Surface tension again?

Question

A molecule in liquid is attracted by molecules from every direction (due to cohesive force)-- so there is no net force on it. But the surface molecules of liquid feel some net downward force coz their position (there is no liquid molecules upward to exert cohesive force).
When something feel a net force it is forced to move to direction of the force. But as we know the surface molecules normally does'nt exchange their position with lower surface molecules---
Now my question is what is the other force that balance the net downward force of surface molecules?
Thx.

Answer 1

When two molecules get close enough together, the attraction turns to repulsion. So it's not an ongoing pull, it's a pull to a specific distance; after which, it's neutral.

This is sometimes modeled using the Lennard-Jones potential. See the reference.

The question isn't really different than asking why a child holding onto a teddy-bear doesn't need a balancing force. There's no problem: the pressure pushes back.

Answer 2

I'm not sure what you are asking? Are you asking which forces keep the surface molecules from exchanging position with the lower surface molecules?
I would say that although you have intermolecular cohesive forces holding the molecules together, you also have forces keeping the molecules apart. To have the surface molecules switch places with the lower molecules, you would have to push the surface molecule in between other lower molecules. The forces between the molecules would make this hard. I am imagining a swimming pool full of tennis balls (no water). Then think of putting a sheet of plywood over the tennis balls and piling weight on the plywood. Are the balls on top going to switch places with the ones below? Probably not.

Sorry, this is not my specialty, so I don't know the name of the force. But I also spend a lot of time pressuring systems with water, and as you know, water is practically incompressible. So the forces that keep the molecules apart must pretty good.

Can you do a free body diagram? Many times I find this helpful. Draw a FBD around around just one surface molecule and think about all the forces, including gravity.


Hope this helps

PS -- A child holding a teddy bear does have a balancing force. It is the force of the bear against the child's arms and chest, which keeps the bear from falling.

Answer 3

All of the molecules at the surface are subject to an inward force of molecular attraction which can be balanced only by the resistance of the liquid to compression. Thus the liquid squeezes itself together until it has the locally lowest surface area possible.

Consider a soap bubble with air inside it. the the liquid surface presses the air inside. The air pressure keeps the surface at centrain distance from the center of the sphere.

When air is removed slowly, the surface shrinks further.

The resistance offered is the answer to your question.

I hope you got the answer.

http://en.wikipedia.org/wiki/Surface_tension

Answer 4

There is a negligibly small shift in position dear......
And hence there is a work is done .So the molecules on the liquid surface have greater potential energy.For a system to be stable,it's potential energy must be minimum.ie,A liquid will have minimum potential energy ,if the no. of molecules on the liquid surface is least.This will happen if the surface area is minimum.Thus the liquid tries to have minimum surface area or a liquid surface tries to contract resulting in Surface Tension.

Answer 5

just thinking of it logically, its probably a interaction with the molecule below it (i.e. a normal force or a repelling force for getting too close) as well as an elastic effect from the molecules in the same plane (like pulling a horizontal cloth down from the center, there is a resistance)