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Name: Anu
Status: educator
Grade: 9-12
Country: USA
Date: Fall 2012


Question:
I have a question on the surface tension of water. When you overfill a glass you see a convex meniscus. Exactly why? My understanding is that the water molecules at the surface experience lateral and downward forces. Therefore the molecules on the surface move closer together laterally as well as push down on the water. I understand why drops/spheres form when the water is in free fall (rain). But when it is in a container, what is happening? Why does the container cause a convex shape? Why not just a straight film above the rim of the glass? Why the curve down at the edges? I am trying to understand what forces are causing this. I have searched a searched but cannot find my answer on the Internet. Either they gloss over it or they are so detailed I cannot understand. I understand the concave meniscus in glass test tube. (adhesion of glass and cohesion of water) I just am not understanding the convex meniscus of an "over full" container. I do not think the container is involved at all in the answer. And the more water you add the more convex the meniscus becomes before bursting instead of just a thicker straight film above the rim.? I hope I conveyed my confusion...

Replies:
Hi Anu,

Q. When you overfill a glass you see a convex meniscus...the water molecules at the surface experience lateral and downward forces. Therefore the molecules on the surface move closer together laterally as well as push down on the water... I understand the concave meniscus in glass test tube. (adhesion of glass and cohesion of water) I just am not understanding the convex meniscus of an "over full" container.

A. As you know, the meniscus can show the interaction between the surface of the container and the (liquid) molecules. For mercury, the attraction between mercury molecules (cohesion) are too strong so you can see a convex of mercury liquids in the container. For water, the attraction between water molecules are weaker than that between water molecule and the surface of the container (adhesion).

However, once you slightly overfill the water in a container, there is no further attraction between the surface of the container and the water molecules, as "extra" water molecules cannot interact with the surface directly. Instead, they are experiencing attraction forces between themselves, which will increase cohesion force and cause more convex. In other words, water molecules want to wet the container surface, but once it's overfilled, there's no surface to wet over the top. So, the molecules can interact with the edge (or the top) of the container, which causes the convex of the liquid.

From Weonkyu Kho, Los Alamos.


Since you understand how attraction to the glass 'pulls' the water up the walls (cohesion/adhesion) of a tube slightly (the meniscus), to understand the over full glass, just think of the water being in container made of air. Water is strongly attracted to itself, but water is not strongly attracted to air. Thus, it will try to maximize contact with itself (a perfect sphere maximizes water-water contact while minimizing water-air contact -- that is why droplets form). But, in the case of an over full glass, the water is subject to gravity. So the best it can do is to form a bulging-up meniscus -- the size of the bulge reflects the balance between gravitational forces and cohesive forces. If you had less gravity, or higher cohesion, you would have a higher meniscus. With more gravity, or less cohesion, it would be flatter (try it with mineral spirits).

Hope this helps, Burr Zimmerman


Anu,

Your question is reasonable and shows good insight. Water tends to stick to many surfaces, and it will crawl up some surfaces in a process called “capillary action.” You can see this happen with a paper towel when you place it in water. The water will slowly crawl up the towel and wet it. A similar process happens with your glass. It is true that water will stick to itself, and this effect creates surface tension. It is like the water molecules are grabbing and pulling on one another. They will also stick to the walls of the glass. You will have in this case the water molecules pulling on each other and holding onto the walls of the glass at the same time. The water will creep up the sides of the glass until all the forces are balanced—the force pulling up on the water from the glass (water sticking to the wall) vs that of pulling down (water pulling on itself + weight of water). This balance creates the meniscus you see in a partially-empty glass. When you overfill the glass, you have the same forces working, but they have shifted somewhat. The water molecules still pull on the sides of the glass, but now they are pulling down. The water molecules also pull on one another, creating the surface tension. The result is a rounded surface bulging out. You will be able to continue to overfill the glass until the forces unbalance, and the water will spill.

Kyle Bunch, PhD, PE


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