Talk:Capillary action
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Article Grading: The following comments were left by the quality and importance raters: (edit ยท refresh) Rated "top" as high school/SAT biology content and important biophysical concept, e.g. water conduction in plant cells. The article could need some expansion with biological examples and context. - tameeria 05:10, 19 February 2007 (UTC) |
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"A natural mechanism whereby a liquid is moved along a substrate. Often referred to as wicking, as in molten wax moving by capillary action up the candle wick. I suppose that trees move sap through capillary action. blood samples are sometimes taken by touching a capillary tube made of glass to a pool of blood on the fingertip after a prick. How does it work though? That's what I came here to find out, and I had to make the entry myself! I don't know, but I wonder if it has something to do with surface tension. Maybe the water molecules are attracted to the sides of the capillary tube. Successive H20 molecules leapfrog eachother up the sides of the tube. Each higher molecule is happy to have found some tube wall it can call it's own. Then surface tension brings up the column a bit more, then another molecule finds the wall a bit higher and so on? It seems like quite a thermodynamics problem though: What is providing the power to lift the column of water? Where is that energy coming from?"
The above text was cut and pasted from the article page. So far the questions raised by the above user have not all been addressed on the article Theresa knott 14:18 May 13, 2003 (UTC)
- I can't answer the question of what's happening on a molecular scale, though the article on adhesion may help. I can give some physical reasoning, though. In any case, it's likely the result of electrical attraction between two molecules. Perhaps each molecule has at any particular time a positively charged region and a negatively charged region, causing the attraction (Van der Waals forces; perhaps it's a more permanent distribution of charge like for polar molecules or ions. In any case, I think it's possible to answer the conservation of energy question by considering a positively charged particle. The total potential energy for this particle will increase if the particle is raised (because of gravity) and it will decrease if the particle is moved towards negative charges. In other words, the work needed for the positively-charged particle going up is positive because you'd have to push it, and the work needed for the particle going towards a negatively charged region is negative because opposites attract. So for a particle to be attached sideways to a surface, the electrical and gravitational potential energies must cancel out, which is certainly possible. So the short answer to the conservation of energy question is that gravitational potential energy gets transferred into the electrical potential energy in between the atoms, so energy is conserved.
- Why do molecules seem to prefer the electrical energy to the gravitational energy? In this case we have to consider forces (derivatives of potential energy) rather than the potential energies themselves. Forces occur in regions where potential energy changes. Consider a thin vertical tube placed just above the surface of the water, at y=0. The gravitational force is d/dy (mgy) = mg, which is true everywhere along the tube. The electrical potential energy is constant from y=b on up at U_e=-C along the inside of the surface of the tube, but there is a sharp drop from 0 at y=0 to -C at y=a. Somewhere between y=0 and y=b the slope of the electrical potential energy will be opposite the force of gravity; we'll call this place a. (See the mean-value theorem). A single particle of water starting at y=0 under these conditions would accelerate upwards until y=a, when the forces cancel out. After y=a the gravitational force wins out, and before y=a the electrical force is stronger. Now, if we were only talking about a single particle, there'd be no drag, so you'd see a particle oscillating about y=a. However, we're talking about a fluid, so instead of a force accelerating a particle, the force pulls a fluid at a constant velocity (with the missing kinetic energy of the accelerating particle being converted into heat).
- One more factor we have to consider is that when a bunch of positively charged particles congregate in an area of negative P.E., the P.E. then rises because the region has become more electrically neutral. This is called saturation. This causes the point at which the forces cancel out to rise a little bit (draw a curve of the P.E. before and after saturation at y=a to see what I mean.) Then the liquid rises to the new y=a, which then gets saturated, and so on. This can't continue forever, though. At some point y=a_f the P.E. function gets stretched out enough along y such that the only way to get a slope great enough to counter the force of gravity is to have a constant slope from y=0 to y=a. (Or you could simply run out of water.) For any height greater than y=a_f the average slope would be too small. 66.189.116.168 21:33, 4 September 2006 (UTC) John S.
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[edit] lift of 3m? not likely!
I find it hard to believe that a 0.1 mm tube could lift water to 3 m. It also appears to disagree with the equation. In addition, at the size, surface imperfections would take over (I would guess) and the theoretical maximum would not be realized. Does somebody have data on this? --Pdbailey 02:04, 23 September 2005 (UTC)
it's plausible. Trees use capilary action to draw water up to their branches, and some trees are dozens of meters tall. --Crucible Guardian 02:00, 13 July 2006 (UTC)
[edit] Merge?
Is capillary action the same thing as Capillarity? And if so, should they be merged? --liquidGhoul 06:59, 16 June 2006 (UTC)
- I agree - Capillarity should become a redirect to Capillary action, as Capillary action is the more widely used term. The Capillarity page seems to add no further information. Ozhiker 11:46, 9 August 2006 (UTC)
[edit] Is gravity necessary?
I thought that capillary action occurred with or without gravity, so shouldn't the definition on the top of the page be changed? 66.189.116.168 19:55, 4 September 2006 (UTC) John S.
[edit] Unreferenced
moved this tage here - because the page has been idle for ages, and this tag is ugly!....
cheers, Petesmiles 00:13, 29 December 2006 (UTC)
