MadSci Network: Physics |
Dear Pio, I looked up capillarity in my trusty Physics book and found an equation describing capillary action. (Physics for Scientists and Engineers, 2nd edition by Douglas Giancoli pg. 302). The equation is as follows: h=(2*y*cos(phi))/pgr where: h= height of capillary action in a tube y= surface tension of the liquid phi= angle of contact of liquid p= density of liquid g= gravitational constant r= radius of capillary tube Now for water contacting a glass tube, phi is essentially = 0, so the equation becomes: h=(2y/pgr) Now for your problem you don't really need the numbers for these, but I will give them anyway in an example from the book. Say the xylem (nutrient carrying tube in a plant) has a diameter of .001 cm, how high can capillary action pull water at sea level? At sea level g=9.81 m/s^2 and we will assume the xylem acts like glass with phi = 0. Surface tension of water is given as y = .072 N/m. Density of water is 1000 kg/m^3. Making sure we have all of the units right, our equation becomes: h = (2*.072N/m)/(1000kg/m^3 * 9.8m/s^2 * .00001m) = 1.5m is how high capillary action will pull the water. Now you don't have to understand all of that, it is just an example in case someone who looks at this happens to need it. No, in your case it is a case of mathematical limits. From the second equation h=(2y/pgr), if everything stays the same except the gravity "g", then we get a limit equation. As g approaches zero (as in space), that means h will approach infinity. So, if we have the right set of circumstances, capillary action would work indefinitely given the right set up. The right set of circumstances is important. In space, there would be no reason for the water to head up a tube instead of having the surface tension bond with itself and float off as a blob. So it isn't like you could just have an aquarium full of water on the Space Shuttle and stick a glass tube in it. There would have to be some containment bag, kinda like those silver Mylar grape juice cartons that come in some lunches. Also, you would still need air pressure since water will evaporate at zero pressure regardless of temperature. Lastly, you couldn't get any air bubbles in the tube since the surface tension on one end of the column of water would counteract the effect on the other end. However, if all those criteria are met, I can't see why capillary action wouldn't move in a tube for a very long distance in space. Hope this explanation helps a little. I looked to see if NASA had anything on this but was unsuccessful in finding any. Maybe you can suggest it as an experiment! Anyway, if you have any other questions, please contact Mad Scientist and they can forward it too me. Best of luck! BK
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