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#1
Old 04-30-2002, 11:21 AM
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What liquid has the lowest/highest viscosity?

What substances in their liquid state have the lowest viscosity?
What substances in their liquid state have the highest viscosity?

At what temperatures would these occur? Does the viscosity also indicate the point at which a substance becomes a solid or a gas? If so, would the lowest viscosity for water be Scotch Mist (drizzle) in Aberdeen?

Specific applications require specific properties from a liquid (such as motor oil), what applications would need a super-low viscosity liquid?
#2
Old 04-30-2002, 12:25 PM
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Well take a look out your window. Thats the lowest viscosity component, I think. Glass is in it's liquid state when you look through it. It is very very very slow..... Highest, would most likely be mercury or liquid teflon maybe...not sure.
#3
Old 04-30-2002, 12:39 PM
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Quote:
Originally posted by Phlosphr
Well take a look out your window. Thats the lowest viscosity component, I think. Glass is in it's liquid state when you look through it. It is very very very slow..... Highest, would most likely be mercury or liquid teflon maybe...not sure.
There is some debate over whether glass is actually a liquid, in the sense cantara is referring. If you do consider it a liquid, according to that same column it takes 10 million years for any real movement to take place.
#4
Old 04-30-2002, 12:40 PM
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Superfluid helium has exactly zero viscosity, so I think it wins for the lowest. Helium becomes liquid below 4 degrees Kelvin (-269 C), and superfluid below 2.17 K. i.e. really cold.

The highest is probably a bit trickier. Phlosphr mentioned glass, but it's not clear if it is truly a liquid. Some say glass is more of an amorphous solid than a true liquid. If you think of it as a liquid, then it is very viscous.

In general, the difference between a solid and a liquid is in the arrangement of the atoms. Solids have atoms arranged in crystalline arrays, and don't have viscosity because they don't flow. They can break, but not flow -- they're fundamentally different beasts than fluids.

As for applications, viscosity is friction, so any application with moving parts (e.g. a car) can benefit from low viscosity fluids being used as lubricants.
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#5
Old 04-30-2002, 12:43 PM
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Helium at temps below 2.17K is a superfluid. It can move in ways contrary to our usual notion of liquids. It is my guess for the least viscous liquid.

For most viscous, there probably isn't any single candidate. Take tar and chill until it is on the verge of completely hardening. "Verge of" can be taken to be arbitrarily close. Ditto lots of other fluids.
#6
Old 04-30-2002, 01:05 PM
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I was going to post about superfluids too but there were already two posts before I got to preview!

Of the commonly available liquids, alcohols probably have the lowest viscosity. Diethyl ether is the lowest in the references I have at hand. The other day I attempted to wash something with alcohol, using a sponge. I was amused to find that the sponge didn't hold the alcohol at all - it simply flowed through the sponge. (Though maybe that has more to do with surface tension than viscosity)

Lubricants need a reasonably high amount of viscosity, otherwise it'll quickly flow away from whatever it is you're trying to lubricate. It's a tradeoff between friction and durability. For example, grease on bicycle hubs last thousands of miles. Some racers use oil to reduce friction, but they have to replenish the oil before each race.

I can't think of any application where lower viscosities are always better. Coolant, perhaps? The lower the viscosity, the less energy you need to pump it. But heat capacity and conductance may be more important.
#7
Old 04-30-2002, 01:40 PM
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Quote:
Originally posted by Giraffe
As for applications, viscosity is friction, so any application with moving parts (e.g. a car) can benefit from low viscosity fluids being used as lubricants.
No. Viscosity in simplest terms is resistance to the shear stresses of flow. Complicating matters is the possibility that a material can be non-Newtonian, including liquids which are thixotropic which is sheer thinning and rheopectic which is sheer thickening.

The best lubricant is not necessarily the material with the lowest viscocity. Motor oil, at approx 200 centipoise reduces friction better than water at 1 centipoise.
#8
Old 04-30-2002, 01:52 PM
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Quote:
Originally posted by Giraffe
Solids have atoms arranged in crystalline arrays, and don't have viscosity because they don't flow.
Oh, man. You are on a roll. Glass is most definitely a non-crystalline solid. Then we also have ceramed glasses.

“Crystals result from our failure to a achieve glass…” A bit of very unfunny materials science humor for you.
#9
Old 04-30-2002, 04:50 PM
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Quote:
Originally posted by ftg
Helium at temps below 2.17K is a superfluid. It can move in ways contrary to our usual notion of liquids. It is my guess for the least viscous liquid.

For most viscous, there probably isn't any single candidate. Take tar and chill until it is on the verge of completely hardening. "Verge of" can be taken to be arbitrarily close. Ditto lots of other fluids.
I always thought of those horrible Hallowe'en waxy/toffee/caramel/tarry candies were solids, not viscous liquids.

I also remember a news item about liquids with memories. They could stir them in one direction and get them back to the same state by sitrring them in the other direction (I think the stirring mechanism was a cylinder within a cylinder). I suppose those would have been very viscous.

Would a low viscosity also mean a low surface tension? It seems to logically follow.

How does a superfluid behave?
#10
Old 04-30-2002, 06:03 PM
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How is glass a liquid? This is confusing to me. I've heard of this before but the explanation of it never clicked with me.
#11
Old 04-30-2002, 06:10 PM
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I seem to remember reading something about superfluids siphoning themselves out of open vessels (like a meniscus gone crazy).
#12
Old 04-30-2002, 06:18 PM
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Ultracold helium isn't the only superfluid, either. Another good example is neutronium, the stuff of which neutron stars are made.

There's no clear boundary line between "very viscous liquids" and "amorphous solids", but most folks would put glass on the "solid" side of the line.
#13
Old 04-30-2002, 06:33 PM
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Quote:
Originally posted by cantara
I also remember a news item about liquids with memories. They could stir them in one direction and get them back to the same state by sitrring them in the other direction (I think the stirring mechanism was a cylinder within a cylinder).
You can demonstrate this with glycerine and food coloring. See a description of demo 13-08 in this pdf. There is also a brief mention in the description of the book Turning the World Inside Out and 174 Other Simple Physics Demonstrations by Robert Erlich and Jearl Walker.
#14
Old 04-30-2002, 06:34 PM
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Quote:
. . .but most folks would put glass on the "solid" side of the line.
Solidly.
#15
Old 04-30-2002, 06:47 PM
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Chill out, Waverly. Both of my (inaccurate) statements were designed to convey a general idea to an obvious layperson.

In simple terms, viscosity is a dissipative force, just like friction. Hydrodynamics is rarely simple, though. Perhaps you could explain what properties define a good lubricant, if not viscosity?

As for glass, what part of "amorphous solid" bothered you? More specifically, how about you give me your definition of a fluid and your definition of a solid? Glasses aside (which I said were a bit more tricky), the conventional definition for a solid is crystalline order. Fluids don't have crystalline order, nor do glasses, which is why some still lump glasses in with viscous fluids. (I would call a glass an amorphous solid.)

cantara, superfluid helium has several cool properties. It has exactly zero viscosity when moving below some critical velocity. It has infinite thermal conductivity, and heat transfer (heat spreading throughout the fluid) only happens via mass transfer (atoms physically moving from one end of the liquid to the other). And superfluidity is purely a quantum effect, on a macroscopic scale. (This last one is really the coolest part.)
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#16
Old 04-30-2002, 08:02 PM
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I’m not sure why I’m being called upon to chill here. You make an inaccurate statement, you get corrected. To suggest the inaccuracy was intentional so you could dumb down the subject matter is very much against what this forum is about. No matter how difficult the concept or how much it needs to be simplified, I simply don’t agree with such an approach.

Scr4 already explained a bit about lubricants, and to go much further would be steering this thread off topic, because viscosity does not equal friction. If it did, a zero viscosity superfluid would violate the laws of thermodymics, now wouldn’t it? We can’t very well be sending the OP off into the world thinking the fundamental laws of physics no longer apply.

‘Amorphous solid’ is a fine way to describe a glass. That was not my objection. You had stated: “Solids have atoms arranged in crystalline arrays.” Which, of course, is a direct contradiction to your latter statement that you would categorize glass as an amorphous solid. Solids do not necessarily have atoms arranged in crystalline arrays.

Glass does undergo a 2nd order phase transition at the Tg, and at this point is solid. It is this transition that allows us distinguish between a liquid and an amorphous solid.
#17
Old 04-30-2002, 08:41 PM
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Quote:
Originally posted by Waverly
I?m not sure why I?m being called upon to chill here. You make an inaccurate statement, you get corrected. To suggest the inaccuracy was intentional so you could dumb down the subject matter is very much against what this forum is about. No matter how difficult the concept or how much it needs to be simplified, I simply don?t agree with such an approach.
I didn't object to being told I was wrong (which I was), I objected to the obnoxious tone.

Quote:
Scr4 already explained a bit about lubricants, and to go much further would be steering this thread off topic, because viscosity does not equal friction. If it did, a zero viscosity superfluid would violate the laws of thermodymics, now wouldn?t it? We can?t very well be sending the OP off into the world thinking the fundamental laws of physics no longer apply.
I actually didn't read scr4's post carefully enough and missed his discussion on lubricants. So, yeah, that was already answered. (Good explanation, scr4, by the way.)

Since you mentioned it, could you elaborate on how a superfluid with zero viscosity and zero friction would violate the laws of thermodynamics?

Quote:
?Amorphous solid? is a fine way to describe a glass. That was not my objection. You had stated: ?Solids have atoms arranged in crystalline arrays.? Which, of course, is a direct contradiction to your latter statement that you would categorize glass as an amorphous solid. Solids do not necessarily have atoms arranged in crystalline arrays.

Glass does undergo a 2nd order phase transition at the Tg, and at this point is solid. It is this transition that allows us distinguish between a liquid and an amorphous solid.
Again, I used sloppy language. By saying "In general, solids are..." I was trying to convey "For almost all things people usually think of as solids, e.g. metal, ice, rocks, etc."

It wasn't long ago that glasses were considered fluids. And in some situations, calling them solids is just an arbitrary lablel -- they really behave like a hybrid of the two. The biggest problem with amorphous solids is not having something like a structure factor which defines a given configuration of atoms as fluid or solid, without knowing the full phase diagram.
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#18
Old 04-30-2002, 09:56 PM
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Leaving glass aside (obviously very controversial), I once head about an Australian who began an experiment maybe 80 years ago, putting a hunk of tar in a funnel and watching it dip. I don't recall exactly, but there was a drop something like once every decade. I don't think the guy who started it is still alive, but someone else is continuing the experiment. I'll bet that by now someone has put a webcam on it and you can probably catch the next drip (if you are patient enough). Of course, you could cool it even more and make it once a century, so I don't think the question has an answer.
#19
Old 05-01-2002, 02:48 AM
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Quote:
Originally posted by cantara
I also remember a news item about liquids with memories. They could stir them in one direction and get them back to the same state by sitrring them in the other direction (I think the stirring mechanism was a cylinder within a cylinder).
Did anyone else just have a flashback to the opening sequence of Ultra-man?

Damn, I'm old.
#20
Old 05-01-2002, 05:52 AM
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What's it like stirring Mercury? It looks quite blobby and jelly-like.

And the glass-not-really-a-liquid-thing: does this mean my physics teacher who told me that ancient church windows were thicker at the bottom than the top because of the liquid "settling down" was wrong?
#21
Old 05-01-2002, 11:41 AM
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istara, in short, yes. In one of my chemical engineering courses, our professor gave us an article that explained that it would take longer than the current age of the universe for glass to flow that much. The window-makers probably just installed the panes so the thicker edge would be pointed down. Unfortunatly, we don't have many "how to" books from that time period to confirm the practice.
#22
Old 05-01-2002, 12:06 PM
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Quote:
Originally posted by istara
What's it like stirring Mercury? It looks quite blobby and jelly-like.

And the glass-not-really-a-liquid-thing: does this mean my physics teacher who told me that ancient church windows were thicker at the bottom than the top because of the liquid "settling down" was wrong?
*sob* Yes, and he should have his credentials revoked. Follow the link given earlier or search this forum to find this myth debunked many times.

Giraffe: I was trying to be good naturedly obnoxious, if that makes sense. Sorry to have appeared to get on your case. Please note that the Tg does give you a non-arbitrary point of reference for the phase change.
#23
Old 05-01-2002, 12:28 PM
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As for applications for superfluids: I used to work for a gas distributor. Every once in a while we would ship some liquid He in little canteen-sized cylinders to a local university (I think UCSD, but it's been a while.) Anyway, the word was that it was used to verify absolutely leak-free vessels and tubing for nuclear/radioactive research. Since the He was a superfluid, any leak, however small, would be apparent as the liquid He could flow through holes of nearly atomic dimensions.

It was a while ago, so if any has a correction please chime in.
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