Why is it when the coffee is dry, the stain is colored in light, in addition to bold?

A super familiar phenomenon, but really ask: Do you know why the stain of coffee in the light is dark?

Pouring coffee, wine, juice . onto cloth, paperwork is definitely not fun. If the stain sticks to the clothing, it can be faded by washing, but it may be dead to the paper.

Observe the stain when dry, do you see something special: the outer edge of the stain is much darker than the inside? Why is that so?

Coffee stains on paper.

It is not until 2012 that scientists can explain the mechanism of this phenomenon.

It is thanks to a coffee circle formation theory (coffee ring) , which was born in 1997 by a group of physicists from the University of Chicago.

The actual mechanism of the phenomenon is like this: Water elements at the outer edge of the coffee streak always evaporate faster than the internal elements.

To take advantage of the faster (more) evaporation speed of the edges, the liquid in the center part will flow outwards. The solid molecules floating in the liquid will be swept away to the outer edge, and when the stain continues to dry, they are piled up at the edge like a snow scene piled up at the fence. And that is the answer to this phenomenon.

The water element at the outer edge of the coffee streak always evaporates faster than the inner element.

Researchers are trying to limit the coffee loop effect by balancing it with the surface tension effect that helps the particle circulate during the dry process.

The work of the University of Pennsylvania has shown that irregularly shaped particles can resist ring formation by bonding large rafts.

However, this effect is not harmful. A number of studies have shown that it can be used to gently place grain arrays on surfaces, thus becoming a tool for micro-production.