Ring Around The Coffee

In science labs and coffee shops
It can’t be helped; some liquid drops
On floor or table.
And as these spills evaporate
On table, napkin, floor or plate
The stuff’s unstable
When coffee dries, it forms a ring
Around its edge—but here’s the thing
We didn’t know—
See, other liquids just dry flat,
Without a ring. But why is that?
And off we go.
The scientists at U. of Penn
Perhaps were drinking coffee, when
They first surmised,
A detail that, before, escaped—
It’s how the particles were shaped
As well as sized.
And coffee’s little, tiny spheres
Flow edge-ward, while a rod adheres
And keeps to center
The little rods deform the drop
And should they try to move, it’s “stop!”
And “do not enter!”

So, anyway, it’s cool. They think
It might be used in paint or ink
For better printing.
With smoother drying, now each letter
Holds its shape and color better—
No more squinting!
The other thing is, through these studies
The particles are seen as buddies,
Fun little critters
We play each morning, our little game
I drink them down; they get the blame
For morning jitters.

Via NPR, a story on the physical properties of coffee rings. This is one of the things I love about scientists. I must have looked at coffee rings thousands of times, and perhaps even noticed that the stain concentrates at the edges of drops. In fact, I am certain I have noticed this, because I thought it looked like an exaggerated Mach Band–the visual perception effect that accentuates edges, as a result of the lateral inhibition of retinal cells. (Maybe I’ll write about that some other time.)

But anyway, these people looked at the stain, and instead of reaching for a sponge, uttered those wonderful scientific words “huh. that’s strange. I wonder….” or words to that effect. And now, we have an answer.

Of course, for me, just as much fun as the physical science of the particles, is the human science of the commenters at the NPR story page. We can predict (and find) the comment we find on every science study: “our tax dollars pay for this?”, others decry the triviality of the topic. But, this being NPR, we get others who (and I would never have made this connection, so kudos to the commenters) draw a connection between this and perhaps the early formation of cells, with some particles migrating to edges (eventually, cell walls) through purely physical processes.

Lastly, something completely orthogonal to practical use–whether worthless or priceless or something between… turns out the process is beautiful, when you look at it just right (viddy link–or look below).


  1. says

    A poem about coffee rings! And this priceless line: “So, anyway, it’s cool.”


    I could just about fall in love with you, if I weren’t already so happily married. :-)

    You are such a nerd!! *wink*

  2. Diana Hickman says

    I heard that story on NPR yesterday and thought “How interesting.” You heard it and wrote a poem, about coffee rings and scientists. Just an observation.

  3. E.V. says

    My god, man – I’m always impressed by your facility with words.

    Speaking of which, have you heard of Murphry’s Law? Any snarky attempt to correct a poster for grammatical errors or misspellings will result in grammatical errors and/or misspellings within the response. The more acerbic the response, the greater the error.

  4. Peter B says

    From coffee rings to blown hay.

    Several weeks ago while driving on I5 south of Sacramento I passed a loaded hay truck. It was shedding hay as such trucks tend to do. “Hey”, I thought to myself ignoring a terrible pun, “that’s illegal — only clean water and chicken feathers are allowed to be dropped by vehicles.” Yet, who would complain? (FWIW, in season I often see tomatoes scattered on Hwy 99 and I5.)

    Then I noticed hay scattered on the median barrier. Random sized piles at random locations. That’s interesting. What combination of prevailing winds and traffic windage could cause such patterns? The more I thought about it the less likely turbulence dynamics seemed to be the answer.

    More studies are needed. Where is NSF grant money when you need it?

    Then I found a more realistic hypothesis: weeds. Specifically weeds growing on dirt trapped under median barriers. Weed growth is common and random. Weeds catch blown hay. Larger weeds trap more hay. Thus my hypothesis is plausible.

    A post doc at UC Davis should get that grant to provide the definitive answer to one of California’s most minor mysteries.


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