A super-familiar phenomenon, but let’s be honest: Do you know why coffee stains are light on the outside?
Spilling coffee, wine, juice… on fabrics and papers is definitely not fun. If the stain sticks to the clothes, it can be faded by washing, but if it fades on paper, it will probably die.
Observe the stain when it dries, do you see something special: the outer edge of the stain has a much darker color than the inside? Why is that?
Coffee stains on paper.
It was not until 2012 that scientists were able to explain the mechanism of this phenomenon.
That’s thanks to a theory of coffee ring formation , born in 1997 by a team of physicists from the University of Chicago.
The actual mechanism of the phenomenon is like this: The water particles at the outer edge of the coffee streak always evaporate faster than the particles inside.
To take advantage of the (faster) evaporation rate of the edge, the liquid in the central part will flow outward. The solid particles floating in the liquid will be carried by the flow to the outer edge, and as the stain continues to dry, they will pile up at the edge like snow piling up at the foot of a fence. And that is the answer to this phenomenon.
The water particles at the outer edge of the coffee streak always evaporate faster than the particles inside.
The researchers are trying to limit the coffee ring effect by balancing it with a surface tension effect that helps circulate the beans during the drying process.
Work by the University of Pennsylvania has shown that irregularly shaped particles can resist ring formation by binding into large rafts.
However, this effect is not only harmful. Some studies have shown that it can be used in gently stacking particles on surfaces, thereby becoming a tool for micromanufacturing.