Expected Results and Explanations
The coffee ring effect is due to the coloured particles suspended in the water. As it evaporates, the water draws the particles towards the edge where the liquid touches the dish. This makes the outer ring darker, and leaves the centre a lighter shade.
If the liquid has a uniform dark colour without suspended particles, then there should be no coffee ring effect.
If particles emerge from solution as the liquid dries, then we expect a ring. Salt is an example where rings should result even though the crystals had dissolved completely.
With the right amount of dish soap, there should be much less surface tension. The first result is that the liquid spreads out over a wider surface. This should also diminish the coffee ring effect.
My tests seem to have been skimpy with the dish soap; I was expecting more of a difference. Only the soya really changed, and was very surprising. It had no visible ring effect without soap, but a ring effect with soap.
Practical Applications from Coffee Ring Experiments
Normally, when we spray or paint a coating onto a surface, we prefer to have a uniform finish. The coffee ring effect makes this difficult whenever there are small droplets on a surface.
Suppose the manufacturer could control the geometry of the particles, and thus make a uniform coating with fewer suspended particles. This should create a better finished effect at lower cost.
How was Your Home Lab Experiment?
Let us know what you tested and how it turned out. Did you make coffee rings? Did soap suppress the coffee rings? Did you find anything that left no rings?
Yunker, Peter J.; Still, Tim; Lohr, Matthew A.; and Yodh, A.G. “Suppression of the coffee-ring effect by shape-dependent capillary interactions“. Nature. Published online Aug. 17, 2011. Accessed Sept. 1, 2011.
Decoding Science. One article at a time.