Questioning My Conception of Surface Tension

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Discussion Overview

The discussion revolves around the concept of surface tension in fluid mechanics, particularly in relation to the behavior of a needle on the surface of water. Participants explore the forces involved when attempting to pull a needle from the water surface, questioning the necessity of an external force despite the upward action of surface tension.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the need for an external force to pull a needle from the water surface, despite understanding surface tension.
  • Another participant describes an experiment where the needle depresses the water surface, suggesting that surface tension has a downward component that requires an external force to counteract.
  • A participant explains that the energy associated with increasing the water-air interface when pulling the needle requires a force, as the water molecules at the surface have higher energy when they are distorted.
  • One participant clarifies that gravity pulls the needle down while surface tension acts upward, preventing it from sinking.
  • A participant provides diagrams to illustrate the forces acting on the needle and the water surface, explaining the relationship between surface tension and the external force applied.
  • Another participant expresses confusion about the necessity of a pulling force when surface tension acts upward, seeking clarification on the dynamics involved.
  • A participant compares the water surface to an elastic sheet, explaining how the forces change when the needle is pulled up versus when it is pushed down by gravity.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the role of surface tension and the forces at play. Some agree on the necessity of an external force to pull the needle, while others remain confused about the mechanics involved. The discussion does not reach a consensus on the conceptual clarity of these interactions.

Contextual Notes

Participants reference experiments and diagrams to illustrate their points, but there are unresolved aspects regarding the interpretation of forces and the behavior of the water surface in relation to the needle.

Ezio3.1415
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I was solving a problem from fluid mechanics... But that led me to question my conception abt surface tension...
The problem stated that the max force needed to uphold a needle(ignore its weight)from the water surface is F... The needle is L m long... What is the surface tension?
They found it by T=F/2L
I know how surface tension works... But I fail to grasp this problem... Why do we need force at all to take out the needle?
 
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I just did an experiment... The needle press down a little on the water... Water gets a little dowm near the edge...so surface tension has a perpendicular downward component... So I need to give an F to pull up the needle... Am I right?
 
The source of surface tension is the fact that water (or other liquid) molecules like to be near to each other. A water molecule at the water-air interface has a higher energy than a molecule that is surrounded by other water molecules. Thus, to increase the area of the water-air interface costs energy. When you pull the needle away from the water, you are doing exactly that - you distort the water surface such that the water-air interface becomes larger. This costs energy, so you need to apply a force.
 
I just did an experiment... The needle press down a little on the water... Water gets a little dowm near the edge...so surface tension has a perpendicular downward component... So I need to give an F to pull up the needle... Am I right?

In this experiment, it is gravity that pulls the needle down. Surface tension is acting vertically upwards, preventing it from sinking.

Edit - I think some diagrams to show the shape of the water surface around the needle might be informative. I'll draw some and add them shortly.
 
http://oi50.tinypic.com/s1p7iv.jpg

In this diagram, I have attempted to show the needle on the water surface, subject to gravity and surface tension. I have shown it from three directions, the two horizontal views are rotated 90 degrees from each other.

Moving the needle down by a small distance distorts the water surface which creates a larger water-air interface. This interface has energy associated with it, E = Aγ, where A is the area of new surface and γ is the surface tension of the water. If you differentiate that equation with respect to the vertical direction, you will obtain the expression for the upward force on the needle due to surface tension.

In your original problem, you were considering pulling the needle up out of the water. In that case, the water would simply bulge up instead of down and the force will hold the needle down instead of pulling it up.
 
I am again back to my previous confusion... If surface tension acts upward,then why do I have to pull with a force when the needle is weightless...
"In your original problem, you
were considering pulling the
needle up out of the water. In
that case, the water would
simply bulge up instead of
down and the force will hold
the needle down instead of
pulling it up."
I didn't understand this part...
 
http://oi50.tinypic.com/dyohup.jpg

In this diagram, I have tried to illustrate the difference between what happens when gravity is pulling the needle down and what happens when you pull it up with some external force. Fst is surface tension and Fext is whatever external force you use to pull at it.

It might be useful to think of the water surface as a sheet of elastic material. When gravity pulls the needle down, it pulls the sheet down a bit with it. The sheet wants to spring back up, so it exerts a force in that direction.

When you pull the needle up with some external force (with a string attached to it or whatever), you pull the surface up with it. The sheet wants to spring back down, so it exerts a force downwards. If the force you exert is strong enough, the restorative force of the sheet can't hold the needle, the sheet breaks and you lift the needle away.
 
Thank you very much for the help... The figure explains it all...
 

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