Capillary forces, intermolecular forces and surface tension questions

In summary: Question #1: as shown in the zoomed-in red square, what is making the top water molecule move up? The water molecule at the top is being pulled up by the cohesive forces of the water molecules around it.
  • #1
Pete2s
6
0
I tried posting a similar question before, however, I've found the 1 reply is actually wrong. Hopefully with better illustrations, someone will know.

The situation in the picture is a drop of water entering a capillary tube. It is located here: http://img207.imageshack.us/my.php?image=capillaryaction2yi2.png

Correct me if I'm wrong, but the drop is drawn into the tube by the adhesive forces of the water to the capillary tube. Once the entire drop is drawn in, the forces equal out and the water stops moving up: the bottom water molecules are pulling down with the same force as the top is pulling up.

Question #1: as shown in the zoomed-in red square, what is making the top water molecule move up? If it isn't moving up, how are other water molecules leap frogging it and pulling more water up with them?

In the bottom diagrams, we have cone-shaped capillary tubes. I know that the capillary pressure is directly related to the radius of the capillary tube.

Question #2: It's my understanding that the water would move up to the top in the cone capillary tube because of the pressure difference. But why is there more pressure when the radius becomes smaller? Why would a water molecule that's stuck to the side at the bottom in the 3rd tube want to move up? It can't be the cohesive forces of the water pulling it up--the adhesive forces are stronger than the cohesive forces, that's why we have the capillary action.

Thanks for any help.
 
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  • #2
Should I post this in the advanced forum? Does anyone know someone who might be able to answer?
 
  • #3
One conceptual problem here is mixing continuum concepts (interfaces, wetting) with atomic concepts (molecules 'leapfrogging').

There has been some work on molecular modeling of wetting (Joel Koplik), but don't concern yourself with that yet.

The curved interface represents a pressure jump across the interface, and the contact angle represents a force balance between the two fluids and the two fluid-solid interfaces. Don't worry about what individual molecules are doing, it's not relevant.
 

1. How do capillary forces work?

Capillary forces are the result of the attraction between molecules at the surface of a liquid and the molecules of the material that the liquid is in contact with. This creates a 'meniscus' or curved surface at the boundary between the liquid and the material, which is caused by the differences in intermolecular forces between the two substances.

2. What are intermolecular forces?

Intermolecular forces are the attractive and repulsive forces that exist between molecules. These forces are responsible for the physical properties of substances, such as their boiling point, melting point, and viscosity.

3. How does surface tension work?

Surface tension is the result of cohesive forces between molecules at the surface of a liquid. These forces pull the molecules at the surface inward, creating a 'skin' on the surface of the liquid, which allows it to resist external forces and form droplets.

4. How does surface tension affect the behavior of liquids?

Surface tension affects the behavior of liquids by making them form droplets and creating a barrier that allows insects to walk on water. It also allows liquids to 'climb' up narrow tubes through capillary action, and can be affected by factors such as temperature and the presence of surfactants.

5. How are capillary forces, intermolecular forces, and surface tension related?

Capillary forces, intermolecular forces, and surface tension are all related to the interactions between molecules. Capillary forces are the result of intermolecular forces at the surface of a liquid, and surface tension is caused by cohesive intermolecular forces. All three concepts are important in understanding the behavior of liquids and their interactions with other materials.

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