A drop of coffee dries and leaves a dark ring at the rim with an almost empty middle. If the liquid evaporated evenly, the particles would stay evenly spread. What moves them to the edge?
A current that only . happens when the edge of the drop catches on a tiny roughness and does not retreat as it dries. Water near the thin edge, so liquid from the middle flows outward to replace it. The flow carries the coffee particles with it and leaves them at the perimeter.
A drying coffee stain leaves a dark ring at the rim and a pale middle, which looks like the opposite of what a contracting liquid should do.
The first condition is that the edge of the drop catches on a tiny roughness and does not retreat. Had it retreated, the stain would be filled.
The second condition is that evaporation is faster at the thin edge, because the vapour there disperses in every direction while in the middle it is blocked by itself.
Together these produce a current: the water lost at the rim arrives from the middle, and the current carries the coffee particles and leaves them at the perimeter. The same effect harms printing, coatings and biological samples, where a layer has to come out , which is why people try to disrupt it with surfactants, with particle shape and with heating.
wat. A coffee ring forms when the edge of the drop stays pinned and flow from the middle replaces water evaporating at the rim, sweeping particles there.
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A small kitchen stain reveals an engineering problem: evaporation alone can sort material and ruin a layer that is supposed to be uniform.
How do we know?
The researchers filmed drying drops, tracked particles inside them and built a model connecting the shape of the drop, the rate of evaporation and the flow to the edge.
What is still unknown
Surfactants, particle shape, heating and other internal motion can weaken the ring or prevent it.
Learn something?
Where to next
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