The Coffee Ring Effect is a well-known phenomenon. A puddle of coffee leaves behind a dark ring, instead of a uniform brown stain. This video explains why— and how this phenomenon resembles what happens in an avalanche.
Ever notice that, as the sides of droplet of coffee evaporate, the droplet doesn’t shrink? The droplet’s circumference doesn’t get any smaller at all. Instead, the droplet flattens down. As the sides evaporate, liquid from the rest of the droplet gets drawn out to the sides—along with all the particles that were swimming around in the liquid. The water continues to evaporate and the particles pile up on the side of the droplet, making the ring.
What’s cool about this video is it shows how the particles pile up. At first, there are just drops randomly sticking to the side, then a definite larger movement, and then a fast charge of all the available particles. What we’re seeing is a horizontal avalanche of particles getting sucked towards the edge of the ring, the same way we’d see a vertical avalanche of snowflakes falling down a mountain.
[Source: Álvaro Gómez Marín]