capillary pump

[satheeshchandran]

can i put a capillary pump like the illustration? my home is far from the farm.

[Enthalpy]

Sorry it won't work. Capillary action can suck water in some tube with proper filling, but the water will be equivalently harder to extract from the tube's top. It won't flow spontaneously out of the tube.

Donkey, wind mill, solar cells...?

[mistermack]

Trees can do it. Up to a maximum height of 115 metres. The way that they manage it, I believe, is by evaporating the water in the leaves, reducing the pressure in the tubes, so the liquid is forced up by root pressure. Obviously, the water evaporates at the top so it wouldn't be usable. I don't know how the roots create enough pressure to push water to the top of a Coastal Redwood, or Australian Mountain Ash at over 100m tall. Maybe energy manufactured in the leaves is used to create the root pressure, so it might not be possible to copy the process. 

[StringJunky]

1 hour ago, mistermack said:

Trees can do it. Up to a maximum height of 115 metres. The way that they manage it, I believe, is by evaporating the water in the leaves, reducing the pressure in the tubes, so the liquid is forced up by root pressure. Obviously, the water evaporates at the top so it wouldn't be usable. I don't know how the roots create enough pressure to push water to the top of a Coastal Redwood, or Australian Mountain Ash at over 100m tall. Maybe energy manufactured in the leaves is used to create the root pressure, so it might not be possible to copy the process. 

The xylem tubes (water carriers) are narrow enough for transpiration from the roots to the leaves, by negative pressure due to evaporation in the leaves, is sufficient. https://www.scientificamerican.com/article/follow-up-how-do-trees-ca/ 

Edited September 19, 2018 by StringJunky

[mistermack]

 

I was thinking along those lines, but I couldn't reconcile the height of tall trees with the partial vacuum caused by evaporation. From memory, going back to my school days, atmospheric pressure will only lift water approximately ten metres, at sea level in response to a nearly full vacuum. So in trees that are 100m tall, you would need pressure from the roots to take it the rest of the way.

If the evaporation was able to reduce the pressure in the tubes close to zero, you would think that the water would boil.

[StringJunky]

52 minutes ago, mistermack said:

 

I was thinking along those lines, but I couldn't reconcile the height of tall trees with the partial vacuum caused by evaporation. From memory, going back to my school days, atmospheric pressure will only lift water approximately ten metres, at sea level in response to a nearly full vacuum. So in trees that are 100m tall, you would need pressure from the roots to take it the rest of the way.

If the evaporation was able to reduce the pressure in the tubes close to zero, you would think that the water would boil.

 

Quote

Root pressure is the transverse osmotic pressure within the cells of a root system that causes sap to rise through a plant stem to the leaves.[1]

Root pressure occurs in the xylem of some vascular plants when the soil moisture level is high either at night or when transpiration is low during the day. When transpiration is high, xylem sap is usually under tension, rather than under pressure, due to transpirational pull. At night in some plants, root pressure causes guttation or exudation of drops of xylem sap from the tips or edges of leaves. Root pressure is studied by removing the shoot of a plant near the soil level. Xylem sap will exude from the cut stem for hours or days due to root pressure. If a pressure gauge is attached to the cut stem, the root pressure can be measured.

Root pressure is caused by active distribution of mineral nutrient ions into the root xylem. Without transpiration to carry the ions up the stem, they accumulate in the root xylem and lower the water potential. Water then diffuses from the soil into the root xylem due to osmosis. Root pressure is caused by this accumulation of water in the xylem pushing on the rigid cells. Root pressure provides a force, which pushes water up the stem, but it is not enough to account for the movement of water to leaves at the top of the tallest trees. The maximum root pressure measured in some plants can raise water only to 6.87 meters, and the tallest trees are over 100 meters tall.

https://en.wikipedia.org/wiki/Root_pressure

 

10 minutes ago, StringJunky said:

If the evaporation was able to reduce the pressure in the tubes close to zero, you would think that the water would boil.

I think the surface tension of the water column at the top of a capillary (xylem) will act like a solid piston head and the negative pressure above it is enough for that column to rise to fill the void. The xyla (?) are only microns in diameter. The xylem will have evolved to have a diameter that is optimal for the surface tension of the water and the height of the plant/tree.

Edited September 19, 2018 by StringJunky

[mistermack]

There seems to be something missing then. If the maximum that a vacuum can lift water is ten metres, what's lifting it above that? It doesn't come across from the pages that have been linked so far.

Reading a bit more, it appears that the exact mechanism isn't agreed as yet. Wikipedia mentions root pressure, transpirational pull, and "pressure flow hypothesis". 

• Pressure flow hypothesis: Sugars produced in the leaves and other green tissues are kept in the phloem system, creating a solute pressure differential versus the xylem system carrying a far lower load of solutes- water and minerals. The phloem pressure can rise to several MPa,^[11] far higher than atmospheric pressure. Selective inter-connection between these systems allows this high solute concentration in the phloem to draw xylem fluid upwards by negative pressure.

This seems to be the answer to some of the problems I mentioned :

Transpirational pull requires that the vessels transporting the water be very small in diameter; otherwise, cavitation would break the water column. And as water evaporates from leaves, more is drawn up through the plant to replace it. When the water pressure within the xylem reaches extreme levels due to low water input from the roots (if, for example, the soil is dry), then the gases come out of solution and form a bubble – an embolism forms, which will spread quickly to other adjacent cells, unless bordered pits are present (these have a plug-like structure called a torus, that seals off the opening between adjacent cells and stops the embolism from spreading).

It seems to be a lot more complicated than a simple tube.

 

 

 

[StringJunky]

35 minutes ago, mistermack said:

There seems to be something missing then. If the maximum that a vacuum can lift water is ten metres, what's lifting it above that? It doesn't come across from the pages that have been linked so far.

Reading a bit more, it appears that the exact mechanism isn't agreed as yet. Wikipedia mentions root pressure, transpirational pull, and "pressure flow hypothesis". 

• Pressure flow hypothesis: Sugars produced in the leaves and other green tissues are kept in the phloem system, creating a solute pressure differential versus the xylem system carrying a far lower load of solutes- water and minerals. The phloem pressure can rise to several MPa,^[11] far higher than atmospheric pressure. Selective inter-connection between these systems allows this high solute concentration in the phloem to draw xylem fluid upwards by negative pressure.

This seems to be the answer to some of the problems I mentioned :

Transpirational pull requires that the vessels transporting the water be very small in diameter; otherwise, cavitation would break the water column. And as water evaporates from leaves, more is drawn up through the plant to replace it. When the water pressure within the xylem reaches extreme levels due to low water input from the roots (if, for example, the soil is dry), then the gases come out of solution and form a bubble – an embolism forms, which will spread quickly to other adjacent cells, unless bordered pits are present (these have a plug-like structure called a torus, that seals off the opening between adjacent cells and stops the embolism from spreading).

It seems to be a lot more complicated than a simple tube.

 

 

 

Isn't the limit set by gravity i.e. the weight of water vs the vacuum above? The thinner the tube, the higher the water can rise for a given vacuum.

Edit: Further down my link:

Quote

As water is lost out of the leaf cells through transpiration, a gradient is established whereby the movement of water out of the cell raises its osmotic concentration and, therefore, its suction pressure. This pressure allows these cells to suck water from adjoining cells which, in turn, take water from their adjoining cells, and so on--from leaves to twigs to branches to stems and down to the roots--maintaining a continuous pull.

 

Edited September 20, 2018 by StringJunky

[mistermack]

42 minutes ago, StringJunky said:

Isn't the limit set by gravity i.e. the weight of water vs the vacuum above? The thinner the tube, the higher the water can rise for a given vacuum.

 

No, I think it's independent of the size of the tube. The atmospheric pressure at sea level is 760 mm of mercury, regardless of the measuring tube. Which equates to about 10m of water. 

I found one fact on the net, that water will boil at 23 deg C, at a pressure of one fortieth of an atmosphere. So the small diameter of the Xylem seems to come into play, by preventing cavitation at low pressures.

1 hour ago, StringJunky said:

As water is lost out of the leaf cells through transpiration, a gradient is established whereby the movement of water out of the cell raises its osmotic concentration and, therefore, its suction pressure. This pressure allows these cells to suck water from adjoining cells which, in turn, take water from their adjoining cells, and so on--from leaves to twigs to branches to stems and down to the roots--maintaining a continuous pull.

That still doesn't overcome the 30m problem. If you have a simple tube, with one end in water, and the other a perfect vacuum, then the maximum that the water can rise is 30m, because that's the value of atmospheric pressure, in m of water. Because pressure is force/area, if your tube doubles in cross section area, the upwards force is doubled, and the weight of water in the column also doubles. So whatever the diameter, a given vacuum gives the same height of column supported.

If you had a six inch pipe, with a near total vacuum at the top, I would expect the water to be pushed up 30m, and be boiling at the surface, if it was at room temperature. 

So I don't think you can make it work for any tree over 35 m in height, with maximum root pressure, and a 100% vacuum at the top. There must be something else in play for the very tall trees.