# Can capillary forces act upside down?

• philip041
In summary, the conversation discusses the concept of capillary action and its effects on water in different scenarios. It is mentioned that capillary forces can draw water into a straw, but surface tension may prevent it from falling out if the drop is too large or the straw is too short. However, it is also noted that the outcome depends on the fluid properties and dimensions of the capillary.
philip041
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 won't fall out the bottom because there is still some drop left at the top, so no air can get into tube?

Cheers

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).

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 won't 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.

## 1. Can capillary forces act against gravity?

Yes, capillary forces can act against gravity. These forces arise due to the surface tension of liquids and can cause liquids to rise or fall in narrow tubes, even against the force of gravity.

## 2. How do capillary forces work?

Capillary forces occur due to the attraction between the surface molecules of a liquid and the surface of a solid material. This attraction creates a meniscus, or curved surface, which can cause liquids to rise or fall in small tubes.

## 3. Can capillary forces be used for applications?

Yes, capillary forces have several practical applications. They are often used in microfluidic devices, such as lab-on-a-chip technologies, for precise control and manipulation of small volumes of liquids. They are also important in the movement of water and nutrients in plants and trees.

## 4. Is there a limit to how strong capillary forces can be?

Yes, there is a limit to the strength of capillary forces. This limit is determined by the surface tension of the liquid and the contact angle between the liquid and the surface of the material. As the contact angle increases, the capillary forces become weaker.

## 5. Can capillary forces act upside down?

Yes, capillary forces can act upside down. These forces are not dependent on the orientation of the liquid or the material, so they can still occur even when the liquid is upside down. However, the strength of the forces may be affected by the change in orientation.

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