Surface Energy or Surface Tension

[turtleman123]

If you spill your coffee on a flat surface and let it dry, explain why the (dried)

coffee stain is always darker at the rim of the spill than in the center (you can also

add illustrations if you like).

[eric l]

Very good question ! I tried to make some observations, but it is like watching paint dry.

 

Anyway, I tried to compare the situations where "drying" is evaporation exclusively (coffee spilled on a glass plate), or penetration into a support (an unglazed ceramic surface) or a combination of both.

 

In the case of evaporation exclusively, one could think that as the amount of water in the coffee decreases, the size of the "pool" would decrease too (because of surface tension), but this is not the case.

It is not the case because the coffee "wets" the glass plate : it spreads out rather than standing out in droplets like mercury on a glass plate would do. The surface tension (actually interfacial tension on the coffee/air - interface) is low and will get (probably) even lower as the concentration increases by evaporation. This would mean that it will dry up in the middle, and as the dry zone in the middle becomes larger the concentration at the rim will increase. But as stated, trying to observe it is like watching paint dry.

 

In the case of penetration into the support, the stain seems to be evenly coloured, no noticeble difference between the rim and the middle - but I did not make any actual measurements, just a visual appreciation.

[Jay-qu]

Have you ever heard of chromotography? perhaps it has something to do with the solutes affinity towards the liquid and stationary phases. Google "paper chromotography" :wave:

[hallenrm]

I would say it is because of diffusion, the molecules of the solute 'coffee' that is dissolven/suspended in water move together to the drier areas because of diffusion; the water molecules being lighter move faster. Soon the water molecules on the fabric are evaporated away (due to ambient heat) and the solute molecules at the frontier, have no solvent molecules to carry them forward, hence they accumulate.

 

That explains, or I believe it to be so! :wave:

[moo]

Hmmmm... a single droplet is dome shaped. Does a larger "surface puddle" have a microscopic slope toward the edges?

 

If so, perhaps gravity pulls floating molecules (coffee or whatever) toward the sloped sides...

 

Just a thought. :)

 

{EDIT}

Found a similar discussion on the subject (wine stain):

 

http://cr4.globalspec.com/blogentry/103/Stain-Newsletter-Challenge-06-20-06

 

Excerpts: "The ring appears because evaporation at the edge is faster than even a short distance in from the edge, where the surface is flat. This causes flow towards the edge. The higher evaporation at the edge is described by Thomson's law, which says basically that equilibrium vapour pressure increases as the radius of curvature of the liquid surface decreases (as viewed horizontally in this case). It's also a function of surface tension."

---

"Good, but only part of the answer. As the evaporation proceeds from the edge, salt (or sugar),is deposited. The higher concentration of solute draws more solution to the outer 'ring` thus the depost is thicker at the edge of the drop."

 

moo