How do Trees Really lift Water to their Leaves?

Dave: 30/04/2005 14:27:22 »
You see I think your experiment proves that it is possible.

Why isn't the water cavitating at the top of the tube in your experiment? It is the pressure that drives cavitation, if it is below the vapour pressure of water the water will prefer to be a gas than a liquid, so it ought to be boiling.

If you consider the case of your experiment and a sealed tube (made out of the same substance as your tubes were) the pressure at the top will be identical, so they should behave similarly. If the water is over 10m high the pressure will be negative and it should be cavitating in both cases.

Now your experiment has very cleverly shown that water will not always cavitate (this is something I didn't know before talking to you and is very interesting) as although on balance it would prefer to be a gas, breaking the surface tension is difficult so the probability of cavitation is low enough for you to do your experiment without it always cavitating. If the surfaces of the tube are covered with something hydrophillic (water loving) and made very small, the probability of cavitation will be lower.

Now I think this means that the single tube should be similarly stable.

I think you are right putting dry salt on the top of the column would probably mean that air can get to the membrane and you would get cavitation (This may be why trees tend to cavitate more in droughts as the membranes at the top dry out allowing air in at the top -> a cavitation), but if we slightly alter the design to be more like a real tree and have the top of the membrane covered with a strong solution it may well work eg:
As now the concentrated solution at the top will act as a seal over the membrane stopping gasses getting in.

I think I will try a small scale version of this at home using some of the thin stuff you get between onion layers as the membrane- I think making the membrane strong enough to support 10m of water will be difficult but that is a purely engineering problem.

Rosy:
Andrew
I don't understand in what way the link to the "straightdope" site supports your argument. Cecil Adams describes it what happens beautiffuly, and then rejects the idea based on the fact that the water will cavitate above 10m.
As your experiment establishes that it will *not* necessarily cavitate that problem with the syphon vanishes.
The reason why a barometer can only go up to 10m, driven by atmospheric pressure, is because the water, whilst it will cohere to itself, cannot stick to the top of the barometer tube so a vacuum forms against which 1 Atm supports a 10m column.
In a loop of water, if there is no cavitation then there can be a syphon. If cavitation occurs, as you say, the water on both sides falls back to form two 10m columns.
The syphon works because the pressure at the top of loop is due to the weight of the water in the columns of either side. Since this weight is greater in the longer column there is a net force over the top of the loop towards the longer column.
Your system, likewise, has a greater weight of water/solution in the "down" column.
The pressure change in the soft tube on injection of an amount of concentrated solutiondoesn't seem to me to be particularly surprising. The soft tube absorbs the energy of the change in height of the extra weight and then transfers it back to the water by evening out the flow.
I don't think this is particularly relevant... the weight is still providing the energy, there's just then a delay in transferring it back into the system.

And the water in two different cells, when separated by a cell membrane, isn't in contact in the first place to it isn't "torn apart" by the cell membrane.
In order for the water to pass between cells it has to pass through a *very* small pore... too small for a glucose molecule to pass through. This has got to involve breaking some water-water hydrogen bonds!!
This process is due to two factors, osmotic potential and pressure. There'll be an effect due to the weight involved here (a pressure factor), but the osmotic effect works the other way as that's how the concentration gradient runs.
I don't know how much water actually gets out of the bottom of the phloem. Anyone (Andrew? Dave?) want to find some figures? Is it actually known?

Maple trees growing upside down... well, they don't seem to have died of all their solutes falling to the leaves. And of course the branches turn upwrds again as they grow, that's where the sun is!

@ Rosy, the concentrated sap in the phloem that is not actually taken up by the trees increasing growth cycle, is rediluted by incoming water from the soil, under a negative pressure generated by the falling sap.

Be my guest and try to make a siphon work over the 10 metre limit. It does not work!, the weight of the water in the lowered side of the inverted U tube, merely stretches the water until it breaks and a cavity forms, causing the water to fall to remain at the 10 metre mark. As the Brixham experiment demonstrates, it is possible to raise the water in excess of 14 metres, but eventually the water bead will break. When I say a siphon will not work, it is because I have actually tested it!

Rosy:

the concentrated sap in the phloem that is not actually taken up by the trees increasing growth cycle, is rediluted by incoming water from the soil, under a negative pressure generated by the falling sap.

OK, I have absolutely no idea what this is supposed to mean.
How on earth can the falling sap generate negative pressure in the roots? I think I must have misunderstood what you mean by this.

As the Brixham experiment demonstrates, it is possible to raise the water in excess of 14 metres, but eventually the water bead will break. When I say a siphon will not work, it is because I have actually tested it!

I'm afraid I simply don't believe that this is because it's impossible.
If you can (1) have a column of water 14m high, which you've shown you can and (2) you can exert a downward force on one side of the loop by introducing the extra weight of a few ml of salt solution.
I'm very interested to know how you conducted your syphon experiment (whether you set it up with two containers at different levels then raised the syphon tube, or whether you established the raised loop then raised/lowered one of the containers... I'm interested as which you did will influence rises/drops in pressure (I wonder because you refer to the "lowered side" of the inverted U.
Of course, I'm aware that you're in the very difficult position of attempting to prove a negative, and so you can win only by reasoned argument... and I'm not making your life easier by not being convinced

Sadly I'm not currently in a position to carry out the experiment to my own satisfaction at present (or any time between now and mid-June) as I'm in Cambridgeshire (no cliffs) and don't have the kind of access to a high level window/the ground below it I'd need to do anything constructive.

@ Rosy
Ive thought of a way to explain what happens when you try to siphon higher than 33 feet limit.

Picture some play slime /goo stuff that kiddies (and me) play with, when you pinch a little between your finger and thumb and try to lift the rest of the mass, it stretches until it snapps off. This happens with water inside the tube also.

With regards to the falling sap generating a negative pressure at the roots, relates to the suction at the lower part of the tree that draws water in. Even if the roots are removed, the suction is still there, rulling out root pressure as a driving force. What I should have said to be more accurate is; the falling sap generates a positive pressure in the phloem in front of the falling sap, and a negative pressure is caused behind the falling sap.

Hope this helps

Dave:
Just to clear up a quick point that may have been causing confusion:
When me and Rosy are talking about negative pressure, we mean negative absolute pressure - where a vacuum is zero pressure.

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.

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.

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.

Would your syphon work if you added the salt near the bottom of the system? if so how is this different from adding an extra weight of water by lengthening the tube?

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?