Can someone explain how when coffee is spilled a ring is formed around the edge of the spill, while spills of other liquids do not? Is it something only in the coffee that does this?
WAG Surface tension?
The key is probably the botom of the mug - it touches the table as a ring, rather than a disk, as in a typical tumbler or “iced tea” glass.
I don’t have the experience you do. I perceive that most, if not all, fluids leave rings. Maybe the difference is the kind of cup you use. I drink just about everything out of a mug.
A professor in my department actually investigated this question some time back. Here’s an article on it.
Actually, what I alluded to in “coffee rings” wasn’t the ring that is formed by a mug, but the dark ring that appears when a coffee spill dries up.
I think the cup or mug is what creates the ring. Coffee spills down the side of the container then moves along the edge of the ring.
I don’t think you can just pour some coffee on a flat surface and have a ring magically form.
MikeS got it then:Here’s an article on it.
Basically, there’s more evaporation at the edges of the drop than in the middle, so solutes are drawn to that outer ring. I’ve seen the same effect with colored Kool-Aid.
As did I.