Comments on: Coffee rings caused by capillary action?

By: The Coffee Makers

The Coffee Makers — Mon, 31 Dec 2007 12:44:24 +0000

Thanks for the in depth comments, I’ve often wondered, but never found an answer until now.

Thanks again.

By: Kelson

Kelson — Thu, 11 May 2006 20:25:23 +0000

Thanks for the explanation!

By: Brooks Moses

Brooks Moses — Thu, 11 May 2006 20:05:23 +0000

Yup, that’s definitely capillary action, or at least it’s caused by the same physical reasons.

Essentially, what’s happening is that the water is in a lower-energy state when it’s against certain kinds of solid surfaces (such as glass and ceramic) than when it’s in the middle of the drop. So, if you stick a glass tube in some water, there’s this balance between the water wanting to creep up the glass so more of it is against the surface, and gravity wanting to pull it back down, and it ends up in a position where those two things balance out.

In the coffee-ring situation, gravity doesn’t balance it, so it just keeps going until it’s covered as much solid surface area as it can. (What eventually balances it there is the fact that water-air surface has higher energy, and so it doesn’t do things that increase that very much. That’s why it only wets the area where the coffee cup is nearly touching the table.)

There’s a related common experiment, where you put a drop of water on a clean glass surface, and a drop on a waxed surface. The water-glass surface is a low-energy state, and so the drop spreads out to get a lot of it. The water-wax surface is a high-energy state, and so the drop beads up to touch very little of it.

By: Kelson

Kelson — Wed, 25 Jan 2006 00:22:54 +0000

Should’ve gone with my first reaction. Chalk that one up to poorly-worded articles on the Internet and a fuzzy memory of old high school science classes.

By: Me

Me — Tue, 24 Jan 2006 11:38:46 +0000

“capillary action specifically refers to fluid moving against gravity”… eh? A capillary is just a small channel – capillary action could be in any situation. The difference between the lesser surface energy of the mug and the coffee and the greater surface energy of the table and the coffee will be enough to cause the coffee to minimise the energy and flow around the edge of the mug, under gravity.

By: Daniel

Daniel — Tue, 10 Jan 2006 00:12:08 +0000

The mug exerts a stronger gravitational force than the air particles near the surface of the table?

Or… most cups and mugs taper inward, so by the time the drop has raced to the bottom it has centripetal momentum?