ambience: melodic background music
People around the world depend on coffee to start their day, but how many can credit their morning cup with jump-starting experiments in a new branch of physics? I’m Jim Metzner, and this is the Pulse of the Planet. Sydney Nagel, a University of Chicago professor, studies physics problems that exist outside the controlled environment of the lab. One morning looking at some spilled coffee, Dr. Nagel noticed the stain didn’t act the way that he would have predicted.
“What I expected to happen was that, if I have a drop of coffee which lands on a countertop, the drop is actually taller at the center than it is at the edges. And so, my expectation had been that when the drop dries, well since most of the coffee is in the center, the stain should be darkest in the center, but then I looked at all the stains around me, I have never seen a stain that is darker in the center and lighter at the edges. It’s always exactly the opposite.”
So Nagel and his colleagues set out to unravel the mystery of the coffee stain.
“We tried to look at these drops on just about every surface we could think of. We tried glass, we tried metal we tried plastic, and we found the same thing would happen on all these different substances.”
It turns out, that the fundamental reason for the drop’s behavior is a geometric one.
“When the drop dries, the fluid is evaporating, leaving stuff behind, and where the water originally hit the surface of the table, that perimeter does not move as the drop continues to dry. So if any fluid evaporates from the edges of that drop, you have to bring extra fluid from the inside of the drop, this creates flows that flow out to the perimeter.”
These flows concentrate particles at the edge of the drop, causing the stain’s dark ring. This finding offers some new scientific possibilities, including techniques for sequencing DNA. We’ll hear more in future programs.
Pulse of the Planet is presented with support provided by the National Science Foundation.