The Physics of Coffee Rings Finally Explained

It’s Monday morning. Another work week begins; another cup of coffee to the rescue. If you’re not careful, you might spill a bit of that precious coffee and then later wonder (à la Jerry Seinfeld) — What is the deal with that coffee ring on the table? Why does it form a ring with dark, outer edges? You can imagine Seinfeld asking this, right?

Well, it turns out there’s an answer for this. And it comes straight from a laboratory at the University of Pennsylvania’s Department of Physics and Astronomy. Yes, my friends, it all comes down to the shape of the particles in the liquid. Coffee is made up of spherical particles, and they get distributed unevenly, with some pushing outward towards an edge and forming dark rings. Meanwhile, other liquids are made up of oblong particles that get distributed evenly, hence no rings. The UPenn video above breaks it all down for you.

Amazingly, this isn’t our first post on Physics and Coffee. Here’s a quick look at how they drink coffee at zero gravity in the International Space Station. Enjoy!

via Radio Lab

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  1. Jorge Vieyra says . . . | October 29, 2012 / 9:03 am

    This are old news. See, for the 1997 article in Nature

  2. Arjuna says . . . | November 21, 2012 / 10:10 am

    Great video…Very well explained… Have you guys heard of organo gold?? its apparently extremely good for you and healthy..

  3. Furnald Hall says . . . | April 6, 2013 / 10:24 am

    But WHY does the liquid flow from the central part of the drop to the edge as it evaporates, which drives the whole effect for round particles? Does it evaporate more readily from the edges, perhaps due to the convex surface of a drop on a hard surface,leading to gravity pushing more fluid from the central “reservoir” towards the edge? Or is there some other reason? We need to bring MIT into this….

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