Capillary Action (Capillary Depression)

In summary, the conversation discusses capillary action and capillary depression for practical purposes. The main forces acting on water in a typical capillary pressure illustration are gravity pushing down and capillary pressure pulling up due to adhesion. The water will not fall out of the capillary when the base cup is removed because the capillary force is not dependent on the presence of the base cup. The conversation also explores various scenarios involving different types of capillaries and their effects on water movement. Further experiments and illustrations are suggested for a better understanding of capillary action.
  • #1
Pete2s
6
0
Hi,

I have questions regarding capillary action and capillary depression for practical purposes. I'm fairly educated in physics and chemistry, but have some "what would happen if..." questions. Please help, and add any information you'd think helpful!

1) Imagine you have your typical capillary pressure illustration: a glass cup with water in it and a hydrophilic tube sticking out of it. Capillary pressure drags the water up into the tube and above the water line of the cup. What are the main forces acting on the water in the tube?

a) My answer: gravity pushing down, while the capillary pressure from the adhesion of the water to the walls of the tube pulling up.

Then why, when you take away the base cup, does the water fall out the bottom? I understand the water in the cup pushes up, but it also pulls down from cohesion. I thought these forces would cancel.


Now, imagine a hydrophilic capillary of changing radius, like "V" but turned upside down like ^. Now, put a drop of water in the middle of the capillary. We know the smaller the radius, the more pulling power. Is the drop just going to fall out without a base cup of water? Will it move up the walls towards the smaller radius?

Imagine the same scenario, but the capillary is a regular V (not turned upside down) but is hydrophobic, so water needs pressure to get in. Drop a drop of water in the middle of the capillary. What happens, especially without a "base" of water anywhere?

Can there be capillary action/depression without a "base cup" of water on the outside of the capillary?

These questions are actually for fabric analysis of wicking and capillary depression.

Thanks for any help.
 
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  • #2
The capillary force has nothing to do with base cup. Only weight and adhesion involve.

So: if you take out the cup, the water inside the capillary stilll holds.
I'll come back the V expreriments later. Could you please draw it out?
 
  • #3
Illustration

Sure, here are the illustrations, with a 3rd situation added. I think you may have answered my question when you said nothing happens when the cup falls out.

http://img156.imageshack.us/my.php?image=capillaryactiondw4.png"

In the first situation, the water is drawn all the way to the top.

In the second situation, the water is pushed to the top and out.

In the third situation, despite the water being so close to the bottom, because the pressure on the north edge is less than the pressure on the south edge, the drop is pushed all the way to the top and out.
 
Last edited by a moderator:

1. What is capillary action?

Capillary action, also known as capillary depression or capillary rise, is the process by which liquids move through narrow spaces, such as tubes, against the force of gravity.

2. How does capillary action work?

Capillary action is driven by the adhesive and cohesive forces between the liquid and the surface of the tube. These forces cause the liquid to climb up the tube, creating a concave meniscus at the top.

3. What factors affect capillary action?

The rate of capillary action is influenced by several factors, including the diameter and material of the tube, the properties of the liquid (such as surface tension and viscosity), and the strength of the adhesive and cohesive forces.

4. What are some real-life examples of capillary action?

Capillary action can be observed in many everyday situations, such as the absorption of water by plant roots, the movement of ink in a pen, and the ability of a paper towel to soak up spilled liquid.

5. How is capillary action useful in science and technology?

Capillary action has many practical applications, including in medical devices such as microfluidic chips, in the design of water filtration systems, and in the development of new materials for drug delivery. It also plays a crucial role in the movement of fluids in plants and animals.

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