres. Fill one with helium and in the other create a vacuum. Now the density of one is lower than the other. When they are dropped from the same height they should both reach the ground at the same time. As would any other object, unless air resistance prevents this. If the helium was enough to lift its sphere this is not what we would get. Yet by definition the sphere with the vacuum would be less dense than the air around it.

hubble_bubble
Ask yourself the opposite question: why don't stacked solid objects rearrange themselves according to their densities?

If you agitate them they will. If you try this with pebbles and rocks the rocks should end up at the top leaving the more densely packed pebbles at the bottom. better illustrated with a mixture of rocks, pebbles and sand. Granular convection.

http://en.wikipedia.org/wiki/Granular_convection

Mentor
Ok so let's put this another way. Take two identical hermetically sealed spheres. Fill one with helium and in the other create a vacuum. Now the density of one is lower than the other. When they are dropped from the same height they should both reach the ground at the same time. As would any other object, unless air resistance prevents this. If the helium was enough to lift its sphere this is not what we would get. Yet by definition the sphere with the vacuum would be less dense than the air around it.
Helium doesn't provide lift, it still has weight. Buoyancy exists independent of weight. A helium balloon rises not because helium has some magical anti-gravity property, it rises simply because the weight of the helium is less than the buoyant force provided by the surrounding air.

So an evacuated container has a lighter apparent weight than does a helium filled container.
If you agitate them they will. If you try this with pebbles and rocks the rocks should end up at the top leaving the more densely packed pebbles at the bottom. better illustrated with a mixture of rocks, pebbles and sand. Granular convection.
True. When you agitate them, you make them behave like a fluid!

hubble_bubble
What you say about helium is perfectly true. However it is inert and therefore less dense than anything else. I think we get caught up on the idea of weight rather than considering energy state. Hot air does not change its mass. It simply becomes less dense and rises. Dry ice CO2 will spill out of containers towards the floor. The really interesting thing is superfluid helium. When it climbs out of containers is this because it still somehow is less dense in the sense of electromagnetic interactivity?

Going back to the pebbles and rocks. It would be an interesting experiment to arrange spherical balls in a container that very gradually increased in size and were coloured according to their size. Would this ultimately show the same colour banding as separated liquids? Also what if the larger they got the more hollow they were. How would this be predicted to end up? Any gas inside would be less dense then the material around it.

Anyone like to hazard a guess as to how this situation would end up?

hubble_bubble
The reverse Brazil nut effect of granular convection appears to indicate a vertical shaking will act differently to a horizontal shaking. As shown here.

http://coewww.rutgers.edu/~shinbrot/InvBuoy.html

How would this be described mathematically.

Mentor
Helium is not less dense than everything else and it isn't a low density because it is inert. And the superfluid state has nothing to do with anything we're talking about here.

No offense, but you're kinda babbling here.

hubble_bubble
Sorry for the babbling. I am running through different ideas as I type. A bad habit of mine. What I meant by dense is less densely packed. Not that helium itself is less dense. Sorry for not being better at explaining.

Take a look at this work on the problem of pharmaceutical mixtures and granular convection.

Item 4.3 "Ternary Unequal Volume Fractions" is interesting in that the ratio of sizes affects the granular convection process. As in CO2 does not gather at the lowest point and suffocate us so the more intermediate size the beads the less the sorting occurs due to agitation. The question is does the sorting always 'gravitate' towards the most efficient arrangement to increase density on average?

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Staff Emeritus
Homework Helper
Sorry for the babbling. I am running through different ideas as I type. A bad habit of mine.
As Russ said, it's a simple matter of gravitational force being proportional to mass. This leads to a fluid's pressure being higher at low altitudes/greater depths, and the difference in pressure with altitude/depth results in a net upward force.

It's probably best if you look at an introductory physics book that discusses fluids, pressure, and buoyant forces. Then come back here with questions afterwards.

Your apparent strategy of using unrelated random ideas as an explanation could take you years to arrive at a proper explanation. Some might also suspect you of simply trolling.

hubble_bubble
As Russ said, it's a simple matter of gravitational force being proportional to mass. This leads to a fluid's pressure being higher at low altitudes/greater depths, and the difference in pressure with altitude/depth results in a net upward force.

It's probably best if you look at an introductory physics book that discusses fluids, pressure, and buoyant forces. Then come back here with questions afterwards.

I'm a little past that stage.

Your apparent strategy of using unrelated random ideas as an explanation could take you years to arrive at a proper explanation. Some might also suspect you of simply trolling.

These are not random ideas, neither am I trolling. No one has actually expressed an opinion on my initial question. Simply skirted around it. I'd rather the person answering simply said I don't know. Buoyancy doesn't appear to be the simple mechanism people think. There is something else going on. To simply say that it just happens that way, or imply that, is not an answer.

hubble_bubble
The reason I am looking at this is I have been involved in the environmental sector for some years. This link is to research made elsewhere. http://www.epa.gov/ttn/scram/userg/other/degugv1.pdf. This is for pollutant release but not anything I am involved in. As I am learning more I am asking questions to join the dots so to speak. I am generally interested in physics anyway. When I find something that goes counter-intuitive to the expected I find it very interesting. That is why sometimes I ask these questions. I will research and find my own answer. I was looking for opinions or maybe links to where I could find more information or research results.

hubble_bubble
I have found my answer. "Gravitational or buoyant convection resulting from material properties other than temperature."