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# Negative Absolute Pressure

07/09/2021 1:21 PM

Can absolute pressure be negative? For a gas, the answer is no. For a liquid or solid, the answer is yes. Every tree taller than 9.8 meters proves that.

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#1

### Re: Negative Absolute Pressure

07/09/2021 3:02 PM

I think I'm going to have to go over this a few more times....I think we need more diagrams...

https://courses.lumenlearning.com/wm-biology2/chapter/transpiration/

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#5
In reply to #1

### Re: Negative Absolute Pressure

07/10/2021 5:17 AM

In these diagrams there are several pressure references in terms of MPa. The Pa part of this is Pascals, a relatively small unit of pressure. The "a" part of this abbreviation does not mean "absolute pressure" (as it would in bara vs. barg vs. bard, or psia vs. psig vs. psid.)

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#2

### Re: Negative Absolute Pressure

07/10/2021 12:33 AM

Thanks! Like SE, I'm going to have to ponder this a while. I've wondered about this for at least 50 years, thought about it many times, but never felt I understood it completely. For the moment, that remains true!

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#3

### Re: Negative Absolute Pressure

07/10/2021 4:05 AM

Surely you jest, or is it the first of April? There can be no such thing as negative absolute pressure for absolute pressure is Zero or a complete vacuum which exists in space and can be almost achieved with a muftistage vacuum pump. When we used to pull vacuums on chambers and were able to get to values of a fraction of a Torr which was measured by a hot wire instrument.

I atmosphere is 760mm of mercury, and 101.3KPa though this is a theoretical value for an atmosphere. 1 atmosphere is different at sea level to on a mountain top. 1 Torr is 133.322 pascal. Calibrating vacuum gauges was part of the job I did on the Deaerator for the boiler feed water in a power station and there was definitely no negative absolute pressures but there were negative gauge pressures.

One of the instruments in the turbine hall was a mercury barometer and the reading in millimeters of the column gave the atmospheric pressure at the temperature on the mercury thermometer included with the barometer.

So I suspect that the pressure mentioned should be gauge pressure. If a liquid is placed in a absolute vacuum it will boil off or evaporate to increase the gas pressure as for a solid well it will be unaffected except it it is one of those solids which change to liquid at the prevailing temperature, like mercury where a Torricellian vacuum forms above the mercury column, I don't know about Gallium though when it changes state.

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#9
In reply to #3

### Re: Negative Absolute Pressure

07/10/2021 9:56 AM

For gases, negative pressure is impossible. The molecules of a gas move freely and have hardly any effect on each other. The molecules of a liquid or solid have an attraction for each other, especially a polar liquid like water. Negative absolute pressure in a liquid can be maintained as long as there is no nucleation site that acts as a weak point allowing it to convert to gas.

"

• Negative absolute pressures are effectively tension, and both bulk solids and bulk liquids can be put under negative absolute pressure by pulling on them.[11] Microscopically, the molecules in solids and liquids have attractive interactions that overpower the thermal kinetic energy, so some tension can be sustained. Thermodynamically, however, a bulk material under negative pressure is in a metastable state, and it is especially fragile in the case of liquids where the negative pressure state is similar to superheating and is easily susceptible to cavitation.[12] In certain situations, the cavitation can be avoided and negative pressures sustained indefinitely,[12] for example, liquid mercury has been observed to sustain up to −425 atm in clean glass containers.[13] Negative liquid pressures are thought to be involved in the ascent of sap in plants taller than 10 m (the atmospheric pressure head of water).[14]"

Pressure - Wikipedia

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#15
In reply to #9

### Re: Negative Absolute Pressure

07/10/2021 6:13 PM

I think that they are confusing Absolute with measured, there is Absolute Pressure or Absolute Zero pressure where no molecules of gas or liquid exist. Not negative Absolute Pressure just a small value above Absolute Zero.

Then there is Absolute Zero Temperature or -273C or 0 Kelvin, which though almost achievable is still Absolute Zero, not negative values just small values above zero.

Mathematics can have values below Zero but then the Zero they use is just an arbitrary value assigned. Their Zero exists somewhere between negative infinity and infinity. There zero is the center of "the piece of string" which we know is just long enough to fit between its ends.

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#17
In reply to #15

### Re: Negative Absolute Pressure

07/11/2021 3:15 AM

An ABSOLUTE vacuum does not exist,except theoretically.

However,scientists have come very close:1E-12 which is 7E-15 Torr.

We come close enough with ordinary vacuum pumps for practical purposes.

Even in outer space there are some single molecules of hydrogen,about 1 per cc.

Absolute pressure on Earth is relative to the atmospheric pressure around us at the time of measurement.

Gauge pressure is the pressure above atmospheric pressure.Pressure minus atmospheric pressure is considered absolute pressure for most situations.

The link I posted in #8 explains and illustrates capillary action very well.

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#12
In reply to #3

### Re: Negative Absolute Pressure

07/10/2021 4:30 PM

In hard vacuum, once you get to zero, pressure is indicated in negative Torr values. A UHV vacuum is something like 1x10-10 Torr, however, this is not negative pressure. The pressure is still zero & the negative Torr values indicate decreasing molecular activity.

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#14
In reply to #12

### Re: Negative Absolute Pressure

07/10/2021 4:52 PM

1x10-10 Torr is NOT a negative Torr value, just a very small one. That 10-10, better written as 10-10, simply means: move the decimal point 10 places to the left.

1x10-10 Torr=0.000,000,000,1 Torr.

That is an exceedingly low pressure, but it is still above zero.

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#27
In reply to #14

### Re: Negative Absolute Pressure

07/12/2021 4:10 AM

Yes, I agree. I explained it poorly.

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#4

### Re: Negative Absolute Pressure

07/10/2021 4:33 AM

The mistake is in equating the tree with a column of water, this is an oversimplification. The hydraulic circuit in the tree is more complicated than this.

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#28
In reply to #4

### Re: Negative Absolute Pressure

07/12/2021 7:28 AM

Biology classes in 1972 merely glossed over it by describing the way water gets up a tree as an "active transport mechanism".

Perhaps a load more science has happened since then.

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#33
In reply to #28

### Re: Negative Absolute Pressure

07/12/2021 8:47 AM

I guess science is happening all the time, and the only thing we really know for sure is that even the greatest scientists will at some point be proven wrong about something. But the best way to ensure that what you say is correct is to keep your definitions as vague and all-encompassing as possible. I like the description "active transport mechanism", it means it's a pump. And being a bit of a tree hugger and formerly a pump guy, I like the idea of a tree being a pump.

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#36
In reply to #33

### Re: Negative Absolute Pressure

07/13/2021 5:11 AM

Science is always "the best model available", and therefore always wrong. A better model comes along, disproving the earlier model [example: Newtonian Mechanics -> Relativistic Mechanics], and the whole process happens again and again. What a wonderful domain to be in.

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#40
In reply to #33

### Re: Negative Absolute Pressure

07/14/2021 1:42 PM

"But the best way to ensure that what you say is correct is to keep your definitions as vague and all-encompassing as possible."

The best way to ensure that what you say is correct is to say nothing!

Then you offend no one, but you also help no one.

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#6

### Re: Negative Absolute Pressure

07/10/2021 6:22 AM

The way the tree gets water to the top was explained to me as the flexing of the trunk was equivalent to a pump, in other words the trunk movement squeezed the water up the trunk...and I do believe you could have neg absolute pressure on glass, which is considered a liquid I think...

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#13
In reply to #6

### Re: Negative Absolute Pressure

07/10/2021 4:36 PM

Would this flexing be due to wind, or to some internal process(es)?

If it were due to wind, then on a windy day, water should be emitted (in addition to transpiration) by the upper leaves, while on a calm day, the upper leaves should wilt from lack of water pressure, regardless of the temperature. An extended calm period would likely kill all the upper leaves. I can't accept wind-based pumping for trees.

An internal flexing (akin to peristalsis?), would require some form of periodic expansion and contraction, however minute, progressing up the tree. That seems plausible, but not obvious.

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#7

### Re: Negative Absolute Pressure

07/10/2021 7:23 AM

So the tree is effectively a pump, with the suction in the roots at the bottom, and the discharge all the way up to the top. Interesting pressure gradients?

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#8

### Re: Negative Absolute Pressure

07/10/2021 7:46 AM

Osmotic pressure is created due to evaporation from the leaves.Evaporation creates a higher concentration of salts,which creates an osmotic pressure that pulls water up from the roots.It flows cell-to-cell.You could think of this as a "bucket brigade" type of action.

Capillary action also is important in the process of moving water up the tree.

Here is a link to help clarify capillary action:

https://www.usgs.gov/special-topic/water-science-school/science/capillary-action-and-water?qt-science_center_objects=0#qt-science_center_objects

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#10
In reply to #8

### Re: Negative Absolute Pressure

07/10/2021 10:13 AM

I agree. No magic or negative absolute pressure in a tree. Simply capillarity and osmose from the different gradients.

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#16
In reply to #8

### Re: Negative Absolute Pressure

07/10/2021 10:34 PM

I think the answer is capillary action.

Derek Muller in his video correctly states that the water will rise only about a meter based on the size of the xylem capillaries. Jurin's law states that the height that water will rise in a capillary is inversely proportional to the capillary diameter. Xylem capillaries are about 25 microns in diameter which equates to 120 cm.

(All dimensions in meters)

Here is an analysis of Derek's video that seems to nail it down.

The Amazing Physics of Water in Trees | Science4All

The bottom line is that the pores on the leaves are maybe 2-5 nanometers wide and can easily support a much higher water column.

Quoting from his article:

"Wait… Didn’t Derek say that capillarity couldn’t get water up more than 1 meter?

As you can imagine when watching at the figure above, a column of water still has to face atmospheric pressure. Now, the larger the tube, the greater the ratio of water molecules which face atmospheric pressure is. The xylem tube is too large to get water up more than 1 meter. But the tube ends with a gigantic number of extremely thin pores in leafs, at the ends of stomata (plural for stoma). These pores are called cell wall pores. While the xylem tube is 20 to 200 micrometers large, the cell wall pores are 2 to 5 nanometers large. This is nearly the scale of molecules and means that the surface of the meniscus is only made of a few hundred molecules!

Does this make a difference?

Yes! The width of the pores is 10,000 times less than the xylem tube. Yet, the height capillarity can suck is inversely proportional to the width of the tube. Thus, the pores can withstand a water column which is 10,000 times taller than the xylem tube! Since the xylem tube could suck water 1 meter up, the pores can suck water kilometers up!"

http://www.science4all.org/article/the-amazing-physics-of-water-in-trees/

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#18
In reply to #16

### Re: Negative Absolute Pressure

07/11/2021 3:33 AM

So,if we had a tube,or a collection of tubes,a mile or so (more or less,according to the application) in diameter,composed of nano tubes,we could pump water over mountains without a pump,then harness the falling water for energy or irrigation or potable water.

The water would be very pure due to the capillary size.

Clogging?(Not the folk dance) Do tree roots get clogged?

Impossible? A tree will bleed sap if you cut it at the top,will it not?

I realize the volume would be limited by the size,and the manufacturing possibilities,but large trees consume thousands of gallons per day.

The numbers here are arbitrary to illustrate a principle.

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#19
In reply to #18

### Re: Negative Absolute Pressure

07/11/2021 7:02 AM

So,if we had a tube,or a collection of tubes,a mile or so (more or less,according to the application) in diameter,composed of nano tubes,we could pump water over mountains without a pump,then harness the falling water for energy or irrigation or potable water.

Unfortunately, not. The same force that pulls the water up will keep it from coming out the end. No free lunch.

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#23
In reply to #19

### Re: Negative Absolute Pressure

07/11/2021 5:49 PM

Drape a piece of lantern wick into a glass of water,up the side,and down the other side.Water will drip out.

Same principle.

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#25
In reply to #23

### Re: Negative Absolute Pressure

07/11/2021 6:27 PM

It's been many years since I last tried that, but as I recall, it only drips if the wick outside extends below the level of water inside. That's what can be considered an inverted siphon. Capillarity wets the wick, but Gravity is the force causing the dripping; there is no "pumping" action.

The few wicks I currently have on hand aren't sufficiently flexible to try it...

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#37
In reply to #25

### Re: Negative Absolute Pressure

07/13/2021 5:46 AM

Use a roll of toilet paper.(not used of course )

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#43
In reply to #25

### Re: Negative Absolute Pressure

08/05/2022 5:18 PM

Try it with a roll of toilet tissue

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#30
In reply to #19

### Re: Negative Absolute Pressure

07/12/2021 7:45 AM

I disagree.The momentum of the moving liquid will propel it off the end of the wick,even if the end is higher than the beginning point.

A tree will bleed sap if you cut it off at the topmost branch,will it not?

Certainly gravity will speed up the flow rate,but it is not the only force at work,Capillary,osmosis.and atmospheric pressure all have a combined effect .

I think that even a little gain is worth exploiting to the fullest.

There is no free lunch,but there may be a free snack.

For instance,install a check valve to prevent back-flow of the liquid.Changes in atmospheric pressure would in effect,pump the liquid out the end.

Atmospheric pressure is always changing,and would function as a pump.

There probably would not be enough for significant power generation,but maybe enough to provide potable water for isolated places.

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#32
In reply to #30

### Re: Negative Absolute Pressure

07/12/2021 8:39 AM

A Nitinol tube ,surrounding the fibers at the exit end,combined with the aforementioned check valve, would also pump the liquid as the temperature changes during the night; Or day as a cloud passes over.

Use tubes with reverse memory characteristics to create a dual phase pump.(One expanding while the other contracts).

This process could be used solo to pump liquids.

Imagine a large array of these bipolar Nitinol tubes?

Pump into a reservoir for use or power generation.

This could also possibly be used for desalination.

Use the bull shark gills as a pattern,they expel excess salt into the water.

These ideas are now released by me into the public domain, so they can be used freely.

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#34
In reply to #32

### Re: Negative Absolute Pressure

07/12/2021 11:08 AM

Unless I'm mistaken, changes in atmospheric pressure and/or temperature could only pump IF there existed a series of one-way check valves that opened and closed at appropriate times. Is there anything in a tree that does something like that?

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#35
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### Re: Negative Absolute Pressure

07/12/2021 1:15 PM

The sap does run down hill sometimes.Trees are swollen at the base because of this.

Read my post carefully.Do you know what Nitinol is?It is a memory metal that changes shape with a change in temperature.

Here is a link to Nitinol:https://www.cdn-inc.com/nitinol/

It can go from a larger form to a smaller form according to design.

It can be formed in any shape desirable,and tuned to different temperatures at formation;the "natural state".

Imagine a tube containing the fiber capillary bundles,wrapped on the outside with Nitinol wire,like a spring.

If half of the Nitinol tubes were naturally in the smaller-than fiber-bundle ,they would expand when heated and contract when cooled.If the other half were in a naturally larger shape,they would contract when heated,and expand when cooled.

A single phase Nitinol wire could be used,with one outside of the tube,and one inside of the tube,in different locations.

With a check valve as I mentioned,you would have a pump that operated due to temperature change.

Imagine a flexible tube wrapped in Nitinol wire,like a spring.

The natural form of the wire is smaller than the tube at a specified ambient temperature.

Below it,on the same tube,is wrapped a wire of Nitinol that is slightly larger than the tube at the same temperature.When the temperature changed,one would contract,and the other would expand.

The tube would be compressed at one point,and released at the other.

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#21
In reply to #16

### Re: Negative Absolute Pressure

07/11/2021 3:42 PM

Here is a hypothetical example of how capillary action could lift water to a great height:

Start with a 25-micron capillary which can lift water 1 meter. After 1/2 meter, it branches into 4 smaller capillaries of half the diameter (which have the same total cross-sectional area). Each of these can lift water another meter. After 1/2 meter, each of these branches into 4 smaller capillaries of half diameter which can lift water 1 meter, and so on. At each branching, the same "1-meter head" and total cross-sectional area remain until the xylem channel system terminates at the pores in the leaves.

Just a thought...

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#29
In reply to #16

### Re: Negative Absolute Pressure

07/12/2021 7:38 AM

If you will look at the illustration and explanation in my link,in #8 it explains very simply exactly what is happening.

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#42
In reply to #8

### Re: Negative Absolute Pressure

08/05/2022 12:27 PM

If you're willing to travel light...

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#11

### Re: Negative Absolute Pressure

07/10/2021 10:25 AM

If you are in a submarine,under water,the absolute pressure will be the water pressure above you plus the atmospheric pressure.When you return to the surface,it will only be the atmoshpreic pressure of the air above you.Higher altitudes result in lower Absolute pressure.Modern vehicles use this factor to adjust the fuel/air mixture for different altitudes.(MAP sensor).

An octopus can provide much more absolute suction at depth than anything on land.

When sensors are placed on whales,scientists do not consider that as the whale goes deeper,the pressure on the sensor to the whale increases,causing pain to the whale.

They always seek help from other whales to remove the sensors because they become painful.

They should put a pressure regulator on the sensors to adjust the differential pressure to compensate for depth.

Perhaps someone will read this and pass the info on.

Another curious,but interesting fact about atmospheric pressure:All gasses in the atmosphere contribute their weight as if they were the only gas in the atmosphere.

Oxygen pressure at normal atmospheric pressure is around 21% of total pressure,or approximately 3psi.

This fact is utilized in gas flame powered refrigeration.

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#20

### Re: Negative Absolute Pressure

07/11/2021 10:03 AM

An anecdote related to the subject matter: In our municipality the public works department is equipped with an ultrasonic probe with which they claim they can determine if the heart of an older tree is rotten and the tree needs to come down. One spring, just before the trees were in leaf, they cut down two mature sugar maples nearby. The remaining stumps were solid all the way through, and for days after the living root system pumped out so much sap that it ran across the sidewalk and into the gutter. Obviously, if wind movement of a tree contributes to the sap pumping action, as posited elsewhere here, it is not the only mechanism. If sap can pour forth from a stump, no above ground structural movement is causing this.

I complained to the city that their ultrasonic probe is shite, because they are cutting down perfectly healthy trees. It wasn't the only time I had complained about this. Their reply was always the same, "Are you an arborist?" My counter-reply was likewise always the same - No, but anyone with eyes can see that the stumps are solid all the way through, and you don't need to be an arborist to know that a dead tree does not pump out gallons of sap for days after it is cut down.

My complaints did have an effect though. Now, immediately after a tree is cut down, a large machine grinds the stump to below grade level, the hollow is filled with topsoil and sodded over. All the wood chips are swept up and trucked away. By the end of the day no evidence exists that just hours before a hundred year old tree stood at that spot.

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#22
In reply to #20

### Re: Negative Absolute Pressure

07/11/2021 5:19 PM

Good point about the sap pouring out. I’ve seen something similar in other trees and plants. Confirms at least that the pressure at ground level is not sub-atmospheric.

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#24
In reply to #22

### Re: Negative Absolute Pressure

07/11/2021 5:50 PM

I suspect the pressure would change when you cut the tree down.

When the tree is intact and healthy, the lowest pressure (most negative) will be at the top of the tree, due to the weight of the water below it.

I suspect the sap coming out of the tree stump is due to osmosis. If the solution inside the roots is more concentrated than the groundwater, water will be drawn in and if the tree is cut it would emerge from the stump.

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#39
In reply to #22

### Re: Negative Absolute Pressure

07/14/2021 9:43 AM

Good point about the sap pouring out. I’ve seen something similar in other trees and plants. Confirms at least that the pressure at ground level is not sub-atmospheric.

Interesting. One day I noticed a wet spot on my driveway. It looked like a vehicle had been parked there, but I was sure that was not the case. There is a river birch tree next to the driveway, and when I looked up I could see water coming out of a high branch. This would seem to indicate a positive pressure at that height.

This thread seems to have two topics: whether negative absolute pressure in liquid is possible, and whether tall trees use capillary action and negative pressure to raise water.

The latter issue is complicated in that there are many different kinds and sizes of trees. Some may use osmosis in the roots, some capillary action, and some both. There are examples for everyone...

Only pressure measurements will settle the question, and I think these measurements were done in 1999 in a test of the Cohesion-Tension theory in Maize plants.

Direct Measurement of Xylem Pressure in Leaves of Intact Maize Plants. A Test of the Cohesion-Tension Theory Taking Hydraulic Architecture into Consideration1 | Plant Physiology | Oxford Academic (oup.com)

Cohesion-Tension theory:

Water Transport in Plants: Xylem | Organismal Biology (gatech.edu)

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#41
In reply to #39

### Re: Negative Absolute Pressure

07/16/2021 8:37 AM

I would argue that capillary action and osmosis are inescapable twins in plants.

Like horses,and horse manure,you can't have one without the other in plants because of their very architecture.

Even seaweed has to move fluids in both directions,against much higher pressure differentials from top to bottom.

Fractal roots and capillaries,all influenced by osmotic pressure and capillary action provide a mutual additive effect in plants of all sizes,from algae to redwoods.

Insofar as the swaying of trees by the wind,it makes the roots and tree stronger,just like exercise does to muscles and bones.Muscles grow along lines of force,becoming longer,adding more muscle fibers,and bones grow across lines of force,increasing in diameter.Same with trees.If hair roots break due to high winds,they branch off from the break and grow more hair roots.The same happens at all sizes for roots and trunk.

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#26

### Re: Negative Absolute Pressure

07/11/2021 6:54 PM

The other mistake that is made here is that it is suction that is ‘pulling’ the fluid up the tree, this whole talk of negative absolute total pressure is total nonsense. Just the mechanical construction of the tree confirms this.

The pressure of the fluid at the base of the tree is clearly more than atmospheric , which is key. The positive gauge pressure at the base of the tree is also key, along with the pumping action of the capillary effect. The fact that pressure at the top of the tree is close to atmospheric is also key.

I once looked at the possibility of pumping reasonable quantities of fluid into the atmosphere at quite high elevations using multiple inline pumps, but it involves relatively high pressures, masses, etc. actually quite complicated.

The tree is a fraction of this.

The important thing to note is that the capillary action is, as I implied earlier, a continuous action so you an consider the tree as a continuous pump from bottom to top. As I said earlier, the tree is a pump. Although people don’t generally understand pressures that is ultimately what it’s all about.

The answer to the original question is that the tree does not ‘suck’ from the top, but rather ’pumps’ to top! So you can ignore the pressure at the bottom, and the pressure gradient is not as shown. Thanks

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#31

### Re: Negative Absolute Pressure

07/12/2021 8:29 AM

I remember considering setting up a CRT rebuilding business,back when CRT's were prolific.It required special pumps, heaters,and gauges(McLeod Gauge) which took the CRT down to about Torr -6

NASA Revealed that the vacuum on the moon was 1X10-9 Torr.

Outer space has approximately 1X10-6 Torr to 3X10-17 Torr.There are always a few hydrogen molecules moving around to prevent a "PerfectVacuum".

Chilling down to liquid helium temperature has produced some almost approaching perfect.

"There is no such thing as nothing"...unknown

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#38

### Re: Negative Absolute Pressure

07/13/2021 7:27 AM

<...For a liquid or solid, the answer is yes...>

<pedantmode>

Ahem, er, actually, most liquids transition into the vapour phase at a constant temperature as the pressure reduces towards zero. The phenomenon is called boiling.

As pressure is a fluid feature and solids are not fluids, the concept is not defined for them. The nearest equivalent feature is tensile stress/strain and compressive stress/strain of the material.

</pedantmode>

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#44

### Re: Negative Absolute Pressure

08/05/2022 5:34 PM

Imagine a large pipe with a smaller pipe inside.Then an even smaller pipe inside of that pipe,and smaller still, another,inside of that pipe.The water will incrementally increase in height as the tubes get smaller.Take this down to the micron size,and you have one of a tree's method of moving water.Combined with osmotic pressure and capillary action it is possible to pump water to the top of the tree.

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