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Hi! I have another question relating to the phenomenon that can be demonstrated in straws or small tubes (being able to contain liquids in those tubes by using "vacuum" and capillary forces). Here it is: In this example, lets imagine that a person has a regular straw. But, that straw is stretched outward in the middle to create a larger tube-like "compartment". Basically, the diameter of the straw is small at the ends but is much larger in the middle. Now for the question: Would such a change in the straw's shape or ability to contain more fluid prevent it from holding in that increased amount of fluid thru the use of capillary forces and a "vacuum" provided by putting one's finger on the top end of the straw? In other, simpler words: Would the straw still be able to hold in, lets say, water, if the amount of water was to be increased substantially due to the introduction of the larger, middle diameter, or compartment, of the straw? Furthermore, how could one determine the maximum amount of water weight that it could hold using capillary forces and vacuum alone in a situation like this? How much could the diameter be increased in the middle, how big could the compartment get, while still allowing the weight of the fluid to be supported by the capillary forces and the pressure of the air outside of the straw? Thanks for your help!
Question Date: 2009-09-28
Answer 1:

The capillary force is responsible for pulling the water up the inside of the straw, while the gravitational force wants the pull the liquid down. The radius of the straw plays a big role in determining the amount of water that is inside the straw. There is an equation which tells you the height of this water column depending on a few variables, such as the density of the liquid, the amount of attraction between the liquid and the surface of the straw, as well as the radius. The height is inversely proportional to the radius, meaning that if you double the width of the straw, the height decreases by a factor of two but the volume of the water column doubles. However, if you increase the width of the straw too much, the height becomes very difficult to measure. For example, if the radius is 1 meter, then the height of the water column would be only 14 microns (for radius of 1 millimeter, then the height is 14 millimeters).

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