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| Hi! Thank you for your help on previous questions. I am still wondering about the phenomenon that is created in straws when one puts his/her finger on the top, and how those properties may be used in different applications. I now know that the diameter of a tube needs to be relatively small to allow the capillary forces/air to keep the liquid inside. Based on that fact, if I were to have, lets say, a solid cylinder, which would be allowed to rotate within a partial enclosure (think of a rectangular-type thing that would go around the cylinder across the middle of the cylinder, across the entire cylinder) The clearance between the cylinder and that enclosure would be very small, as the diameter of a straw is kept small. The clearance, lets say, is 1mm. Now lets say you add some sort of compartment to one side of the enclosure and fill it with water (SO ONE SIDE OF THE CYLENDER IS IN WATER AND THE OTHER IS NOT) Now you put your hand on the open side of the compartment (like you do on the top of a straw to keep the liquid inside) Would the liquid be kept inside, in other words, would the liquid be prevented from flowing thru the 1mm clearance between the cylinder and the enclosure due to the small clearance (much like the small diameter of a straw - to allow the capillary forces to work) and the "vacuum" provided by a person putting their hand on the other end of the compartment? Thanks for your help, it is sort of hard to explain, I wish I could provide an illustration, but I really need your help in figuring out if this application would work. Thanks!
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| Question Date: 2009-09-23 | | Answer 1:
Water can squirt out of a 1mm hole if there is pressure driving (think of a plastic produce bag full of water with a tiny hole in the side). If it would create vacuum by doing so, then it will stay in the container so long as the pressure that would push the water out is lower than the air pressure keeping it in. I don't know how small the hole can be before surface tension breaks down - that would be in the Handbook of Chemistry and Physics, I expect. I don't know how the shape of the hole affects it though - it sounds like you've got a 1mm-wide crack rather than a 1mm circular aperture, and things like that leak in my experience (e.g. trying to hold water in your cupped hands doesn't hold water for too long).
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