When you spill coffee, why does it leave circular discolorations?

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A: If you spill a drop of coffee on a table, you typically see a dark ring with a pale centre after it dries. This pattern emerges from how the drop vaporizes and how coffee particles move. The edge of the drop is typically ‘pinned’ to the surface area and does not move inward, and evaporation is much faster at this edge than at the thicker centre.

To change the water lost at the rim, liquid is drawn outside along the surface area. This sluggish and constant blood vessel circulation brings suspended coffee particles from the middle to the edge. As the water vanishes, the particles are stranded there, developing a thick ring. This is the coffee-ring impact.

The ring can be damaged or removed if the fluid inside flows in a different way. Temperature level or concentration distinctions can develop surface-tension gradients that move particles back towards the centre. Percentages of soap-like particles (called surfactants), included polymers or some glycerol can alter these circulations, sluggish evaporation or free the edge, developing a more even stain.

Particle shape likewise matters: extended grains, like little rods, can jam together and withstand being swept to the rim, producing a consistent movie.