Have you ever noticed that when a drop of coffee hits a surface it dries in the form of a circle? In 1997 Physicists asked themselves that question and they came up with a theory of why it occurs, a theory that led to various problems related to deposition of material, a problem that has finally led the group of researchers to figure out a way to make those rings stop forming.
The ring issue was taken up and solved by a group of physicists at the University of Pennsylvania and here’s what they knew; when a drop of coffee dries the outer edges are pinned, therefore the radius doesn’t change, even at the amount of liquid shrinks. As the volume of the drop decreases from evaporation the contact angle of the edge of the drop also decreases. Because of that string of events a radial capillary flow is formed which carried coffee particles from the center of the drop to the edge, forming a ring.
So how exactly did researchers solve this problem? By making the drop into a non-spherical element. Researchers found that when ellipsoidal particles are transported to the drop edge, they form loosely packed structures that are resistant to capillary flow, as the drop evaporates the particles are evenly distributed, the more elongated they become, the less likely a ring will form.
While to the average person this entire process may seem like a waste of time, the groups understanding of how to deposit materials across a surface could have huge impacts on industries such as printing and various forms of material coating.
Sorry folks, this discovery isn’t going to get rid of coffee rings on that table where you forget to use a coaster.
You can read more about the study here: “Suppression of the coffee-ring effect by shape-dependent capillary interactions.”