Serious Question About Capilary Action

I thought it was trying to find the situation of lowest energy. It is easier for the water to stick to the side than itself, so I imagine at the top of the tube it would form a convex shape and stop.

But then a long time ago I heard a story about vintage motorcycles using this method to get oil to the top of the engine. I'm going away to look...

Actually, the meniscus is concave. If you think about the principle that draws water up the tube it is the electrostatic forces of the molecules between the water and the tube that force it upward. That means the water where it contacts the inside surface of the tube pulls itself upward and the center of the water is dominated by gravity, so it sits lower than the edges of the water where it contacts the tube.

When you push water up a tube (such as when pumping) the meniscus is convex because the pressure forcing the water upward is greater at the center of the water column and the edges that make contact with the tube tend to pull the water downward. That is because the rate that the edges of the water want to rise is slower than the water flow from the pump, but both scenarios experience hydrogen bonding along the contact are between water and tube wall.

Hi anon,

Thanks for your explanation but I would still like the first replier to look at the answer as well. It almost sounded to me he or she wanted the water to try to expell itself from the end of the tube but the meniscus held it back, which would make it convex! This is an interesting thought even though it would be wrong.

Still remains that nagging question in my head and that is, why does it not secretly creep over the edge, it is so near yet so far?

"so it sits lower than the edges of the water where it contacts the tube. "

Surface tension.

In Chemistry way back when, when you read a graduated beaker, you read the lowerest point,

sorry, should have read all the posts, before I regurgitated it

Excellent question! It could be a challenge question.

I had to refresh my memory, but it turns out it wasn't as bad as I though (not worse, either).

The reason water or liquid rises in a tube or paper towel is due to hydrogen bonding between the liquid and the substance that represents the tube or say a paper towel, etc.

In a tube the limit is dependent on gravity (volume of liquid inside the tube) and the area of the inside of the tube. Smaller tubes have better capillary action because the mass of the liquid goes up exponentially with cross section of the tube.

Water can only rise to the height of the tube, at best, because the tube is part of the inter-molecular force to draw the liquid upward. The liquid forms a reverse meniscus inside the tube, so the upward limit is the tube's height, if gravity doesn't overcome it first. I think trees overcome this by actually 'pumping' the water, if you will.

As far as a curved tube goes, I think that the reverse meniscus prevents it from spilling over the curve just as it does at the top of an upright tube.

"In a tube the limit is dependent on gravity (volume of liquid inside the tube) and the area of the inside of the tube. Smaller tubes have better capillary action because the mass of the liquid goes up exponentially with cross section of the tube."

As gravity is a factor, I was wondering whether anyone knows if experiments of this type have been attempted in the space station?

Hi all here at cr4.

First the concave or convex thing, it is difficult to say the least to correctly distinguish between the 2 if you are not relating it to something. It is hard enough in lenses, nigh impossible here so I ask the person who replied first to explain that part of the answer please. Is it hollow in the tube or is it bulging out of the tube?

The second reply I liked as it states this is at least an interesting question. I have however not seen conclusive answers in this reply and it just states the facts as we undoubtedly all know them. I do think that I understand the length of tube being the limit in sofar as the miniscus will go up. Therfore the motion upwards stops if the tube runs out. Question is still why does it not use the top edge of the glass tube and creeps slyly over the edge when you are not looking?

Second part answer, why is the gravity which is now turned the other way in regards of the boddy of water pulling itself up the glass tube (read down the tube), not overcoming the tension between the miniscus and the glass and thus "pulls" the drop out of the tube?

Feels to me we have something more in store about this.

Okay. If you are looking at a cross section of a vertical tube with the higher end straight up, then the meniscus looks like a bowl with the center lower than the edges.

The force between the tube and the liquid is a Van der Waals force (electrostatic) that works like gravity. It is mostly hydrogen bonding, but there may be a small percentage of covalent bonds, too. That is, the magnitude of the force diminishes with the square of the distance between the objects. So liquid at the center of the tube has virtually no attraction to the wall of the tube like liquid that is in contact with it. This effect also produces the surface tension of a liquid because the attractive forces also pull on the liquid molecules and bind them together.

The attractive forces will pull liquid up a tube. When it hits the top it stops because there is nothing to pull it upward any further and over the edge. Surface tension in the liquid counters the tendency to pull liquid over the tubes top because the liquid wants to stay with itself and that tension is greater than the force of the tube's top edge as it veers off perpendicular to the wall of the tube. So the liquid can not follow across the top. Also. look what happens as the area fan out from the inside edge of the tube and the force dissipates between liquid and tube surface.

Does that make sense? I hope I explained it well enough. Now your bent tube.

When you bend a tube in a half circle I suspect that the same principle is at work. The liquid rises to the highest point on the inside edge of the glass tube. Since there is a reverse meniscus there is nothing to push the liquid over the edge or over the bend, so it does not fold over and drain out. In short, the liquid stops short of going over the bend of the tube for the same reason it stops in a straight vertical tube.

"Surface tension in the liquid counters the tendency to pull liquid over the tubes top because the liquid wants to stay with itself and that tension is greater than the force of the tube's top edge as it veers off perpendicular to the wall of the tube."

That means that there must be an equasion to state the force of this surface tension and then you can compare it to the meniscus pull. As the water pulls itself up the tube, this force is greater so at an angle, maybe less than 90deg, it must be feasable to have the water creep along the edge? No? If this 90deg angle makes it too difficult for the water to pull itself along, we must try a smaller angle. Has that ever been done?

"Since there is a reverse meniscus there is nothing to push the liquid over the edge or over the bend, so it does not fold over and drain out."

As we now have gravity pulling the water in roughly the same direction as the capilary action does, it must make sense that again we have an equasion wherby the force of the water tension must equal the force of gravity combined with the capilary action for the water to stay putt! If this makes sense than we should be looking at what that point would be. Reverse meniscus would only be true if the water tension is greater than the gravity and the capilary combined. All other instances you would have the water bulge out of the bottom, clinging on for dear life so to speak. Must get to a point where it "falls off" persuaded by our friend gravity. Maybe this is all true for the water column in the tube is small therefor the gravity is small and it could just be that gravity never gets anywhere near big enough for the water to be enticed out of its hole??? I would like to see evidence of that if it were so.

"it must be feasable to have the water creep along the edge?"

No. Test that for yourself. Apply a drop of water to a horizontal glass plate.

Here is the equation, pulled right from Wikipedia:

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

Given the equation above you can now compute the results for a tapered or flared end of the tube.

"As we now have gravity pulling the water in roughly the same direction as the capilary action does..."

That is not going to happen for the very reason liquid can't seep over the edge of the glass tube. The cross-sectional area is too great to overcome surface tension.

"I would like to see evidence of that if it were so." Use the formulas. First, gravity does not change. The amount of mass in the tube and the tube diameter is what determines how far up a tube water will rise. Again, look at the formula and you will see what the players are that determine what happens.

Capillary action is a very small force. It uses Van der Waals forces (hydrogen bonding) between the glass wall and the water. Specifically, oxygen molecules in the silicon oxide of the glass will bond with the O2 of the water by hydrogen bonds. Hydrogen bonds are the weakest molecular bond, unlike a covalent bond. They are easy to break apart, but fundamental to life.

As an aside, you can add a surfactant to water and it will impede capillary action. Surfactants are used in radiators of cars to improve thermal cooling. They sometimes call it 'water wetter' and it reduces surface tension considerably.

As an aside, you can add a surfactant to water and it will impede capillary action.

Like 'Rinse aid ' in dishwashers... lowers the surface tension helps the water to shed.

Wikipedia has an article(s) on surface tension.

Hyper Physics also explores the subject in more rigid fashion.

A strip of paper or cloth will siphon water or liquid hydrocarbons from a container, whereas a capillary will not. The capillary has nothing to induce the liquid to leave the end of itself where as a fibrous material has multiple capillary structures.

and implied is that the potential energy afterwards is less that that before it was syphoned, so no sign of much 'free' there.

What about pressure differential - inside-the-capillary tube and outside-atmosphere? That might add more light on this topic?

Atmosphere is everywhere, we are talking about an open tube so pressure inside liquid is the same everywhere as is atmospheric pressure.

Even if it had any relevance, I doubt if it ever shines light, lamps normally do that.

Check out the Museum of Unworkable Devices:

http://www.lhup.edu/%7EDSIMANEK/museum/unwork.htm

See the Capillary Motor about 2/3 the way down the home page.

Now that is what I call an explanation! Thanks for that, it shows that knowledge is nothing without being able to explain / understand why.

Cheers

the reason I thought it would go convex is as follows. As I understand it nature is always trying to find the least energy state. The forces which attract water to water and water to glass (for example) mean that less energy is required for water to glass. this produces a concave meniscus where the water level at the glass is higher than at the centre so it's shaped like a modern flat-bottommed skateboard half-pipe from the side. As the section of the glass vessel gets smaller to become a capilliary the surface area inside the glass tube is so small that water will rise to a certain level which can be calculated, the thinner the tube the higher the level. this question was asking about the situation where the tube was truncated, shorter that the level to which the water would rise - so it might seem that the water would want to rise to a particular height and just keep going giving us a free pump. However the reason for its rise is the attraction to the glass surface, for this reason I thought it might continue to be attracted to the glass and rise above the horizontal sticking to the glass top surface and forming a convex meniscus. I imagined it would stop there. perhaps it is more likely that it would stop at the top and form its normal concave meniscus. i don't know. I think the vintage motorcycle story is an urban myth based on someone's poor understanding and repetition. We need a scientist here not an engineer.

I've just looked a PW's link, I could have saved a lot of typing, but I'm clear now. thanks

I shall hold fire until we get onto the whole siphon thing.

PS.(a meniscus won't 'secretly' creep over the edge because it knows you are watching and it won't be secret... If you get Shroedinger's cat to watch it for you, he will be able to tell you if the water goes over...be sure you don't get Hiesenberg's Cat by mistake 'cos that fella is soooo indecisive )

Del

Meniscus' actually do this, deep in the forest, where none are watching, and some thing stirred, and you stepped in it.

Why bother with those cats when we have you? You are close by as well. Can you please watch it for me? I am sure they are all in it together, it's a conspiracy or something.

Any way it is kind of solved now except for the one thing. The article about those none working machines did never mention a bend over capillary tube. I am sure though that with the new found explanation I can work that one out, when it's not watching me.

I think you answered your first question in your 1st sentence. "When water climbs up the tube"...what happens when the tube ends. Since there's no more tube, the water stops rising. It's a combination of surface tension, tube diameter, tube composition, etc and differential air pressure that causes the water to rise in a tube. Water and mercury, for example, have different surface tension and will act differently in a tube. A water meniscus (water wetting the tube walls) is concave in a tube, while mercury (doesn't wet the tube surface) forms a convex meniscus in a tube. Water has a surface tension of 72.8 dynes per sq cm (20 C), which is between alcohol [Ethanol (= 22.3 d/cm sq) and mercury (465 d/cm sq)]. Have you ever seen mercury spilled? It's surface tension causes tiny droplet spheres to form. Water usually puddles and runs (depending on the surface energy of the substrate). A similar situation occurs in siphoning, where the weight of the liquid in the lower part of the siphon (below the reservoir that one is siphoning from) is heavy enough to overcome the differential air pressure, and the weight of the liquid keeps "pulling" more and more liquid out of the reservoir. I'm sure you can find equations to illustrate tube diameter, tube composition, liquid surface tension, liquid density, liquid surface tension, relative air pressure, etc to fully answer your question.

Hi,

surface tension of water to (clean!) glass is around 70x10^-3 N/cm. This is a line force acting along the line that is the interface between glass and water.

(Hydrocarbons typically have only 30.)

So the upward force is constant and the height is limited by the downward force of mass and gravity.

At the upper end you will not have the same small diameter as inside a capillary tube so there is no longer any upward pointing force. This will limit the rise of the fluid.

But you can do better: if you put a piece of woven fabric or any other material with pores smaller than the capillary tube these smaller pores will have a higher suction force and extract some fluid from the capillary.

But this stops by the same mechanism that is acting for the capillary.

Thats why you have to have a sliding surface to extract some of the fluid in order to use it for lubrication or anything else. The sliding surface may be the surface of a rotating shaft or else.

I used this to build a self driven system to transport small amounts of lubricating oil into high precision ball bearings. A bearing assembly with such a lubricating system has the capability of unlimited life if another system of recirculating the oil is added.

Another similar system is for transportation of small amounts of a cutting fluid in microturning or micromilling.

RHABE

with regards to the piece of cotton wool or something, can you remember in the old days people would put a bottle of water next to a plant when they went on holiday. through the lid of the bottle you would thread one or several strings or ropes or wool threads to lay over the plantpot edge into the soil. This would feec the plant its needed water in the absence of its loving owner. Is that the principle you are talking about?

Also does this mean that the soil has to "suck" the water out of the end of the string for it to work? I suppose it is all to do with "wetting". The soil, if very dry, will want to be wetted desperately so it would probably suck if you can call it that.

Thanks for posting tho, even we have answered the original, you still shine more light on the subject by adding new angles.

It is atmospheric pressure that is acting on un even surfaces in capillary tube and surrounding there is less atm pressure compared to outer surface , so water rises propotionatly , suppose you try to shorten the length of the tube to that water level mark , water level will drop down but won`t flow or droop over rim of the tube , also increasing the length of capillery tube will tend to increase the level of water mark , it is not kind of material that is depended it has nothing to do with that , tube diameter and length do characterise this phenomenon , you probably get your second answer now , in our school days our teacher used to try more specific experiments like capillery in capillary tube for calculating atm prss , wide array of pressure application stating from vaccume to high pressure in air tight apparatus , also with other liquids like alcohol , mercury , oils .i think you make observation your self , it is really quiet fascinating when you see results , you must have seen water drinking birdy , where one glass enscapulated bird design tends to drink water , saws , again drinks and repeats . here evaporation , varieble pressure and temperature into different globe, liquid movement through capillary makes all that movement

Hi case491,

In a standards lab where there are oil baths, capillary action can cause the oil to leak out of the bath. Large stranded cables have an extra junction where the cables come over the top of the bath. This is to stop the capillarity of the stranded wire. Without the junctions, the oil (mineral oil) will come up the cable, then go down the outside. I think it settles in a low loop where the cable is coming back up, and goes through the insulation and drips on the floor. Normally there is a connector on the far end of the cable. I don't know whether it will drip from the end with no connector.

There must be a part of "Murphy's Laws" that says "It will only drip where you don't want it too".

S

can you guide me to some more info please?

I like to see the setup and find out how it compares with my original question.

thanks

Case491, I like your thinking, or should I say thought question. I especially enjoyed the link posted for unworkable machines.

It took me some hard thinking on my own to come up with why a thin capillary tube, slowly bent ALMOST in half wound eventually drip in a siphoning effect, but as it usually does, it all comes together (usually with a good smack on the forehead).

Thanks to all for sharing in the exercise. I appreciate the comments from people who still aren't afraid to think and ponder such things.

Hi , thanks for your appreciation, this is what it is all about here I think, bouncing balls and keeping busy ( so we don't have time for those chores the wife gave us).

Hold your horses tho as we have a new angle on the thing and I am sure I can come up with "the next phase" soon thanks to rhabe.

This is the thread that brought me here, I searched the web to find if there was a separate name for the inverse (reverse) meniscus.

Did anything further take place, another thread perhaps. I have a view on this question, but it is from my structural engineering perspective, particularly experience of designing high voltage transmission lines that I, for I must have sinned badly, received after being downsized.