Capillary Action: Exploring Very Thin Capillaries

In summary, according to the speaker, gravity is the only force that balances adhesive forces, and transpiration contributes to the actual flow. Capillary action draws water up until the tube is filled, but evaporation from the top makes the capillary action draw up more to keep it filled.
  • #1
Medicago
14
0
Considering very thin capillaries, such as found in wood to transport water (~100Micron), I understand that the two main factors in play are gravity and the adhesive forces between the water and the surface of the capillary tube.

I understand that gravity is proportional to volume that is (radius)^2 whereas adhesive forces are proportional to inner surface area of tube that is (radius)^1.

So for some small radius adhesive forces are stronger than gravitational pull.

However it seems as if this is independent of length. It seems that since both gravitational pull and adhesive forces, being proportional to volume and surface area, are directly proportional to some ΔL, then the length of the tube is irrelevant and the water will climb up until the tube ends. However, we still define a certain capillary length for capillaries.

Does this capillary length exist for very thin capillaries? Or would water climb indefinitely in a very thin perfect tube?

And if it does exist why would it depend on length anyway?

Thanks.
 
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  • #2
Well I've found the answer myself.

Apparently the opposing adhesive forces act only on the meniscus!

I thought it was a shear force that acts all along the surface of the tube.

But here's another question:

If I take a tube and split it to two tubes somewhere in the middle, creating two menisci, would that raise the water higher? Considering the diameter doesn't change.

I'm simply looking into the mechanics of water transport in trees and I'm really missing some essential fluid dynamics background so I'm trying to make up for it here..

Thanks.
 
  • #3
In trees, a lot of the vertical transport is accomplished by more than pure capillary action. The evaporation (transpiration actually) of moisture out of the leaves also plays an effect. Transpiration causes low pressure at the top of the column which in effect sucks the water up the capillaries. The stomata on the leaves open and close to control transpiration, and therefore vertical flow.
 
  • #4
negligible in terms of pressure gradient, chris.

One can assume that gravity is balanced entirely by adhesive forces, whereas transpiration contributes to the actual flow.
 
  • #5
Yes, a greater length of contact for the same cross-sectional area will support a taller column of water. That's why the capillaries are narrow.
As Medicago says, capillary action draws water up until the tube is filled, but evaporation from the top makes the capillary action draw up more to keep it filled.
 
  • #6
From Wikipedia:

"In taller plants and trees however, the force of gravity can only be overcome by the decrease in hydrostatic (water) pressure in the upper parts of the plants due to the diffusion of water out of stomata into the atmosphere"
 

1. What is capillary action?

Capillary action is the movement of a liquid along a narrow tube or through a porous material due to the forces of adhesion and cohesion. This phenomenon is driven by the surface tension of the liquid, which causes it to rise or spread out in thin spaces.

2. How does capillary action work?

Capillary action occurs due to the intermolecular forces between the liquid molecules and the solid surface of the tube or material. The liquid molecules are attracted to the solid surface, creating an adhesive force, while they are also attracted to each other, creating a cohesive force. These forces cause the liquid to rise or spread through narrow spaces.

3. What is the importance of capillary action in nature?

Capillary action plays a crucial role in many natural processes, such as the movement of water in plants, the formation of blood clots, and the absorption of nutrients in the human body. It also helps to maintain the water balance in soil and contributes to the water cycle.

4. How can capillary action be utilized in technology?

Capillary action has many practical applications, particularly in microfluidics and nanotechnology. It is used in inkjet printing, lab-on-a-chip devices, and drug delivery systems. It can also be harnessed to create self-cleaning surfaces and enhance the performance of batteries.

5. What factors affect capillary action?

The rate of capillary action can be influenced by several factors, including the surface tension of the liquid, the size and shape of the tube or material, and the properties of the liquid and solid surfaces. Temperature, gravity, and the presence of other substances can also impact capillary action.

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