Can capillary forces act upside down?

philip041 · Apr 24, 2009

This is fairly amateur but I'm not an engineer so:

If you have a drop of water on a table and you stick a straw in it some of the water goes up the straw until gravitational force equals the capillary force.

Am I right in saying, (generally):

1) If you have a small drop of water hanging from a ceiling and stick a long straw in it the water starts going in it will keep going until there is no drop left and keep going until it reaches the end of the straw and then fall out the bottom.

2) If you have a large drop with a short straw capillary forces will draw water in but once water reaches the end of the straw it wont fall out the bottom because there is still some drop left at the top, so no air can get into tube?

Cheers

Danger · Apr 24, 2009

Interesting question. My only experience with capillary action was that we relied upon it to suck a thin adhesive between pieces of acrylic for manufacturing purposes. That was a little different than a tube. Regardless of orientation, the liquid would go only to the edge of the material and stop (unless way to much was applied, in which case it would spill out).

ank_gl · Apr 24, 2009

philip041 said: ↑

Am I right in saying, (generally):

1) If you have a small drop of water hanging from a ceiling and stick a long straw in it the water starts going in it will keep going until there is no drop left and keep going until it reaches the end of the straw and then fall out the bottom.

it may or may not fall, surface tension might just hold it in equilibrium. it depends on fluid properties & the capillary dimensions.

philip041 said: ↑

2) If you have a large drop with a short straw capillary forces will draw water in but once water reaches the end of the straw it wont fall out the bottom because there is still some drop left at the top, so no air can get into tube?

No, it ll continue falling down, until condition (1) is reached(small drop left), vacuum isn't holding the fluid in the capillary, surface tension is.

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Can capillary forces act upside down?

philip041

Danger

ank_gl

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