Force due to Surface tension here

[Vivek98phyboy]

Homework Statement

Why does the force due to surface tension act that way it is here?

Relevant Equations

Weight= net force due to surface tension

Why does the force due to surface tension act parallel to the surface here?

I know that surface tension is a result of absence of cohesive force above the surface and thus the water molecules below pulls the surface down and keeps it like a stretched membrane.
If the surface is pressed as shown in the above image,
shouldn't the water molecules along the slope experience a force inward as the water molecules are always pulling it inside?
Shouldn't the resultant force due to surface tension be pointed inwards and collapse the elastic nature as there is no water molecules in the gap created by the leg of insect?

Like this

 

[haruspex]

You are right that the net force on a molecule of water on the surface is into the body of water, but that is not the same as the consequence for the surface as such.
Imagine a ripple in the surface. A molecule at the top of the ripple experiences a greater net force into the fluid than one in the dip, so the effect on the surface is to pull it flat.

 

[Vivek98phyboy]

You are right that the net force on a molecule of water on the surface is into the body of water, but that is not the same as the consequence for the surface as such.
Imagine a ripple in the surface. A molecule at the top of the ripple experiences a great net force into the fluid than one in the dip, so the effect on the surface is to pull it flat.

So, do you mean that the net force is different when look at it as a whole surface from looking at it as a molecule?

 

[haruspex]

So, do you mean that the net force is different when look at it as a whole surface from looking at it as a molecule?

I mean that the observed behaviour is as though there is a force contracting the surface.

 

[Tom.G]

Same effect as putting a bowling ball on your bed (or on a piece of foam rubber).

 

[haruspex]

Same effect as putting a bowling ball on your bed (or on a piece of foam rubber).

I don't see a valid analogy there.
The examples you give are based on forces resisting compression. The challenge with surface tension is to explain how forces of attraction between fluid particles result in a tension in the surface.

There is some attempt to explain it at https://en.m.wikipedia.org/wiki/Surface_tension#Causes, but it's a bit short on detail.

 

[Vivek98phyboy]

I don't see a valid analogy there.
The examples you give are based on forces resisting compression. The challenge with surface tension is to explain how forces of attraction between fluid particles result in a tension in the surface.

There is some attempt to explain it at https://en.m.wikipedia.org/wiki/Surface_tension#Causes, but it's a bit short on detail.

I just checked across few websites and it said that the downward force may sometimes pull the molecules into the bulk liquid but as soon as it goes in, another molecule from the inside rushes to fill the gap. If is it so, then from where does the molecule get energy to move upwards?

 

[haruspex]

I just checked across few websites and it said that the downward force may sometimes pull the molecules into the bulk liquid but as soon as it goes in, another molecule from the inside rushes to fill the gap. If is it so, then from where does the molecule get energy to move upwards?

The forces are electrostatic, and thermal (kinetic) energy keeps the molecules in motion. It all balances out.

 

[Vivek98phyboy]

The forces are electrostatic, and thermal (kinetic) energy keeps the molecules in motion. It all balances out.

I also thought of it in this this way. If the surface is under tension then there could be a situation in which the molecules are unable to move down individually as the whole surface is being pulled down like a ball bouncing from a floor packed with balls.
Is that a right analogy?

 

[haruspex]

I also thought of it in this this way. If the surface is under tension then there could be a situation in which the molecules are unable to move down individually as the whole surface is being pulled down like a ball bouncing from a floor packed with balls.
Is that a right analogy?

No, these are molecules of a fluid. They can slip past each without friction, and they are constantly on the move. Even at 0c water molecules move at about 500m/s.

 

[Vivek98phyboy]

No, these are molecules of a fluid. They can slip past each without friction, and they are constantly on the move. Even at 0c water molecules move at about 500m/s.

So the surface molecules often swapping their positions with the one moving to the surface experiencing stress due to attraction along the surface would be the right way to think?

 

[haruspex]

So the surface molecules often swapping their positions with the one moving to the surface experiencing stress due to attraction along the surface would be the right way to think?

That sounds more reasonable, but I don't think I have a really convincing explanation. Best I can do is what I wrote in post #2.