Water not flow out of capillary

In summary, surface tension keeps liquid in a pipette if it's closed off and has a finger over the top, and it also prevents liquid from breaking up into droplets or bubbles if there's no finger over the top.
  • #1
kapital
19
0
Why does water, in a small glass tube,open at both sides(for example) not flow out if we put it in vertical position and put our finger on the top?(we close it)?
 
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  • #2
Surface tension. The force binding the surface molecules is greater than the weight of the thin coumn of liquid.
 
  • #3
Ok, but why does than have our finger on the top any influence on that?

And why does water not flow out of the pipette, is there the same reason?
 
  • #4
Are you thinking more of a drinking straw? The reason why water doesn't come out when you place your finger over the top of the filled straw is that the air pressure acting on the bottom surface of the water results in an equal (and opposite) force to the weight of the water.

Since you essentially have a vacuum over the top of the column (no air pressure, or very little, acting on the top of the column) the water column has no net forces acting on it.

edit: friction plays a part, too, and is partly contributing to the water column staying still.
 
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  • #5
kapital said:
Ok, but why does than have our finger on the top any influence on that?
If a capillary is really thin and walls are hydrophilic, the water will stay inside even if you don't have your finger on it.

If the capillary is a bit thicker and/or the walls are hydrophobic instead, then surface tension won't actually hold the fluid to the capillary. But it will prevent fluid from breaking up into droplets or allowing bubbles to form. That means, entire column of liquid must move as one. That means, additional air can't get in between the closed end and the liquid. Pressure at the top drops, and atmospheric pressure from bellow holds the liquid in. If you remove your finger, pressure equalizes, and liquid is released.

And yes, pipette works the same way. It relies on ambient pressure forcing or holding liquid in the pipette.

(Some of this is redundant to what DocZaius has said, but it's important to point out that surface tension plays a role either way.)
 

FAQ: Water not flow out of capillary

1. Why is water not flowing out of the capillary?

There could be several reasons for this. One possibility is that the capillary is clogged with debris or air bubbles, preventing water from passing through. Another possibility is that the capillary is not properly connected to a source of water or is not positioned correctly. Additionally, the surface tension of the water may be too strong for it to flow out of the capillary.

2. How can I unclog a capillary that is not allowing water to flow?

If the capillary is clogged with debris, you can try using a small brush or needle to gently remove the blockage. If there are air bubbles trapped in the capillary, you can try tapping the capillary or gently blowing into it to dislodge the bubbles. Alternatively, you can try using a stronger solvent, such as ethanol, to dissolve any debris or air bubbles.

3. What can cause the surface tension of water to be too strong for it to flow out of a capillary?

The surface tension of water can be affected by factors such as temperature, the presence of impurities, and the type of material the capillary is made of. If the water is too cold, it may have a higher surface tension, making it more difficult to flow out of the capillary. Similarly, impurities or contaminants in the water can also increase its surface tension. Additionally, certain materials, such as hydrophobic plastics, can also cause the water to have a higher surface tension.

4. Can the angle at which the capillary is positioned affect the flow of water?

Yes, the angle at which the capillary is positioned can have a significant impact on the flow of water. The steeper the angle, the greater the force of gravity acting on the water, which can help it flow out of the capillary. On the other hand, if the capillary is positioned at a very shallow angle, the force of gravity may not be strong enough to overcome the surface tension of the water, leading to little or no flow.

5. Is there a limit to how much water can flow out of a capillary?

Yes, there is a limit to how much water can flow out of a capillary. This limit is determined by the diameter of the capillary, the surface tension of the water, and the angle at which it is positioned. If the capillary is too narrow or the surface tension of the water is too strong, the water may not be able to flow at all. Additionally, if the angle is too steep, the water may flow too quickly and overflow from the capillary.

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