How do Trees Really lift Water to their Leaves?

01/05/2005 22:00:03 »
Dave:

When me and Rosy are talking about negative pressure, we mean negative absolute pressure - where a vacuum is zero pressure.
Is there any other kind of negative pressure?

quote:The reason why GCSE textbooks tell you that a syphon will not work above 33 feet is that atmospheric pressure is enough to lift water 33feet, so up to this point the water is under compression by the atmospheric pressure. If you go above 33feet the water is in tension (a negative pressure) and should therefore boil or cavitate or something breaking the syphon.

Not sure that I’ve read anything about siphons not working at 33 feet in a gcse biol book? I’ve read it on plenty of other places on the web though. You do appear to be saying the GCSE Biol book is wrong, and that I something we can agree on at least.

Pascal demonstrated that the siphon worked by atmospheric pressure, not by horror vacui, by means of the apparatus shown. The two
beakers of mercury are connected by a three-way tube as shown, with the upper branch open to the atmosphere. As the large container is filled with water, pressure on the free surfaces of the mercury in the beakers pushes mercury into the tubes. When the state shown is reached, the beakers are connected by a mercury column, and the siphon starts, emptying the upper beaker and filling the lower. The mercury has been open to the atmosphere all this time, so if there were any horror vacui, it could have flowed in at will to soothe itself.
source: www.du.edu/~jcalvert/tech/fluids/hydstat.htm#Siph

quote:Now as you have found out real life is rarely as simple as GCSE textbooks, and water can actually survive a negative pressure if it is continuous, there are minimal dissolved gasses (which you removed by boiling) because of the cohesiveness of the water. It is not stable like this and a small bubble will cause it to cavitate. However if there are no gasses this is unlikely enough for you to do your experiment in Brixham.

Agreed



quote:Now if you are using the same liquid in both tubes the pressure in the tube is only dependent on how much weight there is pulling on it, and the chance of cavitation is just dependent on the pressure. So the only difference between a normal syphon and your syphon is that the extra weight is provided by an extra length of water rather than salt.



Dave, I believe there is something else at work in this model, I believe the molecules of the dissolved salts align in conjunction with gravity as they mix with a greater volume of clean water in the same side. I.E. the more dilute the saline becomes and the greater the distance it spreads out, the greater the flow rates achieved, say thrice times the normal rate of decent and accent accordingly, depending on the height. It appears that the higher the experiment goes the greater the flow. Still trying to figure out how to set a scale of flow so that everyone will agree on the formula

On occasions, the saline flow has triggered a very rapid flow, as opposed to the normal stable flow. It can’t be a siphon effect that it is triggering because there is no additional weight or density to the downward flow. When a small amount of saline solution is added in a way that it can flow in both directions over the inverted u centre, you can clearly see the flow at work and the turbulence it causes in the ascending side as well as the descending side. The best place to view this is on a spiral staircase. I’ve often used the one in the local car park for my experiments.

Siphoning is used on a large scale to move huge volumes of water for irrigation from one reservoir to another. However, they find that this does not work if the height is too great and have to install a pump to maintain a positive pressure.

Even the smaller bench top model of the Brixham experiment reveals some amazing properties.

For example: cavitation can be observed, even at the low level. The syringe body filled with saline remains stable while the tube is in the elevated position. After injecting a small amount of coloured saline solution in at the top, the syringe begins to self-empty as the plunger is pulled up against gravity by the descending saline solution. This is pretty amazing when you consider that say 1 mil of saline solution is injected and 5 mils of saline solution are drawn up as the plunger rises from a near vertical down position and joined to a small length of the same tube to the T junction.

I have seen a siphon work on many many occasions. This simply is not a siphon at work here.

Yes the saline solution can be injected into any point on the descending side and the flow will occur. But as I stated earlier, the higher up you add it, the greater the flow rate.

What would you expect to happen to the water levels in the tube, when you remove the both ends of the tube from the bottles, while it remains suspended above the 33 feet limit?

Dave:
I am not quite sure what you mean by "the molecules of the dissolved salts align in conjunction with gravity". Gravity is my molecular standards an incredibly weak force the energy released by rotating all the water molecules to align with water would release less than a nano joule of energy - in comparison heating it up by a degree centigrade would require over 4kJ a difference of a trillion times. So on this scale thermal and intermolecular forces will hugely dominate any gravitational effects.

Gravity only becomes important at larger scales, so we can model water as a fluid affected by gravity rather than worrying about what happens on the molecular scale.

I think you would expect the syringe to be pulled in if you plug it into something with a pressure below that of a vacuum. It may stick in the beginning though. which is why it wasn't moving to start with. Have you tried a similar experiment injecting pure water into the system, and found out what happens to the syringe?

Yes the cohesiveness of water is an effect that is stronger the smaller the tube you use, and the cleaner the water, so syphoning huge amounts of dirty irrigation water is unlikely to work at over 10m.

Just a thought, have you taken into account the momentum the water in the tube will have once it starts moving. (This will not be insignificant if my experiments using a hosepipe to move a level around my parents barn are anything to go by)

Once the water starts moving through the tube it will tend to keep moving. I think this is why you get more flow when you put the saline in at the top, than when you put it in the bottom. When you put the saline in the top it will accelerate under gravity pulling the rest of the water with it. The further it drops the faster the saline, and the whole water column will be going. When it gets to the bottom, now the column is moving it will want to keep on moving because it has inertia, so it will keep syphoning(ish) until it slows down due to friction.

quote:What would you expect to happen to the water levels in the tube, when you remove the both ends of the tube from the bottles, while it remains suspended above the 33 feet limit?


I would guess that the water would start to fall out of the bottom of the tubes as what is known as a slug bubble goes up them (basically what happens if you cover the top end of a tube and lift it out of the water - air goes up the middle and water comes down the sides.

Now through random factors like the exact sizes of the tubes and which one you took out of the bottles first one tube will empty a bit quicker than the other. This will mean that there is more weight on the side of the slower tube, which should start a syphon going which will pull the fast emptying side up faster and faster as the pressure difference between the two sides gets bigger. This would mean that you get more water out of one side than the other, and the difference between the rate of flow out of the tubes should get larger with time.

I am not sure how strong the syphon effect will be relative to them just emptying as I think this is dependent on the diameter of the tube. This is assuming that the fiddling with it doesn't trigger a cavitation.

How close am I? I am really interested to see how good my physical intuition is.

Well, I did have a rather long conversation with a huge group of people on a physics newsgroup about whether gravity is actually a weak force or a strong force, and I did get quite a few people on to my way of thinking eventually, showing that you simply cannot take one pre-defined unit of gravity and measure it against 1 pre-defined unit of say EMF, as the comparison should have been measured against the total pull of the mass of the whole planet, because isolating any part of the mass does indeed result in the measurement of only one tiny part of the mass. Therefore collectively, the mass will always logically be greater than the sum of the smaller part of the mass.

But that’s another argument for another day.


I think you would expect the syringe to be pulled in if you plug it into something with a pressure below that of a vacuum. It may stick in the beginning though. which is why it wasn't moving to start with. Have you tried a similar experiment injecting pure water into the system, and found out what happens to the syringe?

Good, at least we now agree on the negative pressure issue. Actually, I have tried it at the same height of elevation without the solute, and the syringe body remains unaffected, as does the flow. However, as you correctly state, if we go substantially higher with the inverted U tube, the syringe will become sucked in by the negative pressure/tension placed upon the water.

Now using a complete loop of water filled soft walled tube (used to demonstrate how this flow could affect fluids in the body) once the saline solution starts to flow down one side, the turbulence becomes obvious as some of the coloured solution is pulled up one side and down the other side, proving complete rotation / circulation of the loop is taking place.

An obvious and very significant narrowing of the upward flowing tube takes place, and an equally obvious and significant bulging of the opposing downward flowing side of the tube takes place. Indicating the presence of both a negative and positive pressure, generated by the falling salt solution.

I have obviously thought about the momentum of the water. But you have just highlighted something very significant for me, which I believe has just explained my observations on the sudden acceleration of the water in the tubes during flow. Thanks Dave, you’re a star!

What I’ve just gleaned from our conversation is: The water is very elastic and stretches substantially when placed under tension, as does the analogy of using the play slime, mentioned earlier to Rosy.

The sudden acceleration is due to the sudden release of the built up elastic tension, caused by the falling salt solution on the opposing clean water-side of the loop. Just like releasing a stretched elastic band. This fits exactly with my observations when removing the two ends of the water filled tubes out of the two bottles and the water level rises up the tube by half a metre! Proving the amazing elasticity of water. The water remains suspended, even if we blow up one side of the tube, it temporarily alters the level in the side you are blowing up, but the water stays in the tube suspended almost like two weights linked by an elastic band and hung over a wall. More like two weights on an equal length of wire, joined by an elastic band in the middle and hung over a wall. Picture lifting one of the weights gently releasing the tension on the elastic band, but not sufficient as to over balance the weight in the other side. Amazing!
If there is some salt left inside one side of the inverted loop when both ends of the tubes are removed together, the saline side begins to draw up the water in the adjoining side, accelerating as it goes, until all the water flows out of the one side only! 6 mil bore tubing, hard nylon, which resists the negative tension more than adequately.

But you were pretty close Dave. Do you live close to me in Paignton? Perhaps we could meet up and to give you a demonstration of the exp?

@ l_kryptonite
I disagree; your contribution is as valuable as anyone else’s. New-ground in science renders everyone on equal terms. Children being in the best position of all to take on board a new paradigm, as they have not been corrupted by erroneous literature and retain the capacity of having an open mind.

I agree with your statement that: A leaf cannot possibly suck water from the soil because it is not in contact with the soil. I assumed incorrectly that you were talking about osmosis. Although not the main method of fluid movement, it is still an important factor in a plant's survival.

quote]Originally posted by l_kryptonite

Or you could get involved in the incredibly complex discussion being held in the general science section. I need to do about 3 years of study before I get back into that one though. Way out of my league.[/quote]

Dave:
Thre are two things that could be causeing the effect you are referring to as the elasticity of the water. The stretching of the water or the deforming of the tube.

If we start on the stretching of the water. I have found a list of how the volume of water changes as you change the pressure.

temp F(C) 0 atm 500 a 1000 a 2000 a 3000 a
32 (0) 1.0000 0.9769 0.9566 0.9223 0.8954
68 (20) 1.0016 0.9804 0.9619 0.9312 0.9065
122 (50) 1.0128 0.9915 0.9732 0.9428 0.9193

so at 68 farenheit it takes 500 atmospheres to change thevolume by 2%, so 1 atmosphere will change the volume by about 2%/500 = .004% or about .4mm over 10m

Ok tension will be slightly different but probably not hugely so and most of the water in the column is at a positive pressure anyway so I don't think that the expansion of water will be producing a major part of the effect.

Your tube is pretty rigid, but if you squeeze it really hard I expect it will deform a little bit, you would only need a 5-10 percent deformation to cause a .5m movement in the water. Are you using the flexible clear PVC tube or the translucent white much more ridgid stuff?

Hang on a minute - do I understand you correctly in that the bottom half metre of the tube empties and is full of air, and then stops? Have you done anything else other than removed the demijohn? because just removing the demijohn will not alter the pressures at all - so you haven't done anything to the water column apart from let water fall out of the bottom, eg you shouldn't have altered the tension in the elastic band (whether the elasticity is due to water stretching or the tube deforming) as you haven't changed the size of the weights on each end... did you pull the ends out of the demi-johns by lifting the whole apparatus or just the ends of the tubes?

ps. by the way my calculation above was considering the force from the whole earth on the hydrogen atoms in a water molecule. Perhaps I should have said that on a molecular scale the earth's gravity is a very small force.

So even if we calculate the whole length of the tube at 48 metres, that still does not address the half metre rise in the water level on both sides of the tube, when they are lifted out of the bottles containing the water.

Zero water runs out of the tubes at the bottom, as you suggest might be the case. The water is sucked up inside the tube equally by the negative tension placed on the bead of water.

Your tube is pretty rigid, but if you squeeze it really hard I expect it will deform a little bit, you would only need a 5-10 percent deformation to cause a .5m movement in the water. Are you using the flexible clear PVC tube or the translucent white much more rigid stuff?

Yes, it’s the rigid translucent stuff! The softer walled tube will simply neck )( under the negative tension. This rigid stuff does not neck and therefore the diameter internally will not reduce as a result of the negative tension. If I were able to squeeze it and alter its shape, it still would not alter the volume, as in order to do this one would have to compress the tube equally from all directions and this would take a huge force.

Hang on a minute - do I understand you correctly in that the bottom half metre of the tube empties and is full of air, and then stops?

It empties, but empties upwards!

Have you done anything else other than removed the demijohn? because just removing the demijohn will not alter the pressures at all - so you haven't done anything to the water column apart from let water fall out of the bottom,

No water falls out of the bottom until the bead of water cavitates!

eg you shouldn't have altered the tension in the elastic band (whether the elasticity is due to water stretching or the tube deforming) as you haven't changed the size of the weights on each end... did you pull the ends out of the demi-johns by lifting the whole apparatus or just the ends of the tubes?

Just the ends of the tubes!
Not quite correct Dave, there has been a reduction in the weights, because the water in the two bottles has been disconnected, and these do have considerable weight. Consider the water in the bottles as part of the mass of water inside the tubes and you begin to understand how trees draw water and mineral from the surrounding soil into their roots, or into a cut stem or trunk, with no roots.

ps. by the way my calculation above was considering the force from the whole earth on the hydrogen atoms in a water molecule. Perhaps I should have said that on a molecular scale the earth's gravity is a very small force.

I think you still might be wrong with this way of looking at gravity. Try thinking of gravity as being a huge force capable of holding everything in homeostasis.

If there are any lurkers, please feel free to join in with this conversation.

Andrew

Dave:
quote:Yes, it’s the rigid translucent stuff! The softer walled tube will simply neck )( under the negative tension. This rigid stuff does not neck and therefore the diameter internally will not reduce as a result of the negative tension. If I were able to squeeze it and alter its shape, it still would not alter the volume, as in order to do this one would have to compress the tube equally from all directions and this would take a huge force.


An ellipse will have a smaller area than a circle of the same perimeter (think about how a toothpaste tube works), so you can reduce the volume of a tube by squashing it slightly, without compressing it from all directions.

:Just the ends of the tubes!
Not quite correct Dave, there has been a reduction in the weights, because the water in the two bottles has been disconnected, and these do have considerable weight. Consider the water in the bottles as part of the mass of water inside the tubes and you begin to understand how trees draw water and mineral from the surrounding soil into their roots, or into a cut stem or trunk, with no roots.

I am afraid you can't consider the water in the bottles as hanging off the tubes, the way fluids behave is to do with pressure. so as long as the demijohns are not sealed the pressure at the surface of the water is atmospheric. The pressure will increase as you go down the demijohn, but it will reduce as you come back up the tube, so inside the tube, at the bottle water level the pressure will be atmospheric.

So if you pull the tube out of the bottle, unless the level of the end of the tube is different to the level of the water in the bottle nothing has changed.

Did you lift the tubes up or down when you removed the tubes?

I think you still might be wrong with this way of looking at gravity. Try thinking of gravity as being a huge force capable of holding everything in homeostasis.

I am not sure what you mean by homeostasis, as it is not in the oed and the only definition I can find is that it is a biological system that is stable due to negative feedback. Some systems acting under gravity are stable due to negative feedback - eg water in a glass is stable, but to say everything acting under gravity is under negative feedback is ridiculous - there is no way that a cricket ball in the air is going to be held in position... I am confused by what you mean.

Sorry, I did not make myself clear about the requirement of compressing the whole tube equally. I was relating to the negative water causing the tube to collapse equally, as this would be the case with a liquid under tension. Not at all like a finger and thumb compressing it.

Did you lift the tubes up or down when you removed the tubes?

An ellipse will have a smaller area than a circle of the same perimeter (think about how a toothpaste tube works), so you can reduce the volume of a tube by squashing it slightly, without compressing it from all directions.

If the tube was collapsing,” as you state the case might have been", then there should have been a noticeable rise in water level of the two bottles as the tube was hoisted up the cliff. As the bottles were filled almost to the brim on elevating the tube, the only bottle to begin overflowing was the one with the saline solution in it. When no salt is added and the loop is raised, there is almost no alteration in the bottle levels, indicating tube collapse to be minimal if any.

So if you pull the tube out of the bottle, unless the level of the end of the tube is different to the level of the water in the bottle nothing has changed. Did you lift the tubes up or down when you removed the tubes?.

Lifted them up and out of the bottles and let them dangle in the air.

I think you still might be wrong with this way of looking at gravity. Try thinking of gravity as being a huge force capable of holding everything in homeostasis.

I am not sure what you mean by homeostasis, as it is not in the oed and the only definition I can find is that it is a biological system that is stable due to negative feedback. Some systems acting under gravity are stable due to negative feedback - eg water in a glass is stable, but to say everything acting under gravity is under negative feedback is ridiculous - there is no way that a cricket ball in the air is going to be held in position... I am confused by what you mean.

ho·me·o·sta·sis (hm--stss)
The ability or tendency of an organism or a cell to maintain internal equilibrium by adjusting its physiological processes.
The processes used to maintain such bodily equilibrium. Fits ok with this paradigm and discussion on trees and plants?
Your cricket ball is in the air, because gravity holds the atmosphere in place, and it will eventually come back to earth and its ultimate resting place, due to the inevitable effects of gravity, no matter how hard you throw it.
Just like a nuclear explosion is brought back under control by gravity

Dave:
Think back to how the soft tube collapsed when you first tried it, it will neck - go flat. This is because the cross section can reduce in area by changing from a circle to an ellipse - Under a vacuum the stiff pipe will turn into an ellipse slightly, but be strong enough to not actually collapse. However if the level of water didn't change in the bottles the collapse wasn't very significant.

However if the water was stretching then the water level in the bottles should increase as you pull up the tubes too... I am not sure what is happening with the water going up the tubes, I expect that there is something subtle going on with exactly how you are doing the experiment as I don't think either of our explanations work.

Out of interest what happened when you took only one tube out of the bottle?

Not really, a force doesn't necessarily move a system towards an equilibrium (look at how the moon keeps falling towards the earth), a system may be designed to maintain an equilibrium.
You have to look at how the system is set up to find out whether it is stable or unstable

ps The cricket ball is in the air because I threw it there not because of the air - if I throw a ball up in the moon it will be up...