When espresso evaporates: the physics of coffee rings

I’ve spilled a lot of coffee over the years. Usually not a whole cup, just a drop or two while pouring. And when it’s just a drop, it’s easy to justify waiting to clean it up. When the drop dries on the table, it forms a stain with a ring (Figure 1), giving it the look of a deliberately outlined splotch of brown in a contemporary art piece (In this context, the phrase “coffee ring” refers to this small-scale, spontaneously formed circular stain rather than the imprint left on a table from the bottom of a wet coffee cup). But the appearance of these stains is simply a result of the physics happening inside the drop. Coffee is made of tiny granules of ground up coffee beans suspended in water, so the ring must mean that these granules migrate to the edge of the droplet when it dries. Why do the granules travel as they dry? Today’s paper by Robert D. Deegan, Olgica Bakajin, Todd F. Dupont, Greb Huber, Sidney R. Nagel and Thomas A. Witten provides evidence that coffee rings arise due to capillary flow--  the flow of liquid due to intermolecular forces within the liquid and between the liquid and its surrounding surfaces.

Color made from structures inspired by bird feathers

There’s a reason why the word “peacock” has become a verb synonymous with commanding attention. Of course the size of the peacock tail is enough to turn heads, but it wouldn’t be nearly as beautiful without its signature iridescent, or angle-dependent, color. The brilliant colors of the peacock come from the interaction of light with the nanoscale structure of the feathers, which is much different from the origin of color in regular dyes and pigments. In today’s paper, Jason Forster and his colleagues in the Dufresne group developed a simple way to make colors that is inspired by the structures in certain bird feathers. To understand how it works, let’s start from the beginning.