The physics of coffee ring effect has been finally discovered by researchers. Coffee ring effect - a drip of coffee causes dark, well-defined edges.
The research was conducted by professor Arjun Yodh, director of the Laboratory for Research on the Structure of Matter; doctoral candidates Peter Yunker and Matthew Lohr; and postdoctoral fellow Tim Still, all of the department of Physics and Astronomy in Penn's School of Arts and Sciences.
It turns out that liquids, such as coffee, that are made up of spherical particles dry differently than liquids with more oblong particles, scientists have found.
Spherical particles easily detach from the interface, and they flow past one another easily because the spheres do not substantially deform the air-water interface.
Ellipsoid particles, however, cause substantial undulation of the air-water interface that in turn induces very strong attractions between the ellipsoids.
After experimenting with suspended particle shape, the researchers added a surfactant, essentially soap, into the drops to show that interactions on the drop's surface were responsible for the effect. With the surfactant lowering the drop's surface tension, ellipsoid particles did not get stuck at the interface and flowed freely to the edge.
Understanding the impact of particle shape on drop drying could have applications in printing and painting. The principles could also be relevant in biological and medical contexts.
The study was recently published in the journal Nature.