The role of surface tension in capillarity

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SUMMARY

The discussion focuses on the role of surface tension in capillarity, specifically how it interacts with gravity to influence the movement of water in a tube. Participants agree that surface tension exerts an upward force on water, counteracting gravity until equilibrium is reached. Additionally, adhesive forces between water and the container material, such as glass, contribute to the upward movement of the liquid. The conversation highlights the dual nature of surface tension and adhesion in explaining the behavior of liquids in narrow spaces.

PREREQUISITES
  • Understanding of surface tension and its molecular basis
  • Familiarity with adhesive and cohesive forces in liquids
  • Basic knowledge of capillarity and its applications
  • Concepts of continuum mechanics as they relate to fluid dynamics
NEXT STEPS
  • Research the principles of capillarity in different liquids, focusing on water and mercury
  • Explore the relationship between surface tension and adhesive forces in various materials
  • Study the mathematical models of fluid dynamics, particularly in narrow tubes
  • Investigate the applications of capillarity in real-world scenarios, such as soil moisture retention
USEFUL FOR

Students and professionals in physics, chemistry, and engineering, particularly those interested in fluid dynamics, material science, and environmental science.

Shackleford
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From what I've read, the surface tension pulls upward on the given volume of water and when this surface tension force equals the force of gravity (weight), there is no more vertical movement up the tube.

Why is it said that the surface tension pulls upward on the mass of water when surface tension refers to the increased horizontal intermolecular attraction of the top surface layer? Why do you not say that the upward force on the volume of water is the adhesive forces from the container material?
 
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Hi Shackleford! :smile:

I think both views are correct …

a) Surface tension is a force parallel to the liquid surface and pointing towards the solid surface.

So when the meniscus goes up (as with water in a glass tube), the force is up, but when it goes down (as with mercury in a glass tube), the force is down.

In other words …
Shackleford said:
… the surface tension pulls upward on the given volume of water and when this surface tension force equals the force of gravity (weight), there is no more vertical movement up the tube.
b) But the reason why the meniscus is up for water on glass is that water adheres to glass

and so …
Shackleford said:
the upward force on the volume of water is the adhesive forces from the container material.

(And see also http://en.wikipedia.org/wiki/Surface_tension#Liquid_in_a_vertical_tube" for a third argument about minimising the surface area of the water. :wink:)
 
Last edited by a moderator:
Shackleford said:
From what I've read, the surface tension pulls upward on the given volume of water and when this surface tension force equals the force of gravity (weight), there is no more vertical movement up the tube.

Why is it said that the surface tension pulls upward on the mass of water when surface tension refers to the increased horizontal intermolecular attraction of the top surface layer? Why do you not say that the upward force on the volume of water is the adhesive forces from the container material?

I understand the molecular interpretation of surface tension is commonly taught that way, but I'm not convinced it provides any real insight. It's much simpler to see the result in terms of continnum mechanics:

http://www.agron.iastate.edu/soilphysics/a577cap.html
 
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