Negative Pressure: Is There Anything Below Total Vacuum?

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    Negative Pressure
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Discussion Overview

The discussion revolves around the concept of negative pressure and its relationship to vacuum. Participants explore whether negative pressure exists, how it is defined in different contexts, and the implications of these definitions in physics, engineering, and chemistry.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants assert that a vacuum is defined as zero pressure and argue that negative pressure cannot exist below this point.
  • Others clarify that negative pressure can refer to relative pressure, which can be less than atmospheric pressure, and that this distinction is important in engineering and chemistry.
  • A few participants discuss the concept of negative absolute pressure, particularly in the context of fluid dynamics and tree sap, where attractive forces can exceed repulsive forces.
  • There are claims that negative pressure can exist in solids under tension, and that tree sap exhibits viscoelastic behavior that may allow for negative pressure definitions.
  • Some participants reference external sources, such as Wikipedia and scientific articles, to support their claims about negative pressure and its implications in various contexts.
  • Concerns are raised about the interpretation of capillary action and whether it should be classified as negative absolute pressure or merely a mechanical effect.
  • There is a discussion about the conditions under which water can maintain a metastable state and the factors preventing cavitation.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the existence and definition of negative pressure. Multiple competing views remain, particularly regarding the distinction between absolute and relative pressure, and the implications of these definitions in different scientific contexts.

Contextual Notes

Participants highlight the importance of context when discussing pressure, noting that definitions may vary significantly across different fields such as physics, engineering, and chemistry. There are also unresolved questions about the mechanisms that allow for negative pressure in certain scenarios, such as in tree sap.

qitara
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Hi guys

Is there any thing such as Negative Pressure, or is there only total vacuum 0 and nothing below that ?
 
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isn't a vacuum and negative pressure the same thing ?.
 
qitara said:
isn't a vacuum and negative pressure the same thing ?.

No, a vacuum is zero pressure and because you can't have a pressure smaller than that of a perfect vacuum, there are no negative pressures.

What's confusing you is that people often say "pressure" when they mean "difference from atmospheric pressure" - by that definition any "pressure" that is less than atmospheric is negative.
 
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Nugatory said:
No, a vacuum is zero pressure and because you can't have a pressure smaller than that of a perfect vacuum, there are no negative pressures.

What's confusing you is that people often say "pressure" when they mean "difference from atmospheric pressure" - by that definition any "pressure" that is less than atmospheric is negative.

So it's just a way to say that the pressure is below the reference pressure ?
 
In almost all areas of Engineering and Chemistry, yes its relative pressure and it can be negative.
In some areas of Physics, it has an entirely different meaning. This is the second post I've answered in the last 10 minutes where the poster didn't explain in what context the term was being used in. *sigh* Unless you are dealing with quantum field theory, you can NOT have negative absolute pressure and you CAN have negative relative pressure (also known as "gauge pressure").
 
abitslow said:
In almost all areas of Engineering and Chemistry, yes its relative pressure and it can be negative.
In some areas of Physics, it has an entirely different meaning. This is the second post I've answered in the last 10 minutes where the poster didn't explain in what context the term was being used in. *sigh* Unless you are dealing with quantum field theory, you can NOT have negative absolute pressure and you CAN have negative relative pressure (also known as "gauge pressure").

Term used in Engineering, but the picture is clear now thanks to you guys
 
Nugatory said:
No, a vacuum is zero pressure and because you can't have a pressure smaller than that of a perfect vacuum

.


There is no any perfect vacuum because if there is perfect vacuum than it will exert infinite force to fill it.
 
Consider the atmospheric at sea level (1atm) as the reference pressure.
 
  • #10
Varun Bhardwaj said:
There is no any perfect vacuum because if there is perfect vacuum than it will exert infinite force to fill it.
Why do you think that?
 
  • #11
Nugatory said:
No, a vacuum is zero pressure and because you can't have a pressure smaller than that of a perfect vacuum, there are no negative pressures.
Sure there are:
http://en.wikipedia.org/wiki/Pressure#Negative_pressures
When attractive forces (e.g., van der Waals forces) between the particles of a fluid exceeds repulsive forces due to thermal motion. These forces explain ascent of sap in tall plants. Negative pressure must exist at the top of any tree taller than 10 m, which is the pressure head of water that balances the atmospheric pressure. Van der Waals forces maintain cohesion of columns of sap that run continuously in xylem from the roots to the top leaves.
 
  • #13
Chestermiller said:
This is just negative gage pressure (relative to 1 atm.). It is not negative absolute pressure.
This here is absolute negative pressure:
When attractive forces (e.g., van der Waals forces) between the particles of a fluid exceeds repulsive forces due to thermal motion. These forces explain ascent of sap in tall plants. Negative pressure must exist at the top of any tree taller than 10 m, which is the pressure head of water that balances the atmospheric pressure. Van der Waals forces maintain cohesion of columns of sap that run continuously in xylem from the roots to the top leaves.
 
  • #14
If you also want to count solids, and, if you define the pressure in a solid under tension as minus the isotropic part of the stress tensor, then, yes, you can have a "negative pressure" in a solid. Tree sap, which is a polymer solution, exhibits viscoelastic rheological behavior, which is a combination of viscous-fluid and solid behavior. So tree sap can also exhibit, to some degree, what would be defined as negative pressure. But ordinary Newtonian viscous gases and liquids are not viscoelastic, and do not exhibit negative pressure.
 
  • #15
A.T. said:
This here is absolute negative pressure:

That wikipedia article is stretching things a bit by characterizing capillary action (which is what the tree is using to lift its sap above 10 meters) as negative absolute pressure. It's more akin to a mechanical system with a pump at the bottom... except that the pump is distributed all along the surface of the xylem cells. It's would be more accurate to say that at all points in the column the pressure is positive and greater than the weight of the fluid above that point.

However, I think that is all a second-order sort of subtlety when OP is asking "isn't a vacuum and negative pressure the same thing?".
 
  • #16
Chestermiller said:
Tree sap, which is a polymer solution, exhibits viscoelastic rheological behavior, which is a combination of viscous-fluid and solid behavior. So tree sap can also exhibit, to some degree, what would be defined as negative pressure. But ordinary Newtonian viscous gases and liquids are not viscoelastic, and do not exhibit negative pressure.
Pure water can have negative absolute pressure too:
http://www.nature.com/nature/journal/v455/n7210/full/nature07226.html
 
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  • #17
Nugatory said:
That wikipedia article is stretching things a bit by characterizing capillary action (which is what the tree is using to lift its sap above 10 meters) as negative absolute pressure.
No, it's not (just) capillary action (attraction by the walls), but actually negative absolute pressure (attractive forces between the water molecules).
 
  • #18
A.T. said:
Pure water can have negative absolute pressure too:
http://www.nature.com/nature/journal/v455/n7210/full/nature07226.html
That article is amazing, if correct. I've never heard of this type of thing before. Thanks for calling it to my attention. Maybe you can help me. What is it the enables the water to maintain this metastable state, and prevents it from cavitating? Is it somehow related to the small dimensions of the channel?

Chet
 
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  • #19
Chestermiller said:
That article is amazing, if correct. I've never heard of this type of thing before. Thanks for calling it to my attention. Maybe you can help me. What is it the enables the water to maintain this metastable state, and prevents it from cavitating? Is it somehow related to the small dimensions of the channel?

Chet

The article is correct. There is an excellent Veritasium video on youtube called "The Most Amazing Thing About Trees" about that topic. It's worth watching.
 
  • #20
dauto said:
The article is correct. There is an excellent Veritasium video on youtube called "The Most Amazing Thing About Trees" about that topic. It's worth watching.
Here it is:

https://www.youtube.com/watch?v=BickMFHAZR0

Chestermiller said:
Maybe you can help me. What is it the enables the water to maintain this metastable state, and prevents it from cavitating? Is it somehow related to the small dimensions of the channel?

According to the video the key is the lack of air bubbles see (3:33min). The article also discuses probability of cavitation.
 
  • #21
Varun Bhardwaj said:
There is no any perfect vacuum because if there is perfect vacuum than it will exert infinite force to fill it.

That is not correct. the maximum negative pressure (what you are calling force) that can be exerted by a 'vacuum' is Atmospheric Pressure (1Bar). The force on a piston to produce a pressure of 0.01Bar is very little different than the force on a piston producing a pressure of 0.001Bar. The forces needed will be (the Piston area times 0.99 Bar) compared with (Piston area times 0.999 Bar) -i.e. virtually no difference at all.
It's not the need for a massive force that limits how deep a vacuum you can produce; it's the practical matter of how you can extract the molecules of a gas (air) out of a space without more molecules getting back in via the pump. Very low vacuums are limited by contamination by odd molecules coming of the insides of the chamber and pipes.
 

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