What's the energy source for capillarity to lift liquid up?

[makc]

I just recently had a conversation, where we discussed if liquid is getting colder as it moves along thin tube where it "normally" wouldn't. And if it does, doesn't it mean we have a system that makes things colder without warming anything around it? Which I thought wasn't possible.

 

[Dale]

No, the energy source is surface tension, not heat.

 

[makc]

can we store an energy in surface tension, to make liquid go as high as we want? can we extract an energy from surface tension to light up lightbulb, and so produce a liquid with empty surface tension :D I mean, just these two words by themselves do not really explain anything.

 

[makc]

some image or video on what's going on at the edge would be perfect.

 

[Dale]

I don't know of any way to increase surface tension, but you can easily reduce it with soap. You can use that to extract energy and make a boat that is powered by reducing the surface tension:

http://www.youthonline.ca/crafts/soapboat.shtml

 

[makc]

ok, let's begin with simple things:

seems like a right picture to start with. there's non-90° angle, and so net gain of all the forces is upward (and stays so, until enough water is lifted, and gravity cancels it). but, why the hell there's non-90° angle to begin with? why, out of sudden, water molecules climb up onto the surface?

 

[makc]

that's kinda brings another similar question, if we bring metal closer to magnet from above, it will jump up and stick to it. obviously, some work has been made by whatever electromagnetic forces there are, so you would expect this energy to come from somewhere.

I guess the notion of potential energy that doesn't "really" exist but can be added to our equations whenever we need such a mysterious energy source is bothering me :)

same with gravity, an object of certain mass flying slowly through space can potentially attract infinite amount of matter to its surface (ya, I know, that's how planets were made), but where the hell all these unlimited powers come from?

 

[makc]

another question for capillarity effect: imagine circle tube horizontally alligned, with no air inside and one droplet of water. since its surface tension is equal on both ends, it doesn't move anywhere. now if we were to sprink small amount of soap on one of its ends, the surface tension there would be smaller, right? so the droplet starts spinning around the tube. how far will it go?

 

[makc]

(if eventual diffusion bothers you, imagine that we embedded very light "cork" into the water that splits it in two halves that cannot communicate... this is just a matter of clever enginering, right? the question is, what's there to stop the motion)

variation 2:

imagine a tube made from mix of two different materials, and each material share varies from 0 on one end to 1 on another. this tube should accelerate water droplet in it without soap. now if we have sufficiently different materials, and make it sufficiently long, and connect ends, it should be possible for water droplet to slow down at joint insufficiently to halt it completely? again, what would cause it to stop in such a device?

oh, I know: it heats up from friction and evaporate

 

[Andy Resnick]

I just recently had a conversation, where we discussed if liquid is getting colder as it moves along thin tube where it "normally" wouldn't. And if it does, doesn't it mean we have a system that makes things colder without warming anything around it? Which I thought wasn't possible.

Are you referring to thermocapillary flow?

'Very slight surface temperature differences are sufficient to generate subtle fluid flows on the surface of liquids. Such flows, referred to as "thermocapillary," exist on fluid surfaces on Earth. However, thermocapillary flows on Earth are very difficult to study because they are often masked by much stronger buoyancy-driven flows.'

Maybe you mean the actual act of wetting cools the wetting fluid?

If a fluid wets a surface (fills a capillary tube), it's because the interfacial energy between the wetting fluid and the solid is smaller than the *other* fluid and solid. It's not clear if capillary rise is associated with a decrease in temperature, but if it is the effect is very small and very difficult to observe.

 

[Andy Resnick]

can we store an energy in surface tension, to make liquid go as high as we want? can we extract an energy from surface tension to light up lightbulb, and so produce a liquid with empty surface tension :D I mean, just these two words by themselves do not really explain anything.

People have tried to use wetting to store energy (look up superhydrophilic silicon), but the energies involved are *tiny*- pure water has an interfacial energy with air of 78 erg/cm^2, Hg is around 500 erg/cm^2. Contaminating the fluids will only *decrease* the interfacial energy, never increase it.

 

[Andy Resnick]

another question for capillarity effect: imagine circle tube horizontally alligned, with no air inside and one droplet of water. since its surface tension is equal on both ends, it doesn't move anywhere. now if we were to sprink small amount of soap on one of its ends, the surface tension there would be smaller, right? so the droplet starts spinning around the tube. how far will it go?

That's the Marangoni effect, the drop will move as long as there is a gradient of concentration of surfactant. The drop will begin the spread (not roll around). As soon as the gradient disappears, the fluid stops flowing.

You've been asking good questions- there's a lot of cool technology exploiting surface tension effects (wetting) by engineering superhydrophobic or superhydrophilic surfaces. Here's another spiffy trick:

http://physicsworld.com/cws/article/news/2655

 

[makc]

Thanks Andy,

It's not clear if capillary rise is associated with a decrease in temperature, but if it is the effect is very small and very difficult to observe.

Well, this was just my intuition that, if intermolecular forces cause liquid to move, they do some work and so something must lose energy... why not molecules themselves? and if so, the overall effect would be cooling. but then, I thought about magnets and gravity and how they move things as much as you want without losing energy at all... and it got messed up.

That's the Marangoni effect, the drop will move as long as there is a gradient of concentration of surfactant. The drop will begin the spread (not roll around). As soon as the gradient disappears, the fluid stops flowing.

but how can it spread without decreasing liquid density? and if we specifically separate two drops of liquids with different tension by solid object, there will always be a gradient, right?

 

[makc]

here's simplified version of experiment: we have a glass tube, a small piece of glass which friction would be less than capillary forces, and mercury droplet. will it move to the right, and why not, if no?
[PLAIN]http://img43.imageshack.us/img43/962/80143546.jpg

 

[Andy Resnick]

Thanks Andy,
Well, this was just my intuition that, if intermolecular forces cause liquid to move, they do some work and so something must lose energy... why not molecules themselves?


but how can it spread without decreasing liquid density? and if we specifically separate two drops of liquids with different tension by solid object, there will always be a gradient, right?

We do not yet have a microscopic model for wetting- nobody knows how the three-phase line moves, not can we predict what the contact angle is in either equilibrium or dynamic conditions.

So at this point, the best we can say is that wetting occurs when the energy cost incurred by increasing the area of the low energy interface is less than the energy gained by the high energy interface giving up area. Partial wetting is achieved by Young's equation.

I don't understand your second question.

 

[johnuk]

This is an excellent question!

 

[Dale]

Hi makc, Andy has given you some great information about surface tension and wetting, so I won't repeat any of that. However, it sounds like you have a more general and more severe misunderstanding about energy.

if we bring metal closer to magnet from above, it will jump up and stick to it. obviously, some work has been made by whatever electromagnetic forces there are, so you would expect this energy to come from somewhere. ... but where the hell all these unlimited powers come from?

then, I thought about magnets and gravity and how they move things as much as you want without losing energy at all... and it got messed up.

Neither of these is correct. A conservative field has a certain energy density, and in order to do work the energy density of the field must go down. The energy is not unlimited, and energy is lost from the field which is exactly equal to the work done by the field.

A magnet can lift a piece of metal, doing work on the metal, but in doing so the magnetic domains in the metal become aligned against the magnetic field and the field is reduced, it becomes less able to lift more metal. If you wish to lift more metal then you must first remove the piece of metal from the magnet, this requires work and increases the field where the energy is stored.

 

[makc]

A magnet can lift a piece of metal, doing work on the metal, but in doing so the magnetic domains in the metal become aligned against the magnetic field and the field is reduced, it becomes less able to lift more metal. If you wish to lift more metal then you must first remove the piece of metal from the magnet, this requires work and increases the field where the energy is stored.

but you can't play this card with gravity, can you? whatever falls down on the planet, planet's gravity does not get weaker (even if we ignore additional mass of things that fall). you can keep "spawning" objects at some distance, and it will pull them to the surface, ad infinitum.

 

[Dale]

but you can't play this card with gravity, can you? whatever falls down on the planet, planet's gravity does not get weaker (even if we ignore additional mass of things that fall). you can keep "spawning" objects at some distance, and it will pull them to the surface, ad infinitum.

The energy density of gravity is strictly negative, so the more mass you add the more negative it becomes. That is indeed one of the differences between gravity and magnetism. However, it is not different in terms of energy conservation since in order to restore the field to a more energetic state you must always do work which is exactly equal to the change in energy of the field.