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The height limit of a siphon

People might like to consider a recently published paper describing a siphon operating at 15 m, way above the 10 m height limit at sea level. http://www.nature.com/articles/srep16790 (StephenHughes (talk) 13:48, 3 December 2015 (UTC))

@StephenHughes - I posted this at Nature a couple days ago but it says my comment is still pending. That's much too slow a posting rate for a reasonable dialog, so I'll post it here as well.

First, congratulations on some fine research. Although I suspected that water might not boil in vacuum if chilled near freezing, I actually thought it would. I'm a little surprised that it didn't boil, and I'm very surprised that it didn't boil at room temperature. The oil cap is quite clever. I considered a cap made of floating glass or some such, but thought there might be problems with evaporation, freezing, or friction around the edges. The oil cap solves that completely. I think the evaporating water film left on the sides of the reservoir in your vacuum water siphon does not create enough pressure to matter, and that part of the experiment need not be repeated. But others may not agree, and if you wanted to lock that experiment down as well, then maybe you could use flexible glass or nylon tubing, so that after degassing the water you could raise the siphon tube by flexing it up while holding the reservoirs in a constant orientation throughout the experiment. Next it would be interesting to do a siphon to the height of the worlds tallest tree (115m). If that works then your drone or helicopter trick might be in order to see just how high we can go. Wind could be a problem, along with the expense of a helicopter. I don't know if there are any accessible cliffs/bridges/buildings that are tall enough. A non-horizontal siphon running up a mountain side might be good. Another possibility is to drop the tube down into the deep ocean. A sealed chamber at one atmosphere at the bottom would take care of the pressure issue.

But as for Wikipedia, the question is, should we modify the Wikipedia article to primarily reflect the conclusion of the Boatwright/Hughes 2015 article that "This experiment provides conclusive evidence that siphons operate through gravity and molecular cohesion." Wikipedia discussion guidelines suggest that we start by clarifying what we agree on. So I have some questions about how you see siphons working.

1. Have the authors noticed the Flying Droplet Siphon as shown in the Wikipedia siphon article? 2. Does that demonstrate a siphon that operates through atmospheric pressure, rather than molecular cohesion, to push the water up from the higher pressure of the upper reservoir to the lower pressure in the air chamber, sort of like in a barometer?

3. In virtually all practical siphons (except possibly extremely tall ones) is it true that there is positive absolute pressure relative to pure vacuum at all zones in the siphon? 4. Does positive absolute pressure at all zones in the siphon imply that all the liquid molecules are repelling each other rather than pulling any of the liquid up? If so, then you can ignore my next two paragraphs.

If not, then consider a small hollow glass cube moving up the up tube of a water siphon. Assume the source and destination reservoirs are at nearly the same height, so that the water is flowing so slowly that the pressure drop due to velocity is negligible. Assume the cube has an average density equal to water and its hollow center is pure vacuum. Assume the cube floats with its top surface level. Imagine the walls of the cube have small strain gauges embedded in them to measure the pressure or tension force on the walls of the cube.

5. Would you agree that out in the air at sea level, the gauges in the faces of the cube would measure an inward force on the cube faces corresponding to one atmosphere of pressure? 6. Would you agree that at about 15m up a 16m water siphon, the cube would measure an upward pulling force on its top face corresponding to about negative one half atmosphere? 7. Would you agree that if the top face of the cube was at the barometric height, about 10m up the siphon, the cube would measure approximately no force up or down on its top face? 8. Would you agree that at 5m up the siphon, the cube would measure a downward force on its top face corresponding to positive one half atmosphere? 9. Would you agree that just inside the entrance to the tube, assuming it is near the surface of the reservoir, the cube would measure a downward force on its top face corresponding to about positive one atmosphere of pressure? 10. Would you agree that a downward force on the top of the cube when the cube is below the barometric height, would imply that the cube is not being pulled up by liquid cohesive forces, but rather it is being pushed up from below by more than it is being pushed down from above? 11. Would you agree that if water was in the place of the cube in sections of the siphon below the barometric height, that the water would be pushed up by the water below more than it is pushed down by the water above, and that it is not actually pulled up when it is below the barometric height and at a pressure above zero absolute pressure?

I hope these eleven questions are not too many. I tried to make them all quick easy yes/no questions so as not to overwhelm. Even if you feel that good answers require elaborate explanation that you don't have time for, I would hope you would give at least yes/no answers and maybe only on questions where you feel it is needed, at least just short one or two sentence explanations, so I can get some idea of what you're thinking.

Thank you. Mindbuilder (talk) 20:31, 6 December 2015 (UTC)

Thank you mind Mindbuilder. I am happy to answer many of your points where possible, taking them in turn as far as possible. However to keep the flow and save on repeating arguments I won’t always expand or justify every answer

1) Yes I am aware of the flying droplet experiment. No this does not prove that atmospheric pressure is the driving force behind a siphon. As the main article says the word “siphon” refers to a wide variety of devices. This makes the definition of a siphon particularly difficult not least because in the sense of a bent u-tube some devices i.e. coffee siphon are clearly not siphons at all. The fact that this experiment works in atmosphere is fine but I fail to see how this proves that atmosphere is the main requirement. Both ends of the tube are sealed and the top is effectively a vacuum vessel. If I put a hose from an evacuated chamber in a beaker of water the atmosphere will push the water into it. The only difference in this case is there is a tube on the other side which lets water out thus balancing the flow in and out. We know how a barometer works there is no argument that the atmosphere can push water up a tube but this is not the driving mechanism behind a siphon.

2) As above 3) Yes

4) No. Molecules do not repel each other. I’m not sure why people believe this but it is only true when all these criteria are met together – the molecules are charged (not summed to zero) and the charges are like in sign and the charge distribution is similar. Otherwise all molecules will stick when put together. The only reason they don’t in gasses is that they carry too much energy when they collide thus fly apart again. In liquids they do stick together but the bonds are weak so may be broken and remade easily. However the energy in the liquid is low enough that the molecules do not separate unless they are at the surface.

5) Yes 6) Yes 7) Yes 8) Yes 9) Yes 10) Yes 11) Yes

The questions you ask are not and never have been in dispute by myself (I cannot speak for Stephen). However a siphon is still governed by cohesion and gravity, and not atmosphere. None if the questions you pose change this fact. The fact that most siphons are run with atmosphere providing a supporting role does not mean that it is the primary method. The only experiment to possibly counter our argument is the carbon dioxide siphon. However the experiment as performed by Ramette and Ramette did not properly investigate the mechanism of the carbon dioxide siphon. Unfortunately they managed to muddy the water, so to speak, as they published their results before I could report the same experiment so I dropped that from my subsequent high vacuum siphon paper (also submitting papers through peer review requires subtleties that often limit full discussions, sad but true).

The issue is that for a siphon to work it must have laminar flow at the entry and exit of the siphon. The key to laminar flow is that the material must behave in a fluidic way. Sand and other material will not do this but carbon dioxide does, especially when it is cold as in their experiment (dry ice). At low temperature the gas will behave in a very similar way to any other liquid as it can be poured and it will sink to a lower level. However hot (room temperature carbon dioxide) is less liquid like, thankfully, or we would all suffocate. If any normal siphon is run without laminar flow at the entry or exit it will not work. The key point in obtaining laminar flow is the molecular interactions. This is why cohesion is a key part of any siphon.

I would also like you to do a very simple experiment. Try setting up a siphon as normal with the bottom leg just a couple of mm above the surface of a “filled” beaker but one that over flows as the water is added. Now measure the flow rate. Now do the same experiment with the same height differential between the tube entry and exit but with the bottom leg well above (~30 cm) a container so the water will flow out and fall away from the bottom of the tube some distance. See if the flow rate is the same. According to the atmospheric principle they should be the same from the main article “the flow at the output of the siphon is still only governed by the height difference between the source surface and the outlet”, under the cohesion principle they are not. You may need to repeat the experiment carefully many times to get a good average of measurements. AdrianBoatwright (talk) 17:38, 8 December 2015 (UTC)

Your idea, that a siphon will have a greater flow rate if the stream exiting the open end is allowed to fall, rather than quickly entering a water surface, is rather interesting. I'm not set up to do a precise measurement of the situation right now, but you may very well be right about it. I don't deny that water and other liquids can have considerable tensile strength, as proven in your vacuum and 15m siphons. And thank you so much for doing those experiments and recording video of them. I loved seeing them.

If molecules don't repel each other, then what holds the liquid up in a barometer? In particular, consider the top layer of liquid in the barometer. Gravity is exerting a downward force on the molecules, yet they are not accelerating downward at g. If they're not going down, then there must be an upward force on those molecules to balance gravity. There is literally nothing, or almost nothing, above the top layer of liquid, to pull up the molecules, so there must be an upward repelling force on the top layer of molecules by the molecules just beneath, right? You have acknowledged that a glass cube floating in a siphon, below the barometric height, would experience a downward force on its top and an upward force on its bottom. Are those not the repulsive forces of the liquid molecules against the glass cube surface? Mindbuilder (talk) 05:37, 9 December 2015 (UTC)

Molecules don't repel each other unless they meet the conditions I state above. I have no problem with air pressure applying a force. That is not the point, the point is without cohesion you cannot make a barometer or a siphon. If the molecules don't form a seal at the base or exit then both devices will fail. Sand, marbles or any other small objects will not siphon because they don't make a seal and do not hold together. Forces don't just switch on and off at will, if there is cohesion between molecules at the top of a 14 m siphon then they are still there at the base. The only change is the extra force provided by the air. The whole argument is not that atmosphere doesn't affect the environment but that it is not the primary cause. Just repeating the same arguments on the atmospheric pressure does not make the case any stronger. A siphon can/does work above barometric height and in vacuum, also the exit/entrance diameter and height above the surface all affect a siphon. None of these facts are addressed by the atmospheric model. Therefore a new model is needed which is the cohesion model. It works, it makes the right predictions so it is most likely right until proved wrong by a better EXPERIMENT or NEW THEORY. Since neither of these two last criteria are met please let it stand that the cohesion model is the right one.AdrianBoatwright (talk) 16:04, 9 December 2015 (UTC)

I agree that cohesion plays some part in a siphon to prevent leakage like with sand. While forces don't switch on and off at will, they can arise, disappear, or reverse as objects are brought into contact or pulled apart. For example, if you pull two train cars apart, the coupling between them will exert an attractive force pulling them together. Then if you push them together, the coupling force will reverse and exert a repulsive force pushing them apart. I'm not clear on what you think holds the top layer of liquid up in a barometer. You seem to acknowledge air pressure exerting a force, but of course you know that the air pressure is only acting on the surface of the barometer liquid in the reservoir. That force has to be transferred up to the top layer of the barometer. Isn't the atmospheric pressure transferred in a series of repulsions between the liquid molecules up the barometer column? Perhaps we need to discuss force at a more basic level. My understanding is that from a classical mechanics perspective, all forces are either attractive or repulsive, is that correct? Do you think the atmospheric pressure somehow exerts an attractive force on the liquid in a barometer to hold it up? What prevents water from compressing under pressure like a gas does? For example if you blow up a small balloon, tie it off, and push it to the bottom of a 1m deep bucket, its volume will reduce by about 10%. But if you do the same with a small balloon filled with water, its volume will be virtually unchanged. How does the water in the balloon maintain its volume if the molecules in the balloon don't repel each other? Mindbuilder (talk) 16:42, 9 December 2015 (UTC) Modified at Mindbuilder (talk) 16:54, 9 December 2015 (UTC)

Maybe the moving liquid state is confusing our discussion. Do molecules in solids exert repulsive forces? For example, lets say we freeze some water into two ice cubes, and stack one on top the other. Then do the water molecules in the upper face of the bottom ice cube exert an upward repulsive force on the water molecules on the lower face of the upper cube? Mindbuilder (talk) 17:04, 9 December 2015 (UTC)

No, neutral molecules do not exert repulsive forces. In the case of the ice cubes they are not repelling the ice above but rather not undergoing compression. You are not pushing the air up but resisting the compression of air pressure. If the air was removed from above you you would not lift off the ground neither would the lower ice cube if the one above is removed. To resist compression is not the same as repulsion or repulsive forces. When forces balance they are not implicitly repulsive but rather just in balance. On a quantum level the electrons in molecules are repulsive to each other but attracted to the nucleolus, hence both forces are in balance in each molecule. When a new molecule is brought near another the electrons in both will feel both the other electrons and the positive nuclei in both at the same time. The net force is zero. In truth the electron orbitals in the molecules will overlap and find the lowest energy positions where they can co exist. Essentially they cannot pass through each others orbital.

Before I depart for a little while could you also add Simon Puttick and Peter Licence names to the section on modern siphons. They made an honorable contribution and have been somewhat overlooked described as a research group. AdrianBoatwright (talk) 17:43, 9 December 2015 (UTC)

Yes, I'll put in credit for Puttick and Licence. I'll do it after I finish this post so you'll have time to chew on this while I'm adding that.

I think we are making good progress. As it happens so often, it seems our misunderstanding is largely a result of differences in definitions. In particular, you seem to have a definition of the term "repulsive force" that is different from mine. I'm using repulsive force in the rather broad sense as in the OED of simply "a force under the influence of which objects tend to move away from each other". The word "tends" is important here since a force that tends to influence objects to move away may not actually make them move away at all if there is an opposing force. I think I'm using it in a similar sense to this quote about elasticity in solids:

"... when the interatomic spacing is greater than its unstressed value, the attractive forces between atoms must be greater than the repulsive forces (the attractive forces balance both the repulsive forces and the forces you impose). Conversely, (ii) when the interatomic spacing is less than its unstressed value, the repulsive forces between atoms must be greater than the attractive forces." http://www.animations.physics.unsw.edu.au/jw/elasticity.htm

Apparently I'm not the only person to use the term "repulsive force" in this way. Do you think that usage is incorrect? Your definition of repulsive forces seems to be limited to electromagnetic forces (and maybe nuclear forces). But it is my understanding, and apparently at least one other Wikipedia editor's understanding, that contact forces are the result of the electromagnetic interactions between atoms/molecules. It seems that when even neutral molecules get too close to each other, the charges somehow arrange to create a net repulsive force pushing the molecules apart.

"Molecular and quantum physics show that the electromagnetic force is the fundamental interaction responsible for contact forces. The interaction between macroscopic objects can be roughly described as resulting from the electromagnetic interactions between protons and electrons of the atomic constituents of these objects. Everyday objects do not actually touch each other; rather, contact forces are result of the interactions of the electrons at or near the surfaces of the objects (exchange force)." Contact force

But it is not very important what our definitions of "repulsive force" are, so I'm going to drop that terminology.

Do you agree that when two ice cubes are stacked, that the top layer of molecules of the bottom ice cube exert a net force with an upward vector direction on the bottom layer of molecules of the upper ice cube, and that that force is conventionally called the normal force? Or do you think the normal force wouldn't exist in this stacked ice cube example because it is just "not undergoing compression" instead? And do you agree that according to Newton's Third Law, the bottom layer of molecules on the upper ice cube exert an equal and opposite net force with a downward vector direction on the top layer of molecules of the bottom cube?

And do you yet agree that in a siphon less than the barometric height, and therefore at positive pressure, that the molecules have these pairs of contact forces between them which are sort of similar to the normal force and its reaction pair, that are on average NET pushing the molecules apart rather then pulling them together? They may have some attractive forces between them as well, and the forces pushing apart may be balanced by the pressure forces, but aren't the NET forces between them pushing apart, so as to resist the compression of the pressure? Mindbuilder (talk) 07:13, 10 December 2015 (UTC) modified Mindbuilder (talk) 09:09, 10 December 2015 (UTC)

@AdrianBoatwright - You mentioned the Ramette CO2 siphon paper but didn't mention the FlinnScientific video, so I thought you may have missed it, as I did for quite a while. http://www.youtube.com/watch?v=FWybQPxKy1U

Atmosphere Not Doing Work On Constant Height Source Reservoir Surface

@StephenHughes - Do you still subscribe to the theory that atmospheric pressure can't be pushing up the liquid in a siphon because the surface of the source reservoir can be held constant so that the atmosphere does no work on the surface? I don't think I've mentioned the flaw in that argument, so I thought I'd do it now.

It is actually true that the atmosphere does not really push the liquid up the siphon. The pressure of the liquid at the entrance of the siphon is what actually pushes the liquid up the siphon. We usually just leave out this minor detail to simplify. Of course the pressure of the liquid at the entrance is primarily due to atmospheric pressure. For example if the entrance is .1m under the water surface of the upper reservoir, and atmospheric pressure is equivalent to 10m water head height, then 99% of the pressure at the entrance of the siphon is due to atmospheric pressure, and 1% is due to hydrostatic pressure. The atmosphere pushes down on the reservoir surface molecules, those surface molecules push down on the ones below them, and those push down on the ones below them, and so on in a chain of pushing, until the pressure reaches the entrance of the siphon. But that chain of pressure from the atmosphere through the water to the entrance can have other sources of pressure thrown in. Specifically, if you add a stream of water pressurized to atmospheric pressure, or pumped into the upper reservoir, you have added to and complicated the source of pressure at the entrance of the siphon. You can see this must be the case when you consider keeping constant the source reservoir surface of the flying droplet siphon, or even more obviously, a barometer. I hope you would not claim that keeping the source reservoir surface of a barometer at constant height while the pressure at the entrance is pushing liquid up into the evacuated tube, would prove that it could not be atmospheric pressure that pushes the liquid up barometers in general. Mindbuilder (talk) 22:58, 10 December 2015 (UTC)

Top Branch Siphon Demonstrates No Pulling In Siphon At Atmospheric Pressure

I did this siphon sketch a while back and decided I should get it out there. This siphon demonstrates that the liquid in the down tube is not pulling up the liquid in the up tube, because if it was, then the liquid in the top branch would get pulled down, because the liquid in the down tube can't tell the difference between the liquid in each branch and so it can't discriminate between which branch it pulls on. If there was a pull on the liquid in the top branch, it would happily get blown down by the gas pressure and fall down by gravity. There must be some force to push the liquid in the top branch up to keep it from falling. If it is atmospheric pressure pushing the liquid way up, then atmospheric pressure can just as easily push the liquid over the siphon without a pull from the liquid going down. This siphon has been tested with water and it does work steadily and the water does stay in the upper branch and is not pulled down by the water in the down tube.

Note that while the air in the top branch drops to a partial vacuum pressure, it does not suck the liquid up, because even partial vacuum air is a pressurized gas and therefore tends to expand and applies pressure on the surfaces that contain it, such as the liquid surface. For example, if the siphon was 1m tall then the partial vacuum pressure pushing down on the surface of the water in the top branch would be only about 90000 N/m^2 instead of the full normal atmospheric pressure of 100000 N/m^2. Or about 9N instead of 10N downforce for a siphon with a cross section area of 1cm^2. But that is still downforce not a sucking upwards pull. Mindbuilder (talk) 08:46, 13 December 2015 (UTC)

Here is another example perhaps even more clear.

Note that neither of these siphons will work in a vacuum or to heights exceeding the barometric height, even with a low vapor pressure liquid, because as the pressure at the top approaches zero, the gas bubble at top will have nothing to keep it squeezed small and so will expand until it empties the tube. Practical examples of siphons operating with top branches with air bubbles are common because bubbles sometimes accumulate at the top if they are not swept out and because big siphons are often either suction pumped or filled, from a top T fitting. e.g. https://www.youtube.com/watch?v=go0U1n55jCE Mindbuilder (talk) 08:05, 14 December 2015 (UTC)

At Mindbuilder: re your comments:

"But as for Wikipedia, the question is, should we modify the Wikipedia article to primarily reflect the conclusion of the Boatwright/Hughes 2015 article that "This experiment provides conclusive evidence that siphons operate through gravity and molecular cohesion."

1. Have the authors noticed the Flying Droplet Siphon as shown in the Wikipedia siphon article? 2. Does that demonstrate a siphon that operates through atmospheric pressure, rather than molecular cohesion, to push the water up from the higher pressure of the upper reservoir to the lower pressure in the air chamber, sort of like in a barometer?"

One can operate a siphon such that you let a continuous stream of air bubbles in, either by ever so briefly removing the inlet tube from it's water source, and continuously repeating this. Or by drilling a hole in the side of the inlet tube, just above the water level, then covering this hole with your finger, then repeatedly briefly removing your finger and then covering the hole back up again. One could automate this with valve on a side branch.

"This experiment provides conclusive evidence that siphons can operate without molecular cohesion."

Which is the opposite to the conclusion that Hughes has reached

"This experiment provides conclusive evidence that siphons operate through gravity and molecular cohesion."

In relation to your 2nd question:

"2. Does that demonstrate a siphon that operates through atmospheric pressure, rather than molecular cohesion"

The constant air bubble siphon clearly demonstrates molecular cohesion is not at play with such a siphon, however I would suggest that you can't simply replace molecular cohesion with atmospheric pressure. That just adds confusion, since others will say you are wrong, on the basis of the atmospheric pressure claim, and ignore the part that is correct that molecular cohesion is not at play. — Preceding unsigned comment added by 124.186.78.163 (talk) 15:28, 18 December 2015‎ (UTC)

Are you saying that it is neither molecular cohesion nor atmospheric pressure? If not one of those then do you have an idea what it is that makes the water go up even though gravity is pulling it down? I thought those were the only two recognized theories. Do you agree that it is atmospheric pressure that pushes the liquid up a barometer? Mindbuilder (talk) 11:01, 19 December 2015 (UTC)

@ Mindbuilder Re Question: Do you agree that it is atmospheric pressure that pushes the liquid up a barometer? Is that a question or a statement? You have just be shown a siphon operating with a 15 metre height. They could easily have made a barometer with a 15 metre height. Or a barometer in a vacuum chamber.

That was a question. I'm glad I asked it since I appear to have gotten an unexpected answer. You appear to think that a barometer can exceed 15m and work in a vacuum. I don't think that has ever been demonstrated. There is only one way I can think of that it would be possible, which would be if the barometer was filled with a liquid all the way to the top so that it could stick to the top and then inverted into the reservoir. That could probably work for a water in glass barometer, because water sticks well to glass, but I think it's very unlikely to work if the tube had any grease on the walls above the barometric height so the water wouldn't stick, and I think it's unlikely to work much above the barometric height for a mercury in glass barometer because the mercury doesn't stick very well to glass. Although mercury siphons in vacuum have been demonstrated a little above the barometric height, demonstrating some sticking to the glass. So a mercury barometer could exceed the barometric height a little.

But what would happen when the liquid is stuck to the top of the barometer isn't very interesting. The interesting question is what makes the liquid go up the barometer when the barometer starts out with pure vacuum inside, with a closed top, and then a valve at the submerged bottom is opened to allow liquid to go up the barometer. Would you agree that in those circumstances that it is atmospheric pressure that pushes the liquid up the barometer? The very purpose of a barometer is to measure atmospheric pressure after all. And I'm still wondering what your answer to my previous questions are. In particular: Are you saying that it is neither molecular cohesion nor atmospheric pressure that makes the liquid go up? If not one of those then do you have an idea what it is that makes the water go up even though gravity is pulling it down?

I note that some of those doing siphon experiments, often only make conclusions based on a single experiment, totally ignoring other experiments that could question their conclusions. And that many authors, regardless of the results of an experiment, always seem to find a way to explain how it supports their view.

I think I have addressed every experiment related to this subject that has been brought to my attention. And I don't dodge questions on the subject like stubborn people do. Although I do sometimes skip questions unintentionally, and sometimes intentionally, I'll answer them directly if my omission is brought to my attention. It does also amaze me how many surprising explanations people have to maintain their theories. I try to listen carefully to criticisms of my theories and recognize the faults and dump my theories or modify them when needed, but I can't claim to be perfectly immune from confirmation bias.

Maybe the siphon article should list the distinct range of experiments and their primary conclusion, which might encourage those doing experiments to address before they make their own conclusions:

That sounds like it might be a good idea.

Examples:

Siphon with air bubble/s or air break chamber: Siphon can operate without cohesive forces

Fat upleg siphon: Shows the chain analogy to be a poor choice, the lighter weight side is winning

Vacuum siphon: A siphon can operate without atmospheric pressure

But only if the liquid can stay stuck to the tube walls.

15m siphon: Atmospheric pressure can't be pushing water to the top

Although atmospheric pressure isn't pushing water above the barometric height, it is pushing it up *to* the barometric height. From there up, it is being pulled up.

Vacuum barometer: A barometer can operate without atmospheric pressure

I don't think that has ever been demonstrated and I don't think it could if the liquid wasn't stuck to the top of the barometer first. Mindbuilder (talk) 23:43, 28 December 2015 (UTC)

15m barometer: Atmospheric pressure can't be pushing water to the top

Hypobaric chamber siphon: atmospheric pressure regulates the maximum vertical height of a practical siphon

Pascal siphon: Not a siphon as such in the traditional sense, operates on pressure difference due to different density between water, mercury and air. It is effectively a waterfall, or should we say a mercuryfall.

Stephen Hughes response to Mindbuilders questions

Hi Mindbuilder, Apologies for the late reply. I've bundled together my answers to your questions in a single post. Thanks for the 11 questions. I agree with all of Adrian’s answers. I just have a few extra comments on the Flying Droplet Siphon (FDS). In my opinion the FDS is not actually a siphon but essentially the same as a hose spraying water up into the air. Water comes out of a garden hose because the water pressure is greater than atmospheric pressure. In a FDS, the weight of the water in the downward arm creates a low pressure region at the top. The differential pressure drives water into the upper chamber. The water drops down on top of the suspended water column increasing its weight so that it drops down until the atmospheric pressure gradient is commensurate with the weight of the column of water. And so the cycle continues.

You ask whether I still hold the view that stationary reservoir levels indicates no energy transfer. Yes. This is a fundamental physics issue. In this context, energy is the product of force and distance. If there is no change in the reservoir levels the distance is zero and therefore there is no energy transfer.

A siphon with a vertical plug at the top is interesting. In this siphon, the pressure in the tube projecting from the apex is below atmospheric pressure. The pressure difference between the atmosphere and the low pressure region at the top of the tube is sufficient to hold the ascending vertical column in balance. When water is flowing in the siphon the water in the vertical plug connected to the apex of the siphon is essentially the same as the wall of the siphon. StephenHughes (talk) 08:48, 10 January 2016 (UTC)

Thank you for your reply. I really love to understand other people's thinking on this subject.

You have made an excellent description of how the flying droplet siphon works. You have a right to your own definition of a siphon just as much as I do, but I consider the FDS to be a siphon because it has two key features, among others, (1) It gets liquid to flow up over an obstacle, and (2) the upward flow is ultimately powered by the downward force of gravity, rather than pumps. Sure it has an air chamber at top, but then many practical siphons do have air in a top T-fitting or bubbles or such. One of your own siphons that you published had a waterfall through vapor at the top.

What if you have a barometer tube, closed at the top, with a valve at the bottom, with nothing but pure vacuum inside, and you submerge the valve end in a reservoir and open the valve just enough to slowly let the liquid in. And while letting the liquid in you add just enough liquid to the reservoir to keep the surface a constant height so the atmosphere does no work. Then where is the energy coming from to push the liquid up the barometer? Isn't the energy coming from adding the liquid, which is probably pressurized to about atmospheric pressure, to the reservoir? The force part of the energy is coming from the pressure of the added liquid, and the distance part of the energy is coming from the movement of the added liquid into the reservoir, right?

Of my original 11 questions the main difference between Adrian and I seemed to be that he thought there are not repulsive forces between the water molecules. Do you agree with me that this resistance to being compressed is much the same as the normal force, between for example, two ice cubes stacked on top of each other? Do you think this author is wrong to use the word "repulsive" in discussing these forces?:

"... when the interatomic spacing is greater than its unstressed value, the attractive forces between atoms must be greater than the repulsive forces (the attractive forces balance both the repulsive forces and the forces you impose). Conversely, (ii) when the interatomic spacing is less than its unstressed value, the repulsive forces between atoms must be greater than the attractive forces." http://www.animations.physics.unsw.edu.au/jw/elasticity.htm

The above quote is in the context of the elasticity of solids, but you can consider the possibly small but not negligible elasticity of frozen mercury if that simplifies the question. Or do you think repulsive forces do exist between atoms of neutral non-polar solids but not in liquids?

If you think those forces are not repulsive forces then what word or short phrase other than repulsive would you use to label that effect where two molecules, which are pushed together, resist being compressed into the same space? In the classical physics context, is it good for us to describe it as a force of some kind, like the normal force, just not a "repulsive" force, or is it not a force at all, even in the classical physics context? Is the normal force a repulsive force? In the classical physics context, should we count the effects of the Pauli Exclusion Principle and orbital overlaps and such, as two atoms are being pushed together, as repulsive force, for example in a free body diagram of a molecule, or as some other kind of not repulsive force, or not a force at all in the classical physics context?

You and Adrian have said you agree with me on my question 10 and 11, that cubes of glass or water in a siphon are pushed down on from above and pushed up on from below. It seems to me from your answer that you are agreeing with me that the liquid is pushed up rather than pulled up a siphon, so I don't see where our difference is, and I have to ask if you both mistakenly agreed with me on 10 and 11? Mindbuilder (talk) 00:02, 11 January 2016 (UTC)

In the top branch siphon diagrams, are you saying that you think the liquid in the exit tube is simultaneously pulling liquid from the entrance branch and pushing up on the liquid in the top branch at the same time? Mindbuilder (talk) 03:44, 11 January 2016 (UTC)

Hi Mindbuilder. When an ice cube is stacked on top of another Newton's third law is in operation. The downward force of gravity of the top cube on the bottom cube is counterbalanced by the upward force applied by the top surface of the lower ice cube. If this were not the case the top ice cube would sink into the bottom ice cube or levitate. As Adrian said, this does not mean that when the top ice cube is removed there is an net upward force applied by the top surface of the lower ice cube. Once again, I agree with Adrian's description of chemical bonds. Just to reiterate, chemical bonds in some ways are like springs. When the separation of two atoms decreases the bond shrinks and the repulsion increases resisting further compression. When two atoms move apart the bond is in tension. In this sense there is compression between atoms but it is not a force that can do work. In the case of a siphon, the hydrostatic pressure decreases from the inlet to the apex but this pressure gradient cannot propel liquid through the siphon. Siphons work through the linkage between atoms - hydrogen bonds in the case of water.StephenHughes (talk) 13:30, 18 January 2016 (UTC)

It's good that you and I seem to be using the word repulsion in about the same sense. Adrian seems to be using it in a much stricter sense.

But I think that it is this repulsive resistance to compression between molecules that is doing most of the work in a siphon at atmospheric pressure. Consider the two stacked ice cubes. If I pinch the bottom cube between my fingers and lift the two stacked cubes straight up, say a meter, then the top cube has had work done on it, its potential energy has increased. But my fingers weren't touching the top cube. The only significant upward force on the top cube was the upward repulsive force from the top layer of molecules of the bottom cube. That upward repulsive contact force times the upward distance is how the work is done on the top cube, right?

Yes, I agree.StephenHughes (talk) 12:17, 20 January 2016 (UTC)

The same happens in a barometer and siphon. Take the barometer described above where the valve at bottom is opened slowly to let liquid rise up into the vacuum in the barometer. Lets say the liquid is mercury atoms because that is a little simpler than water molecules. Consider one of the atoms at the top of the liquid as it rises up the barometer. Lets simplify and ignore the churn of atoms in the liquid and assume the same atom rides up the barometer at the top surface of the liquid. That atom at the top surface has nothing but vacuum above (or maybe an insignificant amount of vapor) so it is not pulled up by any forces from above. As the atom rides up at a steady speed, its potential energy increases, so there must be some force doing vertical work on it. The atoms to the side have no significant net vertical force component. So it is the repulsive force of the atoms below in the second layer that exert an upward force doing work on the top atoms as they go up the barometer, right?

Now consider the second layer of atoms going up the barometer. Those atoms are also not pulled up by anything, rather the first layer of atoms at the surface bear down with their weight on the second layer, somewhat impeding the upward movement of the second layer. The only net upward force on the second layer is from the third layer below. The third layer pushes up on the second layer, doing work on the second layer as it moves up, and increasing its potential energy. The situation is similar on down through the layers of the barometer, each layer being pushed down on from above and pushed up from below.

You might point out that the top layers of atoms in the barometer do exert an upward force on the second layer. That is true because there are some attractive forces between atoms and molecules when they are near each other. But at the same time the top layer exerts more downward repulsive force on the second layer than upward attractive force. Newton's Second Law is sometimes simplified to F=ma but actually more explicitly it is (the net vector sum of forces)=ma When we ask what makes the liquid go up a siphon or barometer, we're not asking from where are some partial upwards forces coming, but rather, from where is a net upward vector sum of force coming from. For each "layer" in a siphon operating at normal atmospheric pressure, the net upward force comes from the layer below, while the layer above does not exert a net upward pull but rather a net downward push.

I agree that the same thing happens in a barometer, but not a siphon. In the case of a vacuum siphon, there is no atmosphere to do the pushing and so the only way that water can flow through the siphon is by molecules flowing out pulling molecules in. The 15 meter high siphon is a de facto vacuum siphon since atmospheric pressure is not sufficient to support the liquid. If the atmosphere is not involved in the operation of these two siphons, it is difficult to see how the operation of a siphon could switch to a different method in the presence of an atmosphere. StephenHughes (talk) 12:17, 20 January 2016 (UTC)

There is one question from my previous post that I hope you won't skip again. Do you realize that in a barometer where the reservoir surface height is maintained by adding liquid to the reservoir as the fluid rises up the barometer, that it is the added liquid that supplies the energy that usually comes from the atmosphere pushing the reservoir surface down? The nebulous shape of the added liquid may make it less obvious, so if it makes the idea firmer, consider the similar situation of maintaining the reservoir surface height by pushing a tall skinny frozen (mercury or water) stick down into the reservoir as needed to maintain the surface height as the liquid goes up the barometer. Mindbuilder (talk) 04:51, 20 January 2016 (UTC)

Yes, I think I agree. In the case of a barometer, the rise and fall in the height of the column of fluid is dictated by the variation in the pressure difference between the external atmosphere and the pressure above the column of liquid (the vapour pressure of the liquid at the given temperature). When the pressure of the external atmospheric increases the air pushes down on the water in the barometer reservoir and so pushes more liquid up the column. In this case the increase in the gravitational potential energy of the water does come from the atmosphere. However, in my opinion this does not mean a siphon works in the same way. Siphons do not have air at the top (unless it is a bubble passing through) - they have a continuous column of relatively incompressible liquid at the top rather than a compressible gas. Another major difference is that a barometer is a hydrostatic device whereas a siphon is a hydrodynamic device.StephenHughes (talk) 12:17, 20 January 2016 (UTC)

The forces between the atoms of liquid in a siphon switch method from push to pull depending on the circumstances of the pressure applied to them. It's kind of like the coupling between train cars. When the cars are being pulled, the coupling exerts attractive force pulling the cars together. When the cars are being pushed, the coupling exerts repulsive force pushing the cars apart. It all depends on that external push or pull being applied. In a liquid, there are both attractive and repulsive forces between the atoms all the time, but when the liquid is under atmospheric pressure, pushing the atoms together, the repulsive forces dominate over the attractive forces and the liquid resists compression. When the pressure drops to zero and then goes negative, the attractive forces between the atoms will dominate the repulsive forces (or the liquid will come apart if pulled to hard).

Mindbuilder, I think the train analogy is excellent. A pump is like a locomotive pushing a train and a siphon is like a locomotive pulling a train. I think I've just seen what you are getting at. Do you mean that the atmosphere is required to hold the water molecules together in a siphon so that they can move together - maybe like holding together some wooden blocks with a elastic band so that they move as a whole and don't fall apart? I think that an alternative way of looking at the situation is that the water molecules are like lego blocks stuck together. The atmosphere is not required to keep the water molecules pressed together so they flow as a cohesive whole, but rather to keep the pressure in the fluid above the vapour pressure of water to prevent water vapour bubbles forming. However, the height limit paper demonstrates that the removal of dissolved gas somehow prevents the formation of water vapour bubbles (cavitation). I think it was the chemist Linus Pauling who said that liquids are much closer to solids than gases (hence the relative incomprehensibility of liquids). So, if we imagine a vacuum siphon where molecular bonds are under tension, when an atmosphere is added the bonds don't uncouple.StephenHughes (talk) 12:58, 21 January 2016 (UTC)

The 15m water siphon is like a vacuum siphon, but only above the barometric height of about 10m. If you tap the top of the 15m siphon, it will collapse down to about 10m, but not all the way down. When the molecules are below 10m the repulsive forces dominate as they are resisting the pressure. As they move on up to 10m they repel each other less and less until at some point around 10m their attraction and repulsion can balance to zero because the pressure is zero. Above that, the higher they go, the more weight of liquid below they are supporting and so they're pulled apart harder and the more the attractive force between them dominates.

Sorry, I don't understand this section enough to be able to comment.StephenHughes (talk) 12:58, 21 January 2016 (UTC)

So consider again a glass cube with pure vacuum inside, strain gauges in the walls, and weighted to be neutrally buoyant, going up a 15m water siphon. Would you agree that as the cube enters the siphon, the strain gauges would measure an inward push on all faces corresponding to roughly one atmosphere (if the entrance is shallow and the siphon speed slow)? Then wouldn't that inward push lessen as the cube rose to about 10m? And at about 10m wouldn't the strain gauges measure approximately no inward push or outward pull? And above 10m wouldn't the strain gauges measure an outward pull on the cube surfaces, the outward pull increasing until it reaches the top? (If you don't agree the strain gauges would measure as above, then what do you think the strain gauges would measure just below 10m, at 10m, and above 10m?)

If you agree with those four questions, then doesn't that mean that when the cube is below the barometric height of 10m that it has a net downward pressure pushing down on the top, and a little more pressure pushing up on its bottom? And here is the key question: doesn't that mean it is being pushed up rather than pulled up? Or is it just my mistaken idea that if something has a net downward push from above, then it can't be said that it is being pulled up?

Yes. The pressure at the top of the cube will always be less than at the bottom. Which is why bubbles ascend in water. However, if we fill a bubble with water it is neutrally buoyant and stays where it is. This is true in a glass of water. Although the hydrostatic pressure decreases from the bottom to the top of the glass there is no net flow of water from the bottom to the top of the glass. So it is in a static siphon. Also, I think it is pertinent to note that on both sides of a siphon - up and down - the hydrostatic pressure decreases ascending to the apex and yet the water still flows. Therefore it is important to distinguish between the hydrostatic and hydrodynamic properties of a siphon.StephenHughes (talk) 12:58, 21 January 2016 (UTC)

Note that you can figure all these pressures dynamically with Bernoulli's equation and if the siphon is running slowly, you'll get only a slightly different result than the static situation. For example, if the surfaces of the source and destination reservoirs differ by 1mm then the siphon will run slowly, and the pressure changes due to the dynamic nature of the situation will only change the 10m zero pressure height by 1mm. Mindbuilder (talk) 02:21, 21 January 2016 (UTC) again at 08:31, 21 January 2016 (UTC)

OKStephenHughes (talk) 12:58, 21 January 2016 (UTC)

@Stephen Hughes. Bubbles don't ascend in water because the pressure is lower at the top of a container compared to the bottom, it is because air (or water vapor) bubbles have a lower density. It is the lower density that makes them buoyant. If you put some mercury into water, it does not rise to the top because the pressure is lower there, rather it sinks to the bottom because it has a higher density.

If you put some water, mercury, oil and air in a jar then shake it up, it will settle in layers based on the density of each. This is an important issue in relation to siphons, and is one of the things what is happening at the outlet, the higher density water wants to settle at the bottom in place of the lower density air, or zero density vacuum. — Preceding unsigned comment added by 124.185.148.142 (talk) 08:56, 22 January 2016 (UTC)

StephenHughes wrote: "Do you mean that the atmosphere is required to hold the water molecules together in a siphon so that they can move together - maybe like holding together some wooden blocks with a elastic band so that they move as a whole and don't fall apart?"

Not really. I was responding to the idea you seem to have that seems to go something like: the siphon has to be in tension or in compression, and since we all agree that the vacuum siphon and the 15m siphon can't be in compression(at least not near the top), and that it is implausible that the atoms would just switch from tension to compression, then therefore siphons must be in tension. Your idea is reflected I think in the following quotes:

"If the atmosphere is not involved in the operation of these two siphons, it is difficult to see how the operation of a siphon could switch to a different method in the presence of an atmosphere."

"So, if we imagine a vacuum siphon where molecular bonds are under tension, when an atmosphere is added the bonds don't uncouple"

What I'm saying is that the operation of the siphon does indeed switch to a different method when you pile a heavy atmosphere on top of it. And while the bonds are under tension in a vacuum siphon, when an atmosphere is added, the repulsion between the atoms increases until it is greater than the attraction. Though they don't uncouple, the repulsion becomes greater than the attraction. It's not unusual or implausible that materials do this kind of switching, depending on the external load applied. Train car couplings do it. Here is a quote from a page describing the interatomic forces of attraction and repulsion. Although it is written about solids, it's basically true about liquids as well:

Take a short piece of metal wire (e.g. a straightened paper clip), and try to stretch it along its length. Unless the wire is very thin or you are very strong, the amount of stetching will be small, and the wire will not break. What has happened here is that you have slightly increased the average distance, r, between the atoms. However, the attractive force between pairs of atoms has been able to resist the tensile force you applied.

Now try to shorten the metal by exerting a compressive force along its length. Here, you have slightly decreased the average distance between the atoms but the repulsive force between pairs of atoms has been able to resist the compressive force you applied.

From this you can deduce that, (i) when the interatomic spacing is greater than its unstressed value, the attractive forces between atoms must be greater than the repulsive forces (the attractive forces balance both the repulsive forces and the forces you impose). Conversely, (ii) when the interatomic spacing is less than its unstressed value, the repulsive forces between atoms must be greater than the attractive forces. http://www.animations.physics.unsw.edu.au/jw/elasticity.htm

You should look at that page, although you maybe don't need to bother studying the equations. In particular look at the purple line in the third and fourth graphs showing total force and total energy as a function of separation difference to see how the attraction and repulsion add up to try to keep the atom separation distance in the bottom of that little droop in the purple line of the fourth graph.

I'll give this paragraph another try, being more explicit.

The 15m water siphon operating at sea level is only like a vacuum siphon in the part of the siphon above the barometric height for water of about 10m. If you bang on the top of the 15m siphon, the attractive forces between the water and the walls will be insufficient and the water will come un-stuck from the walls of the tube and the water will collapse down to about 10m. But the water will not drop all the way to the bottom of the siphon. It will be like a double column barometer with about 10m of water remaining. When a molecule is at some height below 10m in the siphon, either operating or collapsed, it will have the weight of many molecules above it bearing down on it and atmospheric pressure pushing molecules up against it from below, and that will reduce the interatomic spacing slightly, and so the repulsive forces between it and the other molecules will become greater than the attractive forces. As the molecules move on up from the entrance toward 10m, they repel each other less and less, until at some point around 10m their attraction and repulsion can balance to zero because the pressure is zero. When the water molecule gets to the barometric height, the molecules above it are getting pulled up and it is attracted to those molecules above it, so it gets pulled up with them. The interatomic spacing increases, decreasing the repulsive forces between the molecules, leaving the attractive forces greater than the repulsive.

Hopefully we won't have to complicate this discussion with an analysis of the acceleration of the glass cube or the water at the entrance or exit of the siphon. Lets consider a 15m siphon that has reached a very slow and steady speed, and a neutrally buoyant glass cube with pure vacuum inside that has just entered the siphon tube and already reached that steady speed. The cube will continue up the siphon at that constant speed. Since it has zero acceleration, Newton's Second Law says the vector sum of the forces on it must add to zero. We know there is a downward force of gravity on it, so there must be a corresponding upward force to sum with gravity and get to zero.

(1) Would you agree that at all zones within the siphon on the entrance side from the entrance up to just below the barometric height of about 10m, there is positive pressure, even if you calculate the pressure using a dynamic formula like Bernoulli's equation? (2) Is it true that up to, but below that barometric height, the pressure will exert a net downward force on the top of the cube? (3) Do you think most people would say the cube is being pulled up when there is a net downward pressure force pushing down on its top while it is below the barometric height? (4)Do you think the attractive forces between the water and the top of the cube mean the cube is being pulled up even though the water is exerting downward repulsive forces on the top of the cube even greater than the upward attractive forces? Mindbuilder (talk) 10:30, 22 January 2016 (UTC)

Re question 3: Objection. Speculation. The witness can not know what most people would say unless they interview them all directly.

The question was more meant to find out what the witness himself thinks, and thinks other people would think, rather than to find out what most people actually think.

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@StephenHughes - Here are some train analogies to the compression and tension between the liquid atoms in a siphon. Assume there are no locomotives in these trains, just free rolling cars.

In this first diagram, all the train couplings are in compression. Lets say we thourougly glue the train couplings to make the entire train effectively one long solid piece. Now the atoms in the glue in the couplings will have plenty of strong attractive forces between them, and the atoms of the glue will have plenty of attraction to the metal of the couplings and vice versa, and the metal atoms will have plenty of attraction to the other metal atoms in the couplings, just like the atoms in the liquid of a siphon are attracted to the other liquid atoms. But the atoms in the couplings will have even more repulsive forces between them at the same time as the attractive forces (at least in the direction along the length of the train). So even though the atoms of car 8 are exerting plenty of uphill attractive forces on the atoms of car 7, we don't say car 8 is pulling (or holding) car 7 up, because car 8 is exerting even more downhill repulsive force on car 7 than uphill attractive force. It's the net force that car 8 applies to car 7 that determines whether car 8 is pulling up car 7.

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This second diagram is actually more analogous to a siphon. Something more like a 25m water siphon.

In this diagram cars 1 to 4 and 23 to 26 loosely represent atmospheric pressure. Cars 5 to 8 and 19 to 22 represent liquid in the siphon below the barometric height, i.e. below 10m, and cars 9 to 18 represent cars in the siphon above the barometric height. Car couplings below the dotted line are in compression and the couplings above are in tension. The couplings on the dotted line have approximately zero tension or compression. Again, if we glue all these couplings together then there will be great attractive force between all the cars, but the couplings below the dotted line will have even more repulsive forces than attractive, just as how in a siphon there will be much attractive force between all the liquid atoms, but there will be even more repulsive force than attractive force between the atoms below the barometric height. And again, although there will be much uphill attractive force by car 8 on car 7, we don't say car 8 is pulling (or holding) up car 7, because there will be even more downhill repulsive force by car 8 on car 7 than uphill attractive force.

And note that if the couplings weren't all glued together rigidly and this train was actually rolling up and down hills, the couplings would "switch to a different method", that is, they would change their pull or push depending on the loads applied in certain uphill or downhill sections. Likewise a siphon will "switch to a different method" of pull or push depending on if it is in a vacuum or has a bunch of cars, I mean atmosphere, pushing down on it.

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This third diagram is analogous to a siphon where the entire siphon is shorter than the barometric height.

In this diagram, as in perhaps all practical siphons, all the couplings are in compression. None of the cars are being pulled up. And again, even though there are attractive uphill forces exerted on car 10 by car 11, we don't say car 11 is pulling up car 10, because car 11 exerts even more downhill repulsive forces on car 10 than uphill attractive forces. Mindbuilder (talk) 09:16, 25 January 2016 (UTC)

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StephenHughes wrote: ...if we fill a bubble with water it is neutrally buoyant and stays where it is.

But that water has the downward force of gravity on it, and if it stays where it is instead of falling, then there must be some upward force on it to counter gravity. And if the siphon is under atmospheric pressure, then that upward force is not a pull from above, but rather a push from below. And while the water that fills that bubble will stay where it is if it starts out not moving, if it is given a brief extra upward push, as at the entrance of a siphon, it will continue moving up at a steady speed in a straight line, according to Newtons First Law, if the forces of gravity with others add to zero. And that other upward force that adds with gravity to zero, and allows the water to continue up instead of falling down, is an upward push from below the water rather than a pull from above, since the net force from above, including pressure and liquid cohesion, is a downward force. Mindbuilder (talk) 07:32, 27 January 2016 (UTC)

Sorry, but I haven't got time to answer all these queries. So I'll just make a few comments. First off, bubbles do ascend because of the pressure gradient across the vertical length of the bubble, otherwise bubbles would not move. The fact that a bubble is less dense than the surrounding liquid creates a net upward force. I think that train analogy of a siphon is a good. If a train without an engine is traveling downhill along an undulating track, the links between the cars could be in tension and compression. If the cars were not linked then in the tension sections of the track cars would separate and the train would break (analogous to cavitation). Trains with linked cars generally do not 'cavitate'. The cars under compression push the cars in front. They are doing the pushing, not the atmosphere. In my opinion, even in a siphon below 10 m in height the water at the top is under tension since the two columns of either side of the apex are pulling down on each other. So the bonds at the top are slightly stretched - essentially like stretching an elastic band. In the case of the tallest trees the column of water molecules can generate negative pressures of -15 atmospheres. (please see Youtube clip The Most Amazing Thing About Trees). We might argue that since there is negative pressure at the top of the trees atmospheric pressure is able to push water molecule in - equivalent to a pump with a 150 m head. However, I don't think this is the case because from a fundamental physics point of view the negative pressure is generated by the weight of the column of water molecules from the roots to the leaves. Extra water is pulled into the roots by transpiration. As a water molecule leaves the leaves all the water molecules get lifted upwards and another gets pulled into the roots. Water does not boil since it is in a metastable state - as it was in our 15 m high siphon. I might be wrong here, but in my opinion the -15 atm negative fluid pressure cannot be 'seen' by the atmosphere. Gas pressures cannot go below zero - i.e. hard vacuum - no gas molecules. For a gas, negative pressures don't make sense. Maybe an analogy is that temperatures below 0 K don't make sense.StephenHughes (talk) 13:54, 27 January 2016 (UTC)

The one question I'm really trying to get your answer to that you've skipped several times, is the question about the force on the top surface of a cube going up a siphon. The neutrally buoyant, pure vacuum filled, glass cube, with strain guages in the faces, going up the siphon below the barometric height, at a steady speed, very slowly. You seem to acknowledge that there is pressure on the top of the cube, but still think it is being pulled up. I don't see how it can have positive pressure on its top, so that its strain guages measure the top face getting flexed down, and yet it could still be considered to be getting pulled up. That seems contradictory to me. Maybe you think there is not in fact positive pressure pushing down on the top of the cube. Or maybe you think that even though there is pressure pushing down on the top, that there is also upward attractive pull from the water molecules on the top, and you consider that force to be distinct and that it qualifies as the force pulling the cube up. So to state it another way, the one question is: how do you reconcile the seeming contradiction that the top face of the cube is being flexed down by the pressure at the same time that the cube is being pulled up? Mindbuilder (talk) 19:39, 27 January 2016 (UTC)

@Mindbuilder. OK, sorry I'll try and answer. I agree that in the case of a neutrally buoyant cube slowly ascending in a flowing siphon, the strain gauges would register hydrostatic pressure on all sides wof the cube with the pressure being slightly lower on the top surface of the cube than on the sides and bottom - the difference of course depending on the size of the cube. I agree that at 10 m at sea level the strain gauges would read 0 since there would be no net inward push. Higher than 10 m the strain gauges would read negative. For example at 20 m the gauges would read - 1 atm. Above 10 m the water molecules around the cube woulds be trying to stretch the cube in the vertical direction.

Excellent. We're in agreement up to here. I think we may be getting close to a full agreement.

In the case of the siphon, the neutrally buoyant cube would move due to the general flow of the surrounding fluid, due to the dragging effect of the fluid flowing out of the siphon. I don't see this as a contradiction. In the situation of a glass of water being lifted off a kitchen bench, the atmosphere bears down on the surface of the water as the water is being lifted. In the case of a siphon the water flowing out is providing the energy to lift the water - in essence we have a molecular pulley.StephenHughes (talk) 13:44, 29 January 2016 (UTC)

I'm not sure exactly what kind of drag you're referring to here. If the cube is neutrally buoyant, it should be moving exactly the same speed as the surrounding water, so there should be no viscous drag forces on the sides of the cube. But maybe that's not the kind of drag you're talking about. The fluid flowing out the exit of the siphon doesn't have any direct pull on a cube half way up the up side. There is no significant net force field reaching out over the distance. In a vacuum siphon, the fluid at the exit pulls on the fluid touching it above, and that fluid pulls on the fluid touching it above and so on, from molecule to molecule all the way over to the cube on other side. But the only things that affect the movement of the cube on the up side are the forces applied to it by the molecules directly touching it (and gravity). We seem to agree that the force from the molecules directly touching the top of the cube is net downward (when it's below the barometric height). Is there some other way that an upward drag strong enough to match the considerable weight of the cube, can be applied to the cube? Are you saying that the cube is being dragged up by the same molecules touching its top that are flexing the top down at the same time? Or are you saying that the drag force is somehow reaching around to the sides and/or bottom of the cube? Mindbuilder (talk) 02:26, 30 January 2016 (UTC)

@Mindbuilder. Sorry I didn't mean drag in the conventional sense, I meant that the cube is being moved along in the general flow of water molecules, although at the exit of the siphon the water molecules flowing out drag the molecules behind them. I think a good analogy to a siphon would be a buoy floating on a river. The water molecules in contact with the front of the buoy are applying a hydrostatic pressure, as are all the other surrounding water molecules. However, there is the bulk flow of the river. No net propelling force is required to move the buoy, although it the buoy bumped into an obstruction the water flowing behind would push on the buoy.StephenHughes (talk) 02:53, 30 January 2016 (UTC)

That seems like a good analogy to get across what you're saying. I think you are right that no net propelling force is required to get the cube to move along with the general flow of water up the siphon - only the regular force associated with neutral buoyancy to cancel gravity is required. I think you acknowledged above that the cube will have a little more pressure force from below than above, and I think you would acknowledge that the vector sum of the pressure force from above and below would add to be an upward force equal to the cube's weight, as that is the recognized situation in the case of neutral buoyancy. So wouldn't you agree that some force is required to counter gravity so that the cube can have zero net vertical force applied to it and thereby continue up at a constant speed with zero acceleration? And wouldn't you agree that the force needed to counter gravity is coming from the difference in pressure between the top and bottom of the cube? Or are you saying the general flow of the water molecules makes the cube go up against gravity without any other force but gravity applied at all? Because if no force at all is required to make the cube go up with the general flow against gravity, then that would seem to imply that both of us are wrong because then there is no need for it to be pushed OR pulled up. Or are you saying that the pull up is not a force? But if there is a force pulling the cube up then what faces of the cube is the upward pull force being applied to? Or is it being applied to the cube by some way other than contact forces with the faces? Mindbuilder (talk) 06:36, 30 January 2016 (UTC) again at 15:27, 30 January 2016 (UTC) again at 19:13, 30 January 2016 (UTC) again at 22:51, 30 January 2016 (UTC)

Hi Mindbuilder, thanks for your comments and questions. I am saying that the hydrostatic pressure is a separate issue from what propels the fluid. The pressure (P) in a siphon reduces from the inlet to the apex in accordance with P = -ρgh (where ρ is density of the liquid, g is the acceleration due to gravity and h is the height above the liquid level in the upper reservoir). So as you point out there is a pressure difference between the top and bottom of a cube. When the water is flowing the pressure along the entire length of the siphon is reduced by (ρv^2)/2 where v is the average velocity of liquid in the siphon. A neutrally buoyant cube is effectively like an object in free fall (i.e. in space) and so Newton's first law will be in operation and so no force is required to keep the cube moving - assuming that the siphon is of constant cross-sectional area. In the case of bubbles in a siphon there are two mechanisms whereby a bubble ascends to the apex: (a) hydrostatic buoyancy and (b) hydrodynamic flow. In the case of a low flow siphon, a bubble will ascend to the apex and then get stuck since the flow is too slow to flush the bubble out of the siphon. If we attempt to explain the motion of fluid in a siphon using hydrostatic principles it is difficult to explain how a neutrally buoyant cube, or water flows downhill, to the exit of the siphon.StephenHughes (talk) 12:31, 31 January 2016 (UTC)

Tapered siphons

Has anyone ever done any experiments on a tapered siphon? Seems to me that given that the water holds together due to intramolecular forces analogous to tensile strength, that the same solution used in space elevator proposals to work around them breaking under their own weight - having the cable taper - should work here as well. Has anyone ever tried it? I did a google search and came up empty, just a bunch of unrelated patents and the like. -- 213.176.153.100 (talk) 10:56, 1 February 2016 (UTC)

That's a clever idea. I've never heard of a tapered siphon built or even mentioned. Unfortunately I haven't seen much effort to push the height limits of siphons. The siphon of Adrian Boatwright's team was the first time I know of that anyone published a demonstration of a water siphon higher than 10m. It seems to me that Andrew K Fletcher demonstrated a 24m saltwater siphon on Youtube, but he denied that it was a siphon because the water wouldn't flow without the added density of the salt on the down side. I suspect the salt may have broken up the hot ice crystal structure in the extreme negative pressure zone at the top, if there is such a structure. Z-tubes have demonstrated -280 atmospheres tensile strength of water in, I presume, a constant diameter glass tube, which corresponds to a siphon 2800m tall. Only if someone tried to build a siphon taller than that would they possibly need a tapered tube. But -280 atm is not considered to be even close to the real tensile strength of water. The problem is that -280atm is actually measuring, not the tensile strength of water, but the tensile strength of the water to glass wall adhesion. There have been many studies attempting to probe the limits of water's tensile strength, but the more recent ones have tried to remove the variable of wall adhesion by doing things like using sound waves to create cavitation in the middle of a volume of water far from the walls.

A mercury siphon could possibly make use of a tapered tube at much more reasonable heights. Although there have been many attempts to see how high they could make a mercury siphon go, I think they were almost all satisfied with just a demonstration that very clearly exceeded the barometric height (usually in a near vacuum where the barometric height is of course about zero). Because they were all interested in demonstration, as far as I know, they all used glass tubes so people could see the results. Unfortunately mercury adheres rather poorly to glass, as can be seen by the convex meniscus. So if you wanted to push the tensile strength limits of mercury rather than the mercury to glass adhesion, you would probably want to switch to something like a copper tube. Not as fun to watch though. You could perhaps taper a copper tube by stretching. Mindbuilder (talk) 19:12, 1 February 2016 (UTC)

Stephen Hughes response to Mindbuilders questions continued

I'm starting a new section continuing the previous one just so I don't have to scroll so much.

Your previous paragraph looks all correct, though I don't know why you would try to separate the hydrostatic pressure from what propels the fluid. For example if you have our cube in the middle of a simple glass of water and you lift the glass with your hand straight up from the table, then it is the hydrostatic pressure which lifts the cube along with the glass (if you could still call it hydrostatic while you are moving it).

The liquid gets some speed as it accelerates from the source reservoir into the somewhat lower pressure just inside the entrance of the siphon. Absent friction flowing through the tube and around the corners and absent any other forces to slow it down, that momentum would carry the water and cube all the way through the siphon at constant speed. Of course there are many other significant forces to slow it down (or speed it up), gravity being a particularly prominent one. So it is fair for you to look at it in light of Newton's first law, and say that as long as there is zero net force applied, it will continue in a straight line at constant speed up the siphon. But the critical thing is that in order to invoke Newton's first law to explain the liquid's upward movement, you have to have zero net force applied to it, and gravity is very much not zero. When people ask why the liquid flows uphill against gravity in a siphon, they are keenly aware that gravity is not zero, and they wonder what force counters gravity to allow the liquid to go up. In a vacuum siphon, the cohesive forces between the liquid molecules would apply an upward pull on the top of the cube to counter gravity, but in a siphon below the barometric height, there is no upward pull, rather it's that hydrostatic pressure that exerts an upward push on the bottom of the cube to cancel gravity. Whether the upward force to cancel gravity comes from that hydrostatic pressure or from the cohesive pull, is the critical difference between a vacuum siphon and a normal siphon in explaining that upward movement. I don't see that it is conceptually useful to distinguish those pushes and pulls from what "propels" the fluid up the siphon. Newton's second law makes no distinction between forces that create neutral buoyancy and forces that "propel" the liquid. If someone asks you what propels the liquid up a vacuum siphon, and you explain that the liquid is pulled up by cohesive forces between the liquid molecules, kind of like in the chain model, they may be ok with that explanation. But if you then explain that in normal siphons under atmospheric pressure, there is no pull on the top of a cube, and there is nothing "propelling" the cube up against gravity, it just goes up because of Newton's first law and the fact that there is zero net force on the cube, they are likely to give you strange looks. I don't think they are going to care about the distinction between what forces "propel" the liquid up, and the hydrostatic force that counters gravity to create neutral buoyancy. It's that hydrostatic force that is critical in making it possible to ride Newton's first law up despite gravity. That hydrostatic pressure upward push is what I think they're going to be interested in, even if they don't realize it, so they can understand and predict whether and why the fluid still goes up in siphons with bubbles, the flying droplet siphon, and CO2 gas siphons.

But at any rate, we seem to finally be in agreement that when the cube is below the barometric height and going up, there is no upward pull on its top face, or any other face. Are we in agreement on that yet? To summarize my basic position which is at the root of our disagreement, I would say that in practical siphons, the liquid is not being pulled up, rather it is being pushed up by atmospheric pressure. Though I'm not sure, I think you've come to realize that it's not being pulled up. And you realize there is an upward push from the hydrostatic pressure. So I think I've established the two key elements of my position 1) it's not pulled up and 2) the upward push. That seems like it covers it pretty well, but it is possible that you may still question the third element, that the hydrostatic pressure is ultimately due to atmospheric pressure, as it is for example in a barometer. Do I need to establish that as well, or is it fairly evident by now? Mindbuilder (talk) 17:54, 31 January 2016 (UTC)

Also, you mentioned earlier that the liquid exiting the siphon pulls down on the liquid just inside the exit. But that is not correct. Move our vacuum filled cube to the exit of a siphon operating at atmospheric pressure, either just inside or just outside the exit, whether just outside in the reservoir or just outside in the liquid as it free falls in the air, wherever it is, you can calculate the pressure on each of the faces by as accurate a method as you want, and find the pressure to be positive relative to pure vacuum, on all the cube faces. That means the force on the bottom of the cube will be upward and flex the bottom of the cube upward. The cube will not feel a downward pull from the water below, rather just the opposite, it will feel an upward push, impeding its exit from the siphon, impeding the flow of the siphon. The top of the cube will not pull down on the liquid above, rather the top will push up on the liquid above in reaction to the top being pushed and flexed down, again impeding the flow. You might note that if the liquid free falls from the siphon exit that the siphon may flow at a greater rate than if it exits into a constant height reservoir just below the exit. But that is not because the free falling liquid is pulling down, but rather when the surface of the reservoir is close to the exit, the flow of the siphon may be impeded more, and the flow is impeded less when the liquid is allowed to free fall. But in both cases, the liquid exiting is getting pushed up, just more or less pushed up, but not pulled down in either case. Again you can determine the pressure on the faces of the cube at any point that it may be at in the flow, and find it to be positive, pushing up on the bottom of the cube. The non-existent pull of liquid exiting a siphon operating at atmospheric pressure, can not be used to explain how the liquid is propelled through the siphon. Mindbuilder (talk) 18:37, 31 January 2016 (UTC) again at 18:55, 31 January 2016 (UTC) again at 07:13, 6 February 2016 (UTC)

That's quite a magical cube you have there Mindbuilder.

It appears to have this ability to travel up into the siphon, against the direction of flow.

I also have a magical cube, however, for some unknown reason, it does quite the opposite to your magic cube.

When I put it into a siphon, the pressure force on the top of the cube is downward and flexes the top of the cube downward. My magic cube will not be pulled up, rather just the opposite, it will be getting pushed down, speeding up its exit from the siphon, improving the flow of the siphon. The bottom of my magic cube will not pull up on the liquid below, rather the bottom will push down on the liquid below in reaction to the bottom being pushed and flexed up, again speeding up the flow.

I put these two magic cubes into a siphon, a race of sorts, and mine won, overtaking yours on the exit, as yours was trying to head upwards.

I sat my cube down afterwards, and asked it to explain how Mindbuilder's magic cube could have pressure that pushed it up from below, but some how the pressure on the top didn't push it down in a similar way.

Well, my magic cube replied, it's kind of like the train set that Mindbuilder just loves to play with.

It is supposed to mimic a siphon yet one can put a air bubble in the siphon, which means, in Mindbuilder's most recent example, the train carriages are not coupled together. And put some real heavy carriages on one side of the hill, like you can do with a fat section siphon, and the damn train runs off the hill in the opposite direction to what it is supposed to do.

Don't you just hate that. — Preceding unsigned comment added by 124.177.112.242 (talk) 03:13, 6 February 2016 (UTC)

Thank you for pointing out my lack of clarity. I did not mean to imply that the cube would actually move up against the flow, but just that it would experience an upward force on its bottom face. Where I wrote:

"The cube will not be getting pulled down, rather just the opposite, it will be getting pushed up, impeding its exit from the siphon, impeding the flow of the siphon."

I could have expressed what I meant more clearly with something like this:

"The cube will not feel a downward pull from the water below, rather just the opposite, it will feel an upward push, impeding its exit from the siphon, impeding the flow of the siphon."

I have updated the old text of my comment.

Note that the atmospheric pressure at the exit of a siphon impedes the flow of the siphon. this is obvious if you imagine you had normal atmospheric pressure above the entrance reservoir and no atmospheric pressure at the exit (i.e. pure vacuum). The siphon would of course flow at a much higher rate. And when you analyze a fat up tube siphon, you have to take into account the forces exerted on the liquid and its weight by the "ceiling" surfaces of the fat up section of the tube. When we look at siphons we often ignore the tube wall forces, but you can't do that and have a complete understanding of the fat up tube siphon. Mindbuilder (talk) 07:13, 6 February 2016 (UTC)

I wasn't pointing out your lack of clarity. I was pointing out your "selective" use of information. If water both under and above your cube is trying to expand due to pressure, and thus pushing both upwards and downwards, underneath the cube you choose to use only the upward force, and above, you also choose to use only the upward force. That has all the hallmarks of someone clutching at straws trying to support their argument.

Re your comment: "Note that the atmospheric pressure at the exit of a siphon impedes the flow of the siphon"

Seriously?

If one removes atmospheric pressure all together, (and treats the water so it won't turn to vapor), then the flow rate will not change at all. I can also increase the air pressure at the exit such that the water travels in the opposite direction. Changing the exit air pressure down or increasing the start pressure up both result in an increase in flow due to the increased pressure/total head that dictates the flow rate. This isn't an issue about atmospheric pressure impeding flow. Rather it highlights the issue of how pressure impacts flow.

If one takes two containers, and connects them with a hose near the bottom of a side of each. Just outside one of the containers, fit a tap into the hose and turn it off. Fill the hose and half fill the containers, and put them on the side of a table at the same height, with the hose laying flat across the table. Since the pressure either side of the tap is the same, when the tap is opened, there will be no flow. If you raise the other container, or reduce the air pressure in the container next to the tap, you now have a pressure difference either side of the tap, open it and you will get flow. Raising or lowering the hose doesn't change the pressure difference, so doesn't impact the flow. Raising it creates a siphon. Putting in a fatter section of hose doesn't change the pressure so doesn't change the flow aside from reduced friction in that section. It's extra weight has no impact. One can have one or more air bubbles so there is no cohesive forces and still it will flow. An air bubble will have an impact in relation to it's vertical height and may even stop the flow, since this impacts the pressure generated by the vertical height of the water.

So where are the ceiling surface forces in your train set? And if you have a complete understanding of the fat up tube siphon, why are you using your train set as an example to justify an argument if you are acknowledging the siphon operates under a different set of rules. I also mentioned fat section, it doesn't necessarily have to be fat up section. One can also have a no water ceiling surface fat up section siphon, by having an air pocket at the top. Makes it hard for your "forces" exerted on the liquid and its weight by the "ceiling" surfaces of the fat up section of the tube to do their stuff.

Here's a simple task. Put some ice cubes in a glass full of water. Sit the glass on a table. Turn the glass clockwise or anti clockwise with your hand. What happens to the ice cubes? You will note they don't move until you have stopped. The ice cubes and the water stay in the same position initially. Highlighting they are not attached to the glass walls in any significant way. Same in a siphon, the water can freely move in the tube.

Stephen Hughes made this comment earlier I agree with all of Adrian’s answers. I just have a few extra comments on the Flying Droplet Siphon (FDS). In my opinion the FDS is not actually a siphon but essentially the same as a hose spraying water up into the air. Water comes out of a garden hose because the water pressure is greater than atmospheric pressure. In a FDS, the weight of the water in the downward arm creates a low pressure region at the top. The differential pressure drives water into the upper chamber. The water drops down on top of the suspended water column increasing its weight so that it drops down until the atmospheric pressure gradient is commensurate with the weight of the column of water. And so the cycle continues.

This is where you should be resolving your differences with Mr Hughes. Maybe fine tuned a little, these words describe how a siphon works, about differential pressure. But you probably don't support his view because it doesn't include atmospheric pressure as a driver. And Mr Hughes abandons his own words in relation to not recognizing a flying droplet siphon as a siphon since it shows cohesive forces are not required.

You have to resolve that issue right there. Air bubbles and fat section and flying droplets can all be a form of siphon, but some out there are ignoring this, are even in denial about this because it ruins their cohesive theory. — Preceding unsigned comment added by 124.185.5.238 (talk) 08:52, 6 February 2016 (UTC)

@124.185.5.238 - I see now that you appear to be a poster with whom I have had extensive discussions previously, but you would not even tell me when you agree or disagree on particular points that were important to the discussion. Wikipedia recommends that finding out what is agreed on is an important part of the discussion. I have found that to be very true. When people won't tell me what they agree with, it makes the discussion go very very slowly. That is a waste of your time, not just mine. I don't have that much time to spare on this. Especially when refusing to answer reasonable questions indicates a stronger desire to avoid admitting mistakes than the desire to learn the truth. If you're willing to answer my questions, I'm willing to discuss. I'll try to keep the questions mostly to quick and easy yes/no answers. Of course you can ask me questions too. So here is my first question: Are you generally willing to state clearly whether you agree or disagree with particular statements? Mindbuilder (talk) 22:12, 6 February 2016 (UTC)

Looking for info on automatic self-starting siphons

As a hobby I'm trying to design a gizmo which uses an automatic self-starting intermittent siphon that has no moving parts other than the water, and can begin operating from a completely dry state. This article is extremely lacking in info about such things and how they work.

I'm probably going to try to start adding my own research and citations, though finding good info about the dynamics of how self-starting siphons work is really difficult. They rely on complex dynamic pressure differentials that are not easy to visualize.

I see that intermittent siphons have a quite long patent history in the United States going back about 150 years, for periodic flushing of accumulated wastes from municipal sewers.

But many of the older intermittent siphon designs I have looked at so far seem to rely on a starting condition of water preloaded into some location, and without that starting condition the siphon will act as just a static overflow drain, and not cycle intermittently.

Finding good animations and documentation that properly describe the fluid mechanics of self-starting intermittent siphons is turning out to be quite difficult.

-- DMahalko (talk) 10:55, 6 April 2016 (UTC)

Also does anyone know the name or patent number for this "siphon with a snorkel"? There is no info provided in this video or its description, or what the purpose is of the smaller tube. I don't know if this one is capable of fully dry self-starting.

https://www.youtube.com/watch?v=5wZ9PQepQYI

-- DMahalko (talk) 11:21, 6 April 2016 (UTC)

I haven't looked at auto siphons much, but I did read part of an article by an Indian engineer that looked rather interesting. I think it is the one (number 16) referenced in the siphon section of the Wikipedia Spillway article titled "Design of Volute Siphon". I think that more info about auto siphons would be a good addition to this Siphon article. Unfortunately it will probably be quite challenging to get a good and reliably accurate explanation of how the complex, turbulent air/water interface of the auto siphon works. It's tough to even get agreement on how the much simpler, straight line, constant speed, single fluid, section of an idealized plain siphon works. Good luck. Mindbuilder (talk) 16:14, 6 April 2016 (UTC)

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Bowl Siphon Section

The cited source video in this section in fact shows the opposite of what this section claims about bowl siphons not actually being siphons. It's comparing and contrasting standard American toilets which do function as a siphon, with "Corona" wash-down toilets which function as the Wikipedia entry describes. — Preceding unsigned comment added by 76.14.88.109 (talk) 08:57, 27 November 2016 (UTC)