Playing with oil, water, and glass

[honestrosewater]

Just for fun, I used to play around with liquids, and I noticed something odd. In an uncovered tall glass container, ~20 fl.oz., I made an emulsion of water and olive oil by mixing and blowing bubbles with a straw. (Go ahead, laugh.) I let this sit undisturbed on a window sill for a while and checked it periodically to observe the oil and water separating. But sometimes, little drops of oil would remain stuck to the side of the glass and not float above the water. I would let this sit for a very long time, and after all the other oil had separated, these drops would still remain stuck to the glass. Does anyone know how this happened?

 

[GOD__AM]

The viscosity of the oil makes makes it spread out where it touches the glass making a tight seal against the glass. Since the pressure of the water is more or less even across the bubble of oil it can't get between the glass and the oil.

 

[honestrosewater]

You mean the oil droplet is acting like a suction cup (I don't visit the physics sections often if ever)? So (how) could I use heat to break the seal? Just a hint would be nice. The more targeted, the better - throwing the whole glass in the microwave isn't preferable.

 

[whozum]

Youd just need to break the bonding between the oil and the glass. Its the same idea when you get fizz bubbles stuck on the glass in your coke. Just knock those suckers.

 

[honestrosewater]

Just knock those suckers.

Yep, I'm aware of that option - I'm just curious about other options.

So that's a yes on the suction cup thing? I don't have a clear idea of what's at work here yet. What do you suppose is between the oil and glass - air?

 

[whozum]

Nothings between them and that's the problem. If they were to separate something has to fill in that void that's between them, a pressure pocket builds up as soon as the oil starts letting go. The pressure from the surrounding water keeps the oil on the glass.

 

[honestrosewater]

Okay, I'm trying to understand. Why doesn't the oil slide upwards along the glass? The water is denser than the oil, so the oil should still be pushed upwards, yes? The pressure from the surrounding water is enough to counter that upwards push?

Edit: Sorry, I'm tired. The upward push would be coming from the surrounding water - so why doesn't the oil slide up the glass? Friction between oil and glass?
I think I understand why the oil cannot 'let go' of the glass, but I don't see how that explains the oil not moving at all.

 

[LocktnLoaded]

Although I don't know if this would work, you might try putting a few coats of rainX in the glass?

 

[LocktnLoaded]

Unless you plan to drink this concoction?

 

[honestrosewater]

No, I wouldn't drink any of these concoctions, and I haven't made them in a long time. I'd just like to figure out what was going on. I don't have any Rain-X around, but maybe I'll do it again with saltwater and see whether the same thing happens.

 

[Mk]

Why doesn't the oil go up the sides?

 

[gijsbert2002]

The answer is not so difficult. the oil goes up, because the water above it is heavier and pushes down with more force, moving below the drop of oil and pushing the oil up.

Now the oil is stuck at the side. It is spread out along the glass. the oil drop has a very big surface now, but almost none of it is directed up or downward. that means that the pressure from the water pushing it upwards is almost negligable.

Furthermore, the drop is being pushed against the side of the glass by a not negligable force.

Because of that force, friction begins to play a part. The friction outweights the force of the water on the oil drop in vertical direction and it stays stuck at one height.

 

[honestrosewater]

Okay, that makes sense for the flatter ones. But I did it this morning and some of the droplets (about the size of this --> o) look kind of like a balloon taped by its neck to a wall. They seem to be almost spherical, tilt upwards, and shake as I tilt the glass. They seem to be trying to detach and rise to the top but are stuck to the glass along their bottom at tiny points that look like bubbles. ?? Could the bubbles be suctioning the droplet to the glass and the droplet be held together by forces within the oil?
I skimmed the oil from the top and looked down at the droplets. Most are hemispheres but some are almost spherical with only a small portion flat against the glass.

Oh, and thanks for the explanations.

 

[Danger]

these drops would still remain stuck to the glass. Does anyone know how this happened?

You should have used virgin olive oil; skank olive oil is sticky.

Sorry, Doc & ZZ... it had to be said.

 

[honestrosewater]

You should have used virgin olive oil; skank olive oil is sticky.

Sorry, Doc & ZZ... it had to be said.

Hm, I've steered clear of skanky olive oil since the Crêpe Carnage of '92. I only use the extra virgins; This one must have just been repressed.

 

[gijsbert2002]

That is really interesting.

maybe it's like this:
The oil is hydrofobic. I'm not so sure about glass, but if it is also, than the glass and the oil shoud stick together in a watery environment, because the water wants to stick together with itself.

I think it must be something like this, but I don't know exactly.

 

[Gokul43201]

Rose, what do you have against this explanation ?

Furthermore, the drop is being pushed against the side of the glass by a not negligable force.

Because of that force, friction begins to play a part. The friction outweights the force of the water on the oil drop in vertical direction and it stays stuck at one height.

Below a certain depth (or a certain size) the buoyant force on the oil drops is not enough to overcome friction with the glass wall - and hence, the drop stays put.

NOTE : This is an easily testable explanation. It predicts that most of the stationary drops will be below a certain depth. Any stationary drops near the surface (top) would have to be tiny ones but drops near the bottom can be larger. Do you actually observe this ?

 

[honestrosewater]

Rose, what do you have against this explanation ?

Below a certain depth (or a certain size) the buoyant force on the oil drops is not enough to overcome friction with the glass wall - and hence, the drop stays put.

I don't have a definite problem with it; I'm just still questioning some things.
Some of the drops appear to be almost spherical - I stress appear to be - I don't have great vantage points, they're small, and so on. A relatively small portion of the drop's surface looks to be in contact with the glass. Something like these: http://egweb.mines.edu/tvincent/Welding/DOD/10-6700-2small.jpg
http://www.es.hokudai.ac.jp/labo/nano_device/Droplet-on-surface2.jpg

These drops appear to be relatively stable. So how strong is the friction here?
I can even shake the glass and get the drop to look like, well, part of it is about to detach. Like some of these:
http://ltcm.epfl.ch/webdav/site/ltcm/shared/import/migration/video1.jpg
http://ltcm.epfl.ch/webdav/site/ltcm/shared/import/migration/video2.jpg

NOTE : This is an easily testable explanation. It predicts that most of the stationary drops will be below a certain depth. Any stationary drops near the surface (top) would have to be tiny ones but drops near the bottom can be larger. Do you actually observe this ?

I'll have to try it again.
If anyone else wants to try it in order to get a clearer idea of what I'm saying, I found that I can produce them easily by mixing up the oil until it formed into drops, letting the drops float to the top, sit for several minutes, and slowly adding more water to the glass, causing the rest of the oil to rise and leaving behind the drops that have stuck.

Oh, BTW, if it makes a difference, the glass is, what's the word, not conical... bah, it's a pint glass:
http://topshelfsb.com/PINT-GLASS.gif