If you do not have a tiny opening, the piston remains in place as the weight of the water is counterbalanced by the atmospheric pressure at the bottom of the piston. In this case, the pressure at the top of the water column is formed by the difference of atmospheric pressure and hydraulic pressure p-rho*g*h (h being the height of the column, we neglect the weight of the piston). If the water pressure is less than the steam pressure, you'll get steam (cavitation) and the piston will drop out.

Now you add the tiny hole. If the capillary pressure exceeds the static pressure on the inside of the cylinder, nothing much will change. Some water will get into the opening.

If the hole is too large and the capillary pressure too low, air is sucked into the cylinder and the piston will fall out.

So, if you put a hole into a cylinder, make it very small or the cylinder's performance will drop significantly. You will not improve the cylinder by making a hole.

1. No, there is a limit to 'suction' pressure

2. No.

3. In a water column barometer - it is a setup that demonstrates that steam is formed at low pressure - without a piston.

4. If you have several openings you cannot add the pressure as - as you wrote - the pressure is delivered everywhere in the fluid. In order to add up different capillary pressures, you would have to put several fluids on top of each other, each delivering ist own capillary effect.

OK, I admit I have presented things not very clearly. In fact I mean there is a load hooked  at the bottom of the piston. When there is not an opening the load must overcome just 1 atm. When there is an opening the load can be as much as 10 fold greater as capillary gives 10 atm up. Of course the boundary between water and the piston would be under huge pressure (this can be overcome by applying some voltage as electrostatic force is much greater than gravitation). 

In fact I suppose this is the mechanism that holds water in high trees. It is called cohesion - tension theory. This theory however does not mention the Pascal law, which role I want to make clear. In fact  I think that Pascal law is responsible for sucking water through the roots. Also the water is supposed not to include dissolved air, which maybe is a function of the root itself.

http://www.edinformatics.com/math_science/pascals_principle_hydraulics.htm

Let us do another thought experiment. You have the sealed cylinder with the weight of the piston balanced by the pressure differential (p-rho*g*h-m_piston/A_piston*g). Then you add a capillary tube. The water table in the tube is rising according to the capillary pressure. While the pressure in the cylinder does not change significantly (still governed by p-rho*g*h-m_piston/A_piston*g), the volume of water is decreased by the volume in the tube. And the piston has risen accordingly. So, the capillary tube has done some work on the piston by lifting the weight a tiny, tiny increment. :-)

Hi Ingo Riess 

I am almost convinced that the Pascal law is what helds (but not lifts) the water in high plants. The Pascal law is not only for positive pressures. The capillary ensures negative pressure (pull on the water) and it is transfered everywhere due to Pascal law.

This is seen by the capillary sucking water from a box of water as you also pointed out.

Also when two capillary tubes are connected the level in the thinner tube is higher.

And I beleve it can hold much water (as I showed in previous post that the pressure can be huge).

Also there is no need for a tube but just a capillary opening (which are the stomata in the leaves).

I suppose that if one has a cone with an thin opening the water will not fall out. The base should be covered with paper and the paper should be removed very gently (to avoid waves).

This is a consequence of an trial with two capillary tubes connected and the wider poining down - surely the water will not fall out.

I have a memory of seeing something like this with the cone but not when and where. 

I would be thankfull if you confirm the thought experiment (if you agree) with the cone and can point to such real trial. Maybe I will post a new question to look after such experiment in order to widen the audience.

I am not an expert on capillary pressure, but I am fairly confident that you cannot have a negative pressure in a fluid. The same applies to your thought experiment with the cone. It is the ambient pressure that keeps the paper at the base of the cone. And it works without a capillary on the top. As soon as the hydrostatic pressure exceeds ambient pressure, the paper (and water) falls off.

 Piston will not move down and fall out even if you do not have a hole. Every small moving creates a vacuum up of the piston.

See also.

Ingo,

plant physiologist have a theory, which they call CT theory (cohesion tension theory) which explains why water can climb 100 m up the xylem (tiny water tubes of roughly few micrometer and at the end of these are the nm tiny openings - stomata). The main point is the negative pressure in a fluid, which they measure above  -10 MPa.

The state of water is what they call methastable. The column of water is intact from roots to stomata and must be clear of air otherwise it will build bubles which brake the column. Exactrly these are conditions for Pascal law!

Negative pressure is of course the pressure which the crooked surface in the capillary delivers. It is compensated by the hydrolic pressure (which is down e.g. normal, positive pressure).

I can not see why it can not be transmitted tru the liquid as the normal positive pressure (it will spread tru the H-H bonds the water molecules build with each other). 

About a cone - of corse when the height of water is not much 1 atm can hold it. But this is not the case in plants (sequoia tree is over 100 m). The atmospheric pressure can lift max 10 m high). For 100 you need another reason.

 Alexander see my answer to Ingo.

Ilian, sorry. What do you want to know. I have read all, but can not understand. I see no trouble in this conversation.

Ok, I learned something new today. Found this: While pressures are, in general, positive, there are situations in which negative pressures may be encountered: E.g. when attractive intermolecular forces (e.g., van der Waals forces or hydrogen bonds) between the particles of a fluid exceed repulsive forces due to thermal motion. These forces explain ascent of sap in tall plants. An apparent negative pressure must act on water molecules at the top of any tree taller than 10 m, which is the pressure head of water that balances the atmospheric pressure. Intermolecular forces maintain cohesion of columns of sap that run continuously in xylem from the roots to the top leaves.

My conclusion is that in a tall tree, the forces from hydrogen bonds are balanced locally by the hydrostatic pressure of  the fluid. Effective total pressure is limited. You cannot use this negative pressure for a larger volume of fluid. 

"My conclusion is that in a tall tree, the forces from hydrogen bonds are balanced locally by the hydrostatic pressure of  the fluid. Effective total pressure is limited."

The formula for the height in a capillary is h=1,5.10^-5/R in metres: this gives for the stomata with say 5 nm a readout of 3 km water column; so the hydrolic pressure can not compansate much. 100 m of water is equal to 1 MPa whereas I wrote here about mesuring -10 MPa in trees.

"You cannot use this negative pressure for a larger volume of fluid." I cant not understand do you agree that it is the Pascal law with negative pressure? If you agree you know that in Pascal law the pressure from a small surface is transfered to a indefinately large surface. So on any area the pressure is equal no matter how the volume is. For the plants the xylem tubes are huge in comparison to the opening of the stomata.

Alexander do you agree that

1.The Pascal law is what helds the water in high plants by the capillary force exerted from the nm wide openings called stomata as negative pressure to the base of the tree.

2. Can a opening of 1 micrometer hold water not to fall even in a cone of say 10 cm R.