Why is the dried coffee stain a ring instead of a solid circle?

Perhaps some of us have found this interesting phenomenon-coffee spilled from the cup often does not leave a solid round stain, but presents a ring mark along the edge of the coffee spill. Of course, no matter what shape the stains are, it won't make it harder for you to erase them.

Perhaps some of us have found this interesting phenomenon-coffee spilled from the cup often does not leave a solid round stain, but presents a ring mark along the edge of the coffee spill. Of course, no matter what shape the stains are, it won't make it harder for you to erase them. But when printing, this phenomenon is very annoying. The paint sample, which looks not bad, has changed beyond recognition after being killed. What are we going to do? It's a good thing we have physicists. Scientists at the University of Pennsylvania recently found that the way to prevent this is simple-you just need to change the shape of the particles involved.

Before a drop of water is dried, it is usually not reduced to a smaller drop of water, but flattened. Most children's shoes will observe that the water stains on the coffee table or bar will slowly change from a large drop to a small puddle, and then into a water film, and finally completely dry out. Because water molecules attract each other, they "fix" the surface of the liquid. This means that the water molecules will hold each other close to the surface of the liquid, while the water molecules in the milk will cling to the wall of the container. Because of this, different evaporation rates cause capillary seepage-the liquid at the edge evaporates, and the liquid inside will be replenished. The resulting inner-to-outward flow pushes almost all the liquid to the edge of the droplet, driving the pigment particles to move together. As it evaporates, dark ring marks are left on the outside and transparent in the middle.

Scientists at the University of Chicago first discovered the mechanism of the "coffee ring" effect, and then they observed that spherical particles were more likely to escape from the attraction of the liquid surface and move more easily from the middle of the liquid to the edge. On the other hand, ellipsoidal particles are more resistant to this capillary seepage, and they tend to leave a more flat and uniform imprint. If someone decides to produce coffee with ellipsoidal particles, we will be able to "appreciate" coffee stains of uniform color when we drink coffee. I saw the news from the university today, saying that it solved the problem of the coffee circle. The problem is that the coffee spilled on the table, when dry, forms a circle because the coffee particles gather to the edge in the process of losing moisture. The solution is simple: as long as the particles are lengthened, they will not gather, they will be evenly distributed on the changed surface, and the coffee circle will disappear, which is very useful for printing and paint development, the article was published in the journal Nature.