One of the more famous effects in physics is the Coffee Ring Effect. Leave a drop - or puddle - of coffee on any surface and you'll come back to a dark ring of stain. This video shows you actual footage of your coffee particles making your coffee stain.
If you want to know why your coffee spills always leave dark rings, good news! You're not alone. Physicists have been interested in The Coffee Ring effect for quite some time. The effect happens because the coffe hugs the surface it's on, instead of beading up. As the thin edges of the stain dry, more liquid, along with staining particles, is pulled outwards to take its place.
Scientists have also been intrigued by the "Reverse Coffee Ring Effect." Instead of particles being pulled towards the edges of a pain, and leaving a dark ring, they're pulled towards the center and leave a dark central splotch. After study, researchers have found that they can make this happen by deforming the surface of the droplet, or placing some kind of object in the center of the droplet or puddle of liquid. They can also reverse the effect by changing the shape of the staining particles. Spherical particles get drawn to the edges of the stain, but ellipsoid particles pull together towards the center of the drop. In first part of the video above, you get an actual look inside your coffee spill, as the particles are sucked outwards. In the second, you get a look at ellipsoidal particles making a dash for the center of a drop.
[Via Suppression of the Coffee Effect by Shape-Dependent Capillary Interactions.]