The Theory
A coffee stain is simply a stain left behind after coffee has been spilt on something then allowed to dry. However, coffee stains also have one very interesting property. Instead of forming a circular brown blotch as one may expect, they in fact form a ring-shaped stain or "coffee ring"! The process through which this happens is actually very intriguing, yet also surprisingly simple. Firstly, consider this extremely rough cross section of a coffee spill:

   /                      \   

As you can see with a bit of imagination, surface tension allows a coffee spill to retain a shape similar to that above (albeit with more curved sides). Also, if you look at this cross-section, you will notice that because of the sides being as they are, the coffee stain actually has a much greater surface area around the edges than just on the flat top.

As a result of this, the coffee will naturally evaporate into the air at a faster rate in the corners. Then, as the coffee rapidly evaporates around the edges, the coffee maintains its shape due to an effect referred to as pinning, and instead of shrinking in area, it shrinks in height, with coffee from the centre of the spill flowing towards the outer areas of the spill, allowing the process to repeat itself. Therefore, what you have is a lot of coffee evaporating in the outer portions of the spill, and not much in the centre. As the coffee around the edges evaporates, it is actually just the water portion of the coffee evaporating. The actual coffee particles suspended in the water remain behind, dropped around the edges of the spill once their water has gone. Thus, the coffee ring is formed, being the large concentration of coffee particles left behind by the evaporating water.

The Experiment
This effect can actually be proven in an experiment. All you need is:
- 1 coffee spill (you can easily make your own)
- 100 very very very small polystyrene balls (might not be too easy to find)

All you need to then do is distribute these tiny balls throughout the coffee. Then, as the coffee dries, you will be able to see that they migrate towards the edges of the stain, where they then deposit themselves on the surface that you spilt the coffee on. This is exactly the same as what happens with coffee particles, being drawn to the outside of the spill, then dumped to form the coffee ring.

So the next time you spill coffee, and someone yells at you to clean it up, you now have the right to claim it is a scientific experiment, and have the knowledge to back up the claim.

The Cleanup
Okay, so now that you've finished with your "experiment", you have one beautiful coffee ring. However, some people have no appreciation of science, and will start to demand that you remove the stain from your desk, so here's how:
1. Mix up a paste of bicarbonate of soda and water.
2. Apply this paste to the stain
3. Leave for about 15 minutes
4. Wipe off

There you have it, now nobody knows you're a closet scientist, and you won't get fired for damaging office furniture. Plus, you'll know to be more careful with your coffee next time.

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