Water not flow out of capillary

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    Capillary Flow Water
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Discussion Overview

The discussion revolves around the behavior of water in a small glass tube or capillary, particularly why water does not flow out when the tube is held vertically with a finger covering the top. The scope includes concepts of surface tension, air pressure, and the effects of tube diameter and wall properties on fluid behavior.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that surface tension is a key factor preventing water from flowing out, as the force binding surface molecules is greater than the weight of the liquid column.
  • Others argue that the influence of placing a finger on the top of the tube relates to air pressure, which creates a vacuum effect that balances the weight of the water column.
  • A later reply questions the role of tube diameter and wall properties, suggesting that in thin, hydrophilic capillaries, water can remain inside without a finger on top, while thicker or hydrophobic walls may not retain water in the same way.
  • It is noted that friction may also play a role in keeping the water column stable.
  • Some participants draw parallels to the operation of a pipette, indicating that ambient pressure is crucial in both scenarios for holding liquid in place.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the mechanisms at play, particularly concerning the roles of surface tension, air pressure, and tube characteristics. The discussion remains unresolved with no consensus reached on the primary factors influencing the behavior of water in the tube.

Contextual Notes

Limitations include the dependence on specific definitions of surface tension and pressure, as well as the unresolved nature of how different tube characteristics affect fluid behavior.

kapital
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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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Surface tension. The force binding the surface molecules is greater than the weight of the thin coumn of liquid.
 
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?
 
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.
 
Last edited:
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.)
 

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