How to draw the vectors for cohesive forces and adhesive forces?
July 8, 2013 7:17 PM   Subscribe

How does the adhesive force between the water and the material of the capillary tube exert an upwards force to create the concave meniscus shape of H2O (or the convex meniscus shape of Hg)?

There's something really nonintuitive about capillary action for me. I understand the intermolecular forces attracting the water molecules to each other from all sides, but what I don't understand is how the adhesive force between the water and the side of the capillary tube pulls UP on water molecules, or down on mercury molecules. Isn't the adhesive force vector side to side, and gravity downwards?

Also, my test prep book says the following:
"If the cohesive forces are stronger, a convex meniscus forms and the fluid is pulled downward by the vertical component of the surface tension."
"If the adhesive forces are stronger, a concave meniscus forms and the fluid is pulled upward by the vertical component of the surface tension."

Please help me fix my force diagram drawing! As a visual-spatial learner physics is usually my jam, but this concept is just not clicking.
posted by sunnychef88 to Science & Nature (3 answers total)
Response by poster: Shoddy mspaint drawing of the forces:
posted by sunnychef88 at 7:18 PM on July 8, 2013

I find capillary action nonintuitive also, so I may be confused here, but I think the force that your diagram is missing is the net intermolecular forces on a molecule of water on a curved surface. This will try to pull water into a convex surface (because there's more water below), or pull it out of a concave surface (because there's more water above)— either way, it'll act to smooth the surface (make it flatter). Or in other words, it is energetically unfavorable for the surface of the water to be sharply curved instead of smoothly curved. If the water is in a nonwetting tube, this means it will gradually curve away from the side of the tube instead of making a 90° angle. If the water is wetting the side of the tube, then the tube wall counts as 'inside' the volume, so the water will still gradually curve but in the other direction, so that it smoothly meets the wall of the tube. (If you magically changed the material so that the tube was non-wetting, the water would have a sharply angled peak at the edge which it would immediately pull into itself.)
posted by hattifattener at 7:31 PM on July 8, 2013

Best answer: Mercury bonds stronger to itself than to the glass, so Brownian motion will lead to an aggregate state where contact with the glass is minimized (gravity counteracts this, preventing a tall, thin cylinder from forming).

Water is the opposite, and so random collisions will naturally maximize interactions with the glass. This leads to a concave shape, again counteracted by gravity. Water isn't very heavy though, so interactions with the glass tend to dominate, pulling the liquid up through the tube (counter-intuitively, gravity does help raise the meniscus).

tl;dr: The meniscus isn't due to a single adhesion "force", so much as it's an emergent behaviour of random molecules in the liquid seeking strong intermolecular bonds.
posted by Orange Pamplemousse at 9:16 PM on July 8, 2013 [2 favorites]

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