Densities

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Lady Macaroni
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Densities

Postby Lady Macaroni » Tue Aug 16, 2011 11:07 pm UTC

Because all my best thoughts originate from something food-related.

I was looking at a bottle of salad dressing I acquired as a birthday/graduation present, and I noticed that it stratified in a really cool way, even the solids within it. I know that's basic science right there, the oil is on top because it's less dense than the vinegar, and the hydrophobic properties of the oil make the separation more pronounced.

My question is, if you have two substances that are the same density but still have the hydrophobic/hydrophilic thing going on, how would they maintain the separation since they wouldn't layer? I imagine it would look somewhat like a lava lamp with the glob-type things.
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Re: Densities

Postby thoughtfully » Wed Aug 17, 2011 12:46 am UTC

Or in a gravity-free environment, which might be more practical to achieve.

I'd assume they'd just follow whatever layering is natural from the way they are combined. Shake it up and it'd approach an emulsification. It'd probably settle out into larger and larger globs of each over time, due to random mixing, but I have no clue about the time scale of that.
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Re: Densities

Postby Ibid » Wed Aug 17, 2011 12:59 am UTC

Well, the globing together is caused by brownian motion (IIRC), so the time is probably both liquid and temperature dependant.
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Re: Densities

Postby Lady Macaroni » Wed Aug 17, 2011 1:04 am UTC

Ah, yes. It's been a few years since I took chem (I'm more of a biology person when it comes to science).

Should I find the liquids to test it out, I will so try this to see what happens. In the mean time, salad dressing it is.
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Re: Densities

Postby cpt » Wed Aug 17, 2011 1:44 am UTC

Lady Macaroni wrote:a bottle of salad dressing I acquired as a birthday/graduation present

Lady Macaroni wrote:how would they maintain the separation since they wouldn't layer?


Would not the hydrophobic substance prefer to be on the surface due to the imbalance of forces there? The hydrophobic force away from the water is imbalanced at the surface because the air is not supplying a similar but opposing force. (elsewhere in the mixture, however, the fact that water is surrounding the oil is preventing such an imbalance) This would lead me to believe that the oil would collect on the surface even if the oil and water densities were equal.

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Re: Densities

Postby Ibid » Wed Aug 17, 2011 2:07 am UTC

cpt wrote:
Lady Macaroni wrote:a bottle of salad dressing I acquired as a birthday/graduation present

Lady Macaroni wrote:how would they maintain the separation since they wouldn't layer?


Would not the hydrophobic substance prefer to be on the surface due to the imbalance of forces there? The hydrophobic force away from the water is imbalanced at the surface because the air is not supplying a similar but opposing force. (elsewhere in the mixture, however, the fact that water is surrounding the oil is preventing such an imbalance) This would lead me to believe that the oil would collect on the surface even if the oil and water densities were equal.


Forces always go in two directions. In this case the water will receive an equal force away from the oil as the oil from the water, meaning I could use the same logic to prove that the water would prefer to be on the surface (away from the oil). In fact this shows quite clearly why the two substances would much prefer to be completely separated, instead of "one inside the other", unless something very odd was going on regarding surface tension.
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Re: Densities

Postby cpt » Wed Aug 17, 2011 2:21 am UTC

Ibid wrote:
cpt wrote:
Lady Macaroni wrote:a bottle of salad dressing I acquired as a birthday/graduation present

Lady Macaroni wrote:how would they maintain the separation since they wouldn't layer?


Would not the hydrophobic substance prefer to be on the surface due to the imbalance of forces there? The hydrophobic force away from the water is imbalanced at the surface because the air is not supplying a similar but opposing force. (elsewhere in the mixture, however, the fact that water is surrounding the oil is preventing such an imbalance) This would lead me to believe that the oil would collect on the surface even if the oil and water densities were equal.


Forces always go in two directions. In this case the water will receive an equal force away from the oil as the oil from the water, meaning I could use the same logic to prove that the water would prefer to be on the surface (away from the oil). In fact this shows quite clearly why the two substances would much prefer to be completely separated, instead of "one inside the other", unless something very odd was going on regarding surface tension.


Very true. So the conclusion is, then, that the two substances will collect together somewhere, but where they collect is up to luck?

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Re: Densities

Postby Tass » Wed Aug 17, 2011 10:21 am UTC

cpt wrote:
Ibid wrote:Forces always go in two directions. In this case the water will receive an equal force away from the oil as the oil from the water, meaning I could use the same logic to prove that the water would prefer to be on the surface (away from the oil). In fact this shows quite clearly why the two substances would much prefer to be completely separated, instead of "one inside the other", unless something very odd was going on regarding surface tension.


Very true. So the conclusion is, then, that the two substances will collect together somewhere, but where they collect is up to luck?


Wrong, because there are three forces at play here. The surface tension of oil, the surface tension of water and the interface tension. If you have an oil-watter blob floating in a zero g environment the oil would indeed tend to cover most of the surface over time. Water surface is much more expensive than oil surface (i.e. water has a higher surface tension). Water prefers to play only with itself, it hates oil, but it hates air even more. Oil minds less. It will not cover the entire surface unless the contact angle exceeds 180degrees hovewer, in other words if [imath]\gamma_w>\gamma_o+\gamma_{wo}[/imath], otherwise you will get two truncated spherical blobs, touching each other, with the contact angles dictated by the relative surface tensions and the interface tension. For water and most oils it will tend to be so that the oil blob looks like it "eats" the water blob - partially or fully.

This is of course for equilibrium, without gravity to guide the individual droplets to the others this will take a very long time to achieve by random motion. Try it yourself: Get some food oil and mix alcohol with water until it has the same density (put a little oil drop in the water and ad alcohol gradually until it floats in the middle, ad water if you overshoot). Then try to shake, the oil droplets (or water droplets if there is most oil) will rather quickly reach millimeter size, but after this they will very rarely run into each other and it will be quite stable if you have the density just right. If the density is a little of then they will of course eventually meet up at top or bottom.

Another fun thing to try while you have the jar if you own a neodymium magnet: Water is more diamagnetic than oil, so a powerful magnet will appear to attract the oil (timescale minutes to hours, not immediately visible).

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Re: Densities

Postby Ibid » Wed Aug 17, 2011 12:14 pm UTC

Huh, I was not aware that the surface tension was that strong. I guess my "something very odd" regarding surface tensions should have been "Just...just constantly". At least I got the mechanism right! (Yeah I'm clinging to straws, shut up).
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Re: Densities

Postby Lady Macaroni » Wed Aug 17, 2011 1:34 pm UTC

Wow. Thanks, guys. :)

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(This was the stratification I was talking about. I've never seen it layer like that, but then again, I also never thought this salad dressing bottle would make me feel so stupid...)
Chaoszerom wrote:Someone hit someone else in the face with a fire extinguisher in the first 100 words, I can't let that slide!


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