5 replies to this topic

[Works4TheGood]

Have you folks noticed this?  As your test tube setups begin to lose water, either your cotton ball begins to migrate towards the back of the tube or an air pocket begins to grow behind the cotton until the tube is eventually empty of water.  These behaviors make sense given the physics at work, but what I want to do is control which behavior I experience.  Is the behavior a result of how tightly the cotton is packed into the tube?  Is it random?  Do tapered tubes experience cotton-migration to a lesser extent?  Rather than experiment, I was hoping someone could just tell me when to expect which behavior.

 

Thanks for reading!

[Reacker]

I did some testing with this years ago and I found that the difference seems to be in if there is any air pocket in the tube once plug the water with cotton. If the water chamber has air in it from the start, then the air pocket expands until the tube empties. If the tube has no air pocket then the cotton is pulled back. 

 

Oh and I think the cotton had to be very tightly packed as well for the cotton to pull back. 

Edited by Reacker, October 9 2017 - 6:17 PM.

[dspdrew]

I did some testing with this years ago and I found that the difference seems to be in if there is any air pocket in the tube once plug the water with cotton. If the water chamber has air in it from the start, then the air pocket expands until the tube empties. If the tube has no air pocket then the cotton is pulled back. 

 

Oh and I think the cotton had to be very tightly packed as well for the cotton to pull back. 

 

From what I have seen, tightly packed cotton doesn't move. It's the loosely packed cotton that moves.

[Reacker]

Well it has been 11 years since I did the tests, I'm probably misremembering. 

[sgheaton]

Cotton would allow air flow...

[Works4TheGood]

Ok, so I simply pack the cotton tightly if I don't want the cotton to move as the water is removed. Got it! Thanks